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This�paper�was�submitted�by�the�faculty�of�FAU’s�Harbor�Branch�Oceanographic�Institute.�������

Notice:�©1996�Estuarine�Research�Federation.�This�manuscript�is�an�author�version�with�the�final�publication�available�at�http://www.springerlink.com/�and�may�be�cited�as:��Lapointe,�B.�E.,�&�Matzie,�W.�R.�(1996).�Effects�of�Stormwater�Nutrient�Discharges�on�Eutrophication�Processes�in�Nearshore�Waters�of�the�Florida�Keys.�Estuaries,�19(2B),�422Ͳ435.�doi:10.2307/1352460�

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Estuaries Vol. 19, No 2B, p. 422-435 June 1996

Effects of Stormwater Nutrient Discharges onEutrophication Processes in NearshoreWaters of the Florida Keys

BRIAN E. LAPOINTE!WILLIAM R. MATZIEHarbor Branch Or:eanogmphic Institution, Inc.Division 0I Marine ScienceFlorida Keys Program'3754 Pine ,....treetBig Pine N"!, Florida 3304 '3

ABSTRACT: Rainfall events cause episodic discharges of groundwaters contaminated with septic tank effluent intonearshore waters of the Florida Keys, enhancing eutrophication in sensitive coral reef communities. Our study charac-terized the effects of stormwater discharges by continuously (30-min intervals) measuring salinity, temperature, tidalstage, and dissolved oxygen (DO) along an offshore eutrophication gradient prior to and following heavy rainfall at thebeginning of the 1992 rainy season. The gradient included stations at a developed canal system (1'1') on Big Pine Key, aseagrass meadow in a tidal channel (PC), a nearshore patch reef (PR), a bank reef at Looe Key National Marine Sanctuary(LK), and a blue water station (BW) approximately 9 km off of Big PIne Key. Water samples were collected at weeklyintervals during this period to determine concentrations of total nitrogen (TN), ammonium (NH•• ), nitrate plus nitrite(NO, - plus NO, ), total phosphorus (TP), total dissolved phosphorus (TDP), soluble reactive phosphorus (SRP), andchlorophyll a (chi a). Decreased salinity immediately followed the first major rainfall at Big Pine Key, which was followedby anoxia (DO <, 0.1 mg 1-'), high concentrations of NH. + (-24 J-LM), TDP (-1.5 J-LM), and chi a (-20 J-Lg I-I). Maximumconcentrations of TDP (-0.30 J-LM) also followed the initial rainfall at the PC, PR, and LK stations. In contrast, NH. +(-4.0 J-LM) and chi a (0.45 J-Lg 1-') lagged the rain event by 1-3 wk, depending on distance from shore. The highest andmost variable concentrations of NH. ", TDP, and chi a occurred at PI', and all nutrient parameters correlated positivelywith rainfall. DO at all stations was positively correlated with tide and salinity and the lowest values occurred during lowtide and low salinity (high rainfaIJ) periods. Hypoxia (DO < 2.5 mg I') was observed at all stations following thestormwater discharges, including the offshore bank reef station LK. Our study demonstrated that high frequency (daily)sampling is necessary to track the effects of episodic rainfall events on water quality and that such effects can be detectedat considerable distances (12 krn ) from shore. The low levels of DO and high levels of nutrients and chi a in coastalwaters of the Florida Keys demand that special precautions be exercised in the treatment and discharge of wastewatersand land-based runoff in order to preserve sensitive coral reef communities.

IntroductionProtecting water quality and the long-term health

of the Florida Reef Tract is an issue of national con-cern as this outstanding natural resource reprcsrntsNorth America's only living coral reef ecosystem.Numerous scientific and management workshopshave addressed the effects of expanding human ac-tivities on this resource, which led to passage of theFlorida Keys National Marine Sanctuary Act by theUnited Stales Congress in 1990. While a suite ofhuman impacts have been recognized, water qualitydegradation resulting from nutrient overenrich-ment and eutrophication is considered the most sig-nificant and widespread problem (National Oce-anic and Atmospheric Administration 1988; UnitedStates Environmen tal Protection Agency 1993). Cor-

, Corresponding author.

© 1996 Estuarine Research Federation 422

al reefs have low nutrient thresholds for the onsetof decline from eutrophication, as noted in Hawaii(Smith et al. 1981), Barbados (Tomascik and Sand-el" 1985, 1987), the Florida Keys (Lapointe andClark 1992), and the Great Barrier Reef (Bell1992), thus the concern regarding nutrient over-enrichment.Most studies of coastal eutrophication consider

surface runoff as the major pathway of nutrientinput, although submarine groundwater dischargeaccounts for substantial inputs to many coastal sys-tems (Johannes 1980; Valida et al. 1990). Subma-rine groundwater discharge results from ground-water percolating up through nearshore sedi-ments, driven by the hydraulic head differential be-tween the receiving water and the water table onthe adjacent land mass. Submarine groundwaterdischarge delivers three to five times as much ni-

trate to coastal waters as riverine inputs along thenorth Perth coastline in Western Australia (Johan-nes and Hearn 1985). It is also an important routeof anthropogenic nutrient input in the FloridaKeys, where high groundwater tables and transmis-sive limestone substrata occur along with the wide-spread use of septic tanks, cesspits, and injectionwells for on-site sewage disposal systems (Lapointeet al. 1990; United States Environmental Protec-tion Agency 1992). Although the importance ofwatershed rainfall as a major factor regulating riv-erine nutrient inputs and primary production hasbeen recognized for temperate estuaries (Mallin e tal. 1993), the relationship of rainfall to episodicnutrient enrichment via submarine groundwater'discharge and its ecological effects have not beenassessed in the coastal waters of the Florida Keys.One pervasive impact resulting from the wide-

spread use of on-site disposal systems in the FloridaKeys is the reduction of dissolved oxygen (DO)concentrations in nearshore waters (Lapointe andClark 1992). Such reductions in DO result fromthe contamination of groundwaters and surfacewaters with untreated or pari ially treated wastewa-ter effluents that have high 'levels of biochemicaloxygen demand (BOD) (Mitchell 1974; Bicki et al.1984). In addition, submarine groundwater dis-charge of nutrient-enriched groundwaters increasephytoplankton biomass (chlorophyll a), light atten-uation, and community respiration, all of which re-duce DO levels, particularly during war-m, cloudyperiods (Odum and Wilson 1962; Valida et al.1990; Lapointe et al. 1994). Most pollution studiesmake single-point measurements of DO duringdaylight hours; however, the minimal daily DO oc-curs during pre-dawn hours, so midday measure-ments alone lead to misconceptions of ecosystemstatus (Johannes 1975; Lapointe and Clark 1992).This study tested the hypothesis that rainfall-driv-

en stormwater inputs to nearshore waters of theFlorida Keys results in "pulsed" submarinegroundwater discharge of wastewaters that subse-quently cause transient periods of nutrient enrich-ment, development of Widespread phytoplanktonblooms, and reduced DO. To address this hypoth-esis, our study characterized nutrient concentra-tions, chlorophyll a (chi a), DO, salinity, and tidalstage along an offshore transect from Big Pine Keyto Looe Key National Marine Sanctuary (LKNMS)during the transition pe-riod from the dry seasonto the wet season. Our previous research on effectsof land-based wastewater nutrient enrichment onproductivity and trophic structuring of seagrasscommunities along this transect demonstrated itsuse as a eutrophication gradient (Lapointe et al.1994). As noted by Ketchum (1967), if the totalnutrient concentration of lower salinity coastal wa-

Effects of Storrnwater Nutrient Discharges 423

tn is highel- than that of more offshore, highersalinity waters, then a land-based source of nutri-ents is indicated.

Materials and MethodsENVIRONMENTAL SETTING

The Florida Keys, a 220-mile-long archipelago oflow-lying carbonate islands stretching from KeyLargo to Dry Tortugas, are flanked by the Gulf ofMexico and Florida Bay to the north and west andthe Straits of Florida and Atlantic Ocean to thesouth and east. Channels between the Keys allowfor the net transport of water from the Gulf ofMexico and Florida Bay seaward toward the off-shore bank reefs and the Straits of Florida (Smith1994; Fig. 1). The climate of the Keys is typical ofthe "wet and dry" tropics, with over 80% of theannual rainfall falling between June and October(average annual rainfall is inches yr- 1; Mac-Vicar 1983). Tides in Monroe County are sernidi-urnal on the Atlantic coast and mixed on the Gulfof Mexico coast. Mean sea level varies by 24 ernthrough the year, with maximum tides occurringbetween May and October (Marmer 1954).Human development of the Florida Keys over

the past several decades has signifrcantly increasedwastewater nutrient discharges to coastal waters.During the 1950s and 1960s, extensive canalization(utilizing dredge and fill operations) of the Keysprovided greater boating access for residential andtourism development. Freshwater recharge ofgroundwaters in the Florida Keys has been in-creased by human development as a result of po-table water supplies being pumped into the FloridaKeys from well fields on the South Florida main-land (current freshwater flows into the FloridaKeys are 14 million gallons per day; United StatesEnvironmental Protection Agency 1992). With theexception of the two incorporated areas in theKeys-Key West and Key Colony Beach-that havecentralized wastewater collection and secondarytreatment, the remaining areas of the Keys rely onon-site disposal systems. The extensive canal sys-tems in the Keys. as well as contiguous nearshorewaters, are mixing zones where groundwater nu-trients derived from on-site disposal systems dis-charge into nearshore waters (Lapointe et al. 1990;Lapointe and Clark 1992). Approximately 65% ofthe domestic wastewater in the Florida Keys is dis-posed of by on-site sewage disposal systems(OSDS), which include septic tanks (reg-ulated by Chapter 100-6 of the Florida Adminis-trative Code as administered by the Florida De-partment of Health and Rehabilitative Services)and 11 ,000 illegal cesspits (George Garrett, Mon-roe County personal communication). The re-

424 B. E. Lapointe and W. R. Matzie

GULF OF MEXICO

I10

KilometersIo

"\

BIG PINE KEY

ATLANTIC OCEAN

HAWK CHANNEL

LKNMS

•....1

BW

..-..-..-..I

LK II

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Fig. 1. Map of the study area in the lower Florida Keys adjacent to Big Pine Key showing the water quality monitoring stations.LOOE KEY (LK), N 24°32'84.1", W 81°24'43.8"; PATCH REEF (PR) N 24°36'79.0", W 81°23'67.0"; PINE CHANNEL (PC), N24°42'07.4", W 81°24'52.1"; PORT PINE (1'1'), N 24°43'39.5", W 81°23'94.4"; BLUE WATER (BW), N 24°31'29.8", W 81°24'23.3".

maining wastewater from the OSDS's is disposedof by 1,000 aerobic treatment units that dis-charge into shallow (20-30 m) injection wells reg-ulated by the Florida Department of Environmen-tal Protection (United States Environmental Pro-tection Agency 1992, 1993).

HYOROLAB MONITORING

Our eutrophication gradient extended from in-shore waters of Big Pine Key to km offshore(south) and included four continuously monitoredstations; a canal system directly impacted by septictank discharges (Port Pine Canal, 1'1'), a seagrassmeadow in a tidal channel (Pine Channel, PC), apatch reef inshore of Hawk Channel (PR) , and anoffshore bank reef at Looe Key National Marine

Sanctuary (LK, Fig. 1; Table I). A fifth station(BW) was located 12 km offshore and 3 km southof Looe Key National Marine Sanctuary in blue wa-ter and was monitored at weekly intervals for nu-trients and chI a. Hydrolab Data Sonde 3 multiple-sensor water quality data-logging instruments weredeployed simultaneously at each of the four sta-tions in 1-2 m water depth, and remained in placebetween April 9 and August 6, 1992, to providecontinuous monitoring along the nearshore por-tion of the eutrophication gradient. This samplingr-egime allowed us to assess short-term (diel andindividual storm events) as well as longer term, sea-sonal variability in water quality parameters. By ini-tiating sampling during the dry season (April andMay) and continuing through the wet season in

Effects of Stormwaler Nutrient Discharges 425

TABLE 1. Descriptive statistics for five Hydrolab Datasonde parameters.

Standard StandardLocation Parameter Number Mminuuu Maximum MI:.lI\ Error Deviation

Port PineTemperature n=) 4,31:\ '21 1'2 32.86 29.'28 0.04 '264pH 4,34:, 7.')2 8.37 8.04 0.00 0.16Salinity (%0) 4,3'13 2'U 39.5 :0633 0.03 1.83Dissolved oxygen (mg I ') 4,34', a 8.22 298 0.03 1.92Tide (m) 4,313 1.17 1.82 1.40 0,00 0.12

Pine ChannelTemperature eC) 4,315 2037 3414 2856 0.04 2.53pH 4,345 7.81 1'.99 8.25 000 0.13Salinity (%0) 4,345 '29.8 39.5 :i5.94 0.03 2.09Dissolved oxygen (mg I ') 4,345 1.39 1201 5G8 0.03 1.76Tide (m) 3,347 1.2G 2.14 1.73 0.00 0.14

Patch ReefTemperature COC) 4,34') 22.7 33.31 28.33 0.04 2.33pH 4,345 7.86 9.42 821 0.00 0.10Salinity (%0) 4,345 33."' 38.6 36.55 0.02 1.29Dissolved oxygen (mg I-I) 4,345 1.44 11.45 602 0.02 1.43Tide (m) 4,31S 3.3 4.34 3.83 000 0.17

Looe KeyTemperature (OC) 4,344 2:1.28 32.13 27.94 0.03 2.25pH 4,334 7.1 8.S 8.18 1).00 O.OC)Salinity (%0) 3,0,,0 33.4 37.6 36.40 0.01 053Dissolved oxygen (mg I I) 4,3-13 1.65 12.98 612 002 1.35Tide (m) 3,32:, 1.17 2.2 1.68 000 0.18

June and July, we characterized the effects of the"first flush" on water quality parameters and eu-trophication processes. Rainfall was monitoredwith a gauge at PP throughout the study; rainwatersamples for nutrient analysis were collected inclean high-density polyethylene containers.The Hydrolabs provided continuous records of

water quality parameters, including temperature,specific conductance, salinity, DO, and depth. Con-crete mooring systems were constructed to stabilizethe Hydrolabs; a PVC shield was included to re-duce physical damage, sedimentation, and lightthat contribute to fouling of the sensors, Prelimi-nary experiments showed that the Hydrolabs pro-vided accurate data for only 5-6 d due to biofoul-ing of the sensors. Accordingly, we deployed theHydrolabs for five consecutive 3-wk periods bycleaning the sensors at 4-d intervals; to achievethis, over lOa individual maintenance missionswere performed during the study. The Hydrolabswere programmed to collect ambient water qualitydata at 30-min intervals. Prior to each 3-wk de-ployment, the specific instrument checks and cal-ibration protocols described below were carriedout.

TemperatureThe temperature sensor was factory set and re-

quired no calibration or maintenance; however, ateach calibration, temperature was checked againsta laboratory thermometer (Walter H. Kessler

#20S6 mercury thermometer) as a check for ac-curacy.

SjJecijic Conductance/ SalinityThe Hydrolabs wen' recalibrated for specific

conductance / sal in i ty wi t h cond uctivi ty / salinitystandards prior to each deployment. Ambient sea-water in the sampling area has an average conduc-tance of 50 mS cm -), hence we used a conductivitystandard that "bracketed" that value. A YSI 3165Conductivity Calibrator Solution (100 mS cm")was mixed (I: I volume/volume) with deionizedwater to yield a conductivity standard of 50 mScm-I The salinity of a sterilized, filtered standardseawater sample was also determined with a facto-r-y-calibrated SeaBird CTD and used for intercali-bration of salinity measurements.

Dissolved OxygenDO calibration of the Hydrolabs began with re-

placement of the DO membrane. One day prior toinstrument deployment, a new low-flow membranewas installed on the DO probe, stretched, and con-ditioned in deionized water over-night. The follow-ing day barometric pressure was recorded and en-tered into the DO calibration system while the new,relaxed membrane was fully saturated. The air cal-ibration method described in the Hydrolab Oper-ating Manual was used as well as a common bathin which all four insrruments were immersed si-multaneously. Pre- and postdeployment cross-cali-

426 B. E. Lapointe and W R Malzie

bration checks were performed with a SeabirdCTD and by Winkler titration (using a Hach Dig-ital Titrator) to detect any significant instrumentdrift and to insure accurate DO measurement.Time series DO data derived from the Ilydrolabdeployments were analyzed along with tide and sa-linity data using the distance-weighted least squaresmethod. This allowed us to produce a three-di-mensional statistical representation of the interac-tion among DO, tide, and salinity.

DepthCalibration of the depth sensor was accom-

plished by entering zero for the standard at thewater surface. This system was checked at the studysites by use of a standard tape measure as depthswere within 2-3 m of the surface.

NUTRIENTS AND CHLOROPH\lL A

Water samples were collected for determinationof ammonium (NH4 "), total nitrogen (TN), nitrateplus nitrite (NO] - plus -), total phosphorus(TP), total dissolved phosphorus (TOP), solublereactive phosphorus (SRP), and chlorophyll a (chia) at each of the four Hydrolab stations and theoffshore BW station at approximately I-wk inter-vals between April 27 and July 30, 1992. Our studyfocused on concentrations of NH4 + due to its pre-dominance in groundwaters contaminated by sep-tic tank effluent (Lapointe et al. ]990) and in sur-face waters of the Florida Keys (Lapointe and Clark]992). TOP was measured because this nutrientpool includes dissolved organic phosphorus(DOP), the major dissolved phosphorus pool thatsupports macroalgal blooms in coastal waters ofthe Florida Keys via alkaline phosphatase hydrolysisof organic phosphorus monoesters (Lapointe1989; Lapointe et al. 1994). Chi a concentrationswere measured as an index of phytoplankton bio-mass, the best overall parameter for monitoringeutrophication on coral reefs (Laws and Rcdalje1979).Duplicate water samples were collected adjacent

to the Hydrolab units m depth) and at theBW station (2 m depth) into clean, high-densitypolyethylene bottles and kept on ice in the darkduring return to the laboratory. One aliquot ofeach sample was filtered through a 0.45-j.J-m GFIFfilter. The filter was analyzed for chi a using a mod-ified dimethyl sulfoxide (DMSO)-acetone method(Burnison 1979) for extraction followed by mea-surement of fluorescence on a Turner DesignsModel 10 fluorometer calibrated with known con-centrations of reagent-grade chlorophyll. The fil-trate of each water sample was frozen until analysisfor NH4 + (Slawk and Maclsaak 1972) and TDP ona Technicon Autoanalyzer II; TOP was determined

if)WIo

"

Fig. Weekly rainfall during the study period

by persulfate digestion followed by the molybde-num-blue method for SRP (Menzel and Corwin1965). Whole, unfiltered water samples were pre-digested with persulfate- then analyzed for TN(D'Elia et al. 1977) and TP (Menzel and Corwin1965).

ResultsRAINL\LL

The spring and early summer of 1992 were typ-ical of the transition from the drv to wet season inthe Florida Keys. Between April and the end ofMay, there was no significant rainfall and this pe-riod provided a dry season baseline for our study.The wet season began in the first week of Junewhen a total of 5.0 inches of rain fell; 3.6 inchesfell the second week, 5.0 inches the third week,and 3.0 inches the last week, totaling 16.6 inchesfor the month. During .Juh. '2.5 inches of rain fell(Fig. '2).

J-l\UROLlli MEASUREMENTS

The Hydrolab records at all four stations illus-trated the transition from the cooler, drier springmonths to the warmer, wetter summer months.The minimum, maximum, and mean values of thefive parameters measured with the Hydrolabs areprovided in Table I.

Port Pille CanalA mid-spring cold front in late April, which re-

duced water temperatures to 21.1°C at PP, was fol-lowed by a continuous warming trend inJune andJuly, and a temperature peak of 32.9°C in August.Salinity at this station increased steadily during thespring dry season, to a maximum of 39.5%0 at theend of May; it decreased with the onset of the rainyseason and reached a minimum value of 29.3%0in early July, after which it increased (Fig. 3).The DO concentrations at PP averaged 2.9 2: 1.9

mg 1-1 (n = 4,343) during the study (Table 1). Themaximum DO values, up to 8.8 mg I-I, occurredduring midday in the cooler, drier period in Apriland May. DO values decreased to hypoxic «2.5mg I· I) and anoxic (DO < 0.1 rng 1-1) levels with

38

SALINITYpptI.' 30

TIDEm

3.5

30

25

20

Effects of Storm water Nutrient Discharges 427

DiSSOLVEDOXYGENmq- I

'Or""8: 34J2

'0

otf' ",'" .,ri' .,f- 4''' ",,' 4'" t)....... 1/''' "\,....'0 ,,¢Q <J>

Fig. 3. Dissolved oxygen (mg I-I; n = 4,343) and salinity(parts per thousand, n = 4,343) between April 9 and August 6,1992, at the Port Pine Canal station on Big Pine Key, Florida.

the onset of the rainy season in early June (Figs. 3and 4). Low DO levels at PP began on May 30 con-current with the beginning of the rainy season andreduced salinity; persistent anoxia began on June8 when the first heavy rainfall (>3 inches) of theseason occurred (Fig. 4). Over the course of thisstudy, DO levels correlated negatively with temper-ature (p < 0.001) and positively with salinity (p <0.001) and tide (p < 0.001). Reduced salinity fromrainfall and ebbing tides interacted to reduce DOlevels at PP (Fig. 5).

Pine ChannelThe mid-spring cold front in late April reduced

water temperatures to 20.4°C at PC, followed by acontinuous warming trend through June and July,and a water temperature peak at 34.1°C in August.Salinity at this station increased steadily during thedry season in April and May, and peaked at 39.5%0at the end of May. Salinity decreased with the onsetof the rainy season and reached a minimum valueof 29.8%0 in late June and early July, after whichit increased (Fig. 6).The DO concentrations at PC averaged 5.7 :!:: 1.8

Fig. 5. Surface response curve of dissolved oxygen (mg I-In = 4,343) versus ride (meters, 01; n = '1,:H3) and salinity (pansper thousand, n = at Port Pine Canal during the studyperiod.

mg I-I (n = 4,345) during the COUI'se of the study.The maximum DO values, up to 12.0 mg I-I, oc-curred during midday in the cooler, drier periodin May and decreased with the onset of the rainyseason; the minimum value of 1.4 mg I-I followedthe first major rainfall on June 6. Substantial di-urnal variability in DO occurred in the seagl'assmeadows at PC, with hypoxic values ranging from<2.0 mg I-I before dawn to >6.0 mg I-I at midday(Fig. 6). DO at PC correlated negatively with tem-perature (p < 0.00 l) and positively with salinity (p= 0.004) and tide (p < 0.001).

Patch /1/'{'/The mid-spring cold front in late April reduced

water temperatures to 22.7°C at PR, followed by acontinuous warming trend through June and July,and a water temperature peak at 33.3°(: in August.Salinity at PR increased during the dry season illApril and May and peaked at 38.6%0 in early Junc.

7.0

6.0

ZW 5.0C!

Rainfall event>X 4.0OcC '"W 3.0> E...J0C/) 2.0C/)

151.0

0.0

Fig. 4. Time series dissolved oxygen (mg I I) at Port Pin,Canal between May 22 and June 12, 1992, showing anoxia fol-lowing the initial rainfall event.

Fig. 6. Dissolved oxygen (mg I I; n = 4,345) and salinity(parts per thousand, n = 4,345) at Pine Channel during thestudy period.

428 B. E. Lapointe and W. R. Matzie

75

80,------- "I-n

I::34

a a

zw

Q.c -:-w 0>:; E

C

Fig. 8. Dissolved oxygen (mg I-I, n = 4,345) and salinity(parts per thousand, n = 3,030) at Looe Key National MarineSanctuary during the study period.

39

TIDE

42

3.233

5.5

65DISSOLVEDOXYGENmg-I 6.0

4544

5.0

Fig. 7. Surface response curve of dissolved oxygen (mg 1-1;n = 4,345) versus tide (meters, m; n = 4,345) and salinity (partsper thousand, n = 4,345) at the Patch Reef during the studyperiod.

Salinity decreased with the onset of the rainy sea-son and reached a minimum value of 33.5%0 inearly July, after which it increased.The average DO concentration at PR was 6.0 ::!.:

1.4 mg I-I (n = 4,345) during the study. The max-imum DO values, up to 11.5 mg I-I, occurred dur-ing the cooler, drier period in May. DO concentra-tions began decreasing after May 30 and reacheda minimum value of 1.4 mg 1-1 in late July. Thereduced DO concentrations at this site began afterJune 6 when salinity decreased following the onsetof the rainy season. Over the course of the study,DO at PR correlated negatively with temperature(p < 0.001) and positively with salinity (p < 0.001)and tide (p < 0.001). Reduced salinity (resultingfrom rainfall) and ebbing tides interacted to re-duce DO levels at PR (Fig. 7).

Looe KeyThe mid-spring cold front in late April reduced

water temperatures to a low of 23.3°C at LK, fol-lowed by a continuous warming trend throughJune and July, and a water temperature peak at32.1°C in August. Salinity at LK increased steadilyduring the dry season in April and May andpeaked at 37.6%0 in early August. Salinity droppedsharply in early June with the onset of the rainyseason and reached a minimum value of 33.4%0in early July, after which it increased (Fig. 8).The DO concentrations at LK averaged 6.1 ::!.: 1.4

mg I-I (n = 4,343) during the study. The maxi-mum DO values, up to 12.9 mg I-I, occurred dur-ing the cooler, drier period in May. DO concentra-tions decreased following the onset of rainfall, andminimum values of 1.65 mg I-I occurred betweenJune 9 and 12 (Fig. 9). Following the onset of the

rainy season in June, there was a trend of increaseddiel variability in DO concentrations indicated bylower minima and higher maxima (Fig. 9). Overthe course of the study, DO at LK correlated neg-atively with temperature (p < 0.00l) and positivelywith salinity (p < 0.001) and tide (p < 0.00 1); re-duced salinity and ebbing tides interacted to re-duce DO levels at LK (Fig. 10).

Blue WaterA water column profile of DO and salinity on

July 24 showed the presence of lower salinity, lowerDO water from the surface to a 20-m depth at BW(Fig. ll).

NUTRIENTS AND CHLOROPHYLL A

The TN and TP concentrations of rainfall col-lected on June 12 and June 24 at PP averaged 14.9::!.: 5.6 I-lM and 0.03 ::!.: 0.0 I-lM, respectively. Con-centrations of N03" plus N02- averaged 9.9 ::!.: 0.0I-lM and NH4+ averaged 6.2 ::!.: 0.3 I-lM. SRP con-centrations in rainfall were <0.03 I-lM on both sam-plings.NH4 + concentrations increased at all stations fol-

lowing the initial rainfall in early J line. For exam-

zwC)>-x0,0;'g; E..J

°UlUlCi

Fig. 9. Time series dissolved oxygen (mg 1-- 1) at Looe KeyNational Marine Sanctuary between May 22 and June 12, 1992,showing hypoxia following the initial rainfall event.

Effects ot Stormwater Nutrient Discharges 429

7.5,--- "T"?" ----, PORT PINE

mg·I"DISSOLVED OXYGEN

7/97/16/236/1 7

PINE CHANNEL

6/105/29

3.6

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:E:::> 2.0

Z 1.60:E 1.0:E

0.6

3,6

3.0

2.5

2.0

1,5

1,0

0.5

Fig. 12. Ammonium concentrations (fLM) at three stations(Port Pine, Pine Channel, Looe Key) between May 29 and July9,1992. Values represent means :!:1 standard error (n = 3).

1 B

1.

1 •

12

10

occurred at PC on June 17 and at LK on July 1(Fig. 12); the maximum NH4 + concentrations oc-curred at the BW station on July 24. Over the en-tire study, NH4+ concentrations at PP correlatedsignificantly with rainfall (r = 0.37, P = 0.023).NH4 + averaged 5.48 ::!:: 5.1 J-lM at PP, 2.25 ::!:: 0.69J-lM at PC, 1.86 ::!:: 0.97 J-lM at PR, 2.02 ::!:: 0.88 J-lMat LK, and 1.1 2:: 0.61 J-lM at BW. Nitrate plus ni-trite averaged 1.09 ::!:: 0.77 IJ.M at PP, 1.38 ::!:: 0.50J.LM at PC, 0.70 ::!:: 0.31 IJ.M at PR, 0.84 ::!:: 0.51 J-lMat LK, and 0.26 ::!:: 0.09 J.LM at BW.In contrast to NH4 ' , TOP concentrations in-

creased at all stations immediately following theinitial rainfall and then decreased over the remain-der of the study. TOP concentrations at PP duringMay were <0.3 IJ.M and increased to >0.9 J.LM con-current with rainfall in early August; over the en-tire study, TOP concentrations at PP correlated sig-nificantly with rainfall (r = 0.37, P < 0.001). Theincrease in TOP following the rainfall was observedat PC, LK, and BW; following this increase, TDPdecreased over the remainder of the study at thesestations (Fig. 13).

39

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70

30.0

Fig. 10. Surface response curve of dissolved oxygen (mg I-I,n = 4,343) versus tide (Ill, n = 4,343) and salinity (parts perthousand, n = 4,343) at Looe Key National Marine Sanctuary.

60

6.5

Fig. 11. Dissolved oxygen (mg 1-') and salinity (parts perthousand) depth profile at the offshore blue water station onJuly 24, 1992,

pie, NH4 + concentrations at PP during May were<2.0 J-lM and increased to over 20 J-lM concurrentwith the onset of rainfall and reduced salinity onJune 10 (Fig. 12). Maximum NH4+ concentrations

PP

Fig. 14. Mean total nitrogen (upper) and total phosphorus(lower) concentrations (Jl-M) at five monitoring stations (PortPine, PP; Pine Channel, PC; Patch Reef, PR; Looe Key, LK; bluewater, BW) during the study period. Values represent means Istandard deviation (n = 22).

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7/97/16/236/1 76/105/29

430 S. E. Lapointe and W. R. Matzie

PORT PINE1.0

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0.70.6 .

0.5 .

0.4 .

0.3 .:2: 0.2 .:::1.

0.1(/):::l0: PINE CHANNEL0 0.3J:0-(/)0J: 0.20-CW> 0.1...J0(/)(/)

i5...J<{ LOOE KEYI- 0.30I-

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Fig. 13. Total dissolved phosphorus concentrations (I--lM, n= 22) at three stations (Port Pine, Pine Channel, Looc Key)between May 29 and July 9,1992. Values represent meansstandard error (n = 3).

As a result of temporal changes in dissolved in-organic nitrogen (DIN = NH4 + plus N03" plusN02-) and SRP concentrations, the DIN:SRP ratiosincreased from low values in the dry season tomaximum values immediately following the initialrainfall event. For example, the DIN:SRP ratio atPP increased from 7:1 on May 22 to 70:1 on June10, which was 4 d after the first major rainfallevent; following this increase, the ratio decreasedto 12:1 by July 24. A longer period was observedfor the rise in the DIN:SRP ratio in nearshore wa-ters, where values increased from 11: I to 27: 1 atPC and 11: 1 to 17: 1 at PR 11 d following the initialrain event. At the most offshore sites, a 3-wk periodpassed following the June 6 rain event before theDIN:SRP ratios increased from 9:1 to 26:1 at LKand from 5: 1 to 9: 1 at BW.Over the en tire study, the average TN concen-

trations along the offshore g,'adient were highestand most variable at PP and decreased with in-creasing distance from shore, with the lowest val-ues at BW. TN averaged 36.6 2:: 9.5 J.LM at PP, 24.7

2:: 5.8 J.LM at PC, 17.2 2:: 6.9 fLM at PR, 12.7 2:: 9.3fLM at LIZ, and 9.1 2:: 6.9 fLM at BW (Fig. 14). TPconcentrations showed a similar pattern, averaging0.49 2:: 0.30 fLM at PP, 0.17 2:: 0.06 fLM at PC, 0.] 72:: 0.07 fLM at PR, 0.14 2:: 0.08 fLM at LK, and 0.132:: 0.08 fLM at BW (Fig. 14).Concentrations of chi a increased at all stations

along the eutrophication gradient [rom the dry pe-riod into the rainy period. Chi a concentrations atPP during May were <2.0 fLg I-I and increasedmarkedly to >20 fLg 1. 1 within days following thefirst major rainfall on June 6 (Fig. 15); over theentire study, chi a concentrations at PP correlatedsignificantly with rainfall (I' = 0.57, P = 0.001). Chia also increased after June 6 at both PC and PR,where it remained elevated for the remainder ofthe study; maximum chl a values occurred at LKin late June (Fig. 15). Over the entire study, chi aaveraged 2.94 2:: 0.49 fLg I-I at PP, 0.26 2:: 0.10 /-lgI-I at PC, 0.372:: 0.13 fLg I-I at PR, 0.262:: O.ll fLgI-I at LIZ, and 0.18 2:: 0.09 J.Lg I-I at BW.

DiscussionDECREASED SALlNl'lY AND NUTRIENT ENRICHMENT

This study demonstrates the ecological effects ofepisodic stormwater nutrient discharges on coastal

Effects of Stormwater Nutrient Discharges 431

PINE CHANNEL

PORT PINE detrimental to the health of sensitive coral reefcommunities irrespective of nutrients or other pol-lutants carried by the fresh water. Jokiel et al. (1993)described how storm floods in late December 1987reduced salinity in the surface waters of KaneoheBay, resulting in a massive freshwater "reef kill";corals, echinoderms, and crustaceans all had ex-tremely high mortality rates and virtually all coralwas killed to a depth of 1-2 111. Similarly, Sakai andNishihira (1991) reported reduced salinity from ter-restrial runoff near a river mouth in Okinawa, Ja-pan, caused severe mortality of corals following anextended dry season. Even relatively minor salinityreductions can have negative effects on reef coralrecruitmen t. Richmond (1993) conducted experi-ments with coral gametes and showed that a 20%decrease in salinity from :15%0 to 28%0 resulted inan 86% drop in fertilization rate; success of larvaldevelopment following fertilization was also inhib-ited by freshwater runofT, by as much as 50% (Rich-mond 1993).The salinity issue is of particular conce-rn to the

Florida Keys National Marine Sanctuary Program assome scientists have speculated that high salinitieswere a primary factor causing stress and die-off ofseagrasses over baywidc scales in the late 1980s (e.g.,see M. Durako cited in Mcivor et al. 1994). To date,no scientific evidence is available to support this hy-pothesis (Boesch et al. 1993). The "hypersalinity hy-pothesis" was recently extrapolated to the offshorebank reefs of the Florida Keys as a mechanism toexplain recent coral reef stress and die-off (Porteret al. ] 994). Om data from Looe Key showed thehighest salinities were <38%0 and averaged 36.4%0,close to that of oceanic seawater, which is the opti-mal salinity for growth of most corals. Despite a lackof supporting evidence for the hypersalinity hypoth-esis, the Water Quality Protection Program for theFlorida Keys National Marine Sanctuary (UnitedStates Environmental Protection Agency 1993) de-veloped strategies aimed at restoring freshwaterIlows (i.e., quantity, not quality) through the Ever-glades. Water managers in South Florida began in-creasing freshwater d e live ries in 1991 throughShark River Slough (Everglades) and into the Flor-ida Bay-Florida Keys region, an action that has con-tinued to the present. The freshwater inputs toShark River Slough come from agricultur'al areasnorth of the Everglades and contain high concen-trations of nitrogen f..I.M, data on file withSouth Florida Water Manageuren t District); most ni-trogen taken up by the wei lands vegetation and mi-(Tabes of the Everglades is re-released as NI I.' andlittle net uptake or denitrification occurs (Gordonet a!. ] 986; Urban et al. 1993). Accordingly, the re-r cnt 30W}('J increase in large-scale water !lows hasconsiderably increased N loads to N-limited waters

7/16/23

__ __---4

6/1 7

LOOE KEY

6/10

0.4

OJ::1.

lUI 0.3

-l-l>- 0.2J:a.0CC 0.10-lJ:U

0.6

0.5

0.4

0.3

0.2

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1 5

waters of the Florida Keys. Decreased salinity fol-lowed the rainfall at all stations in June, includingLK which is located km from shore and wheresalinity dropped from 37%0 to 31%0 over the 3 wkfollowing the onset of rainfall. The 16.6 inches ofrain that fell in June during this study was greaterthan the long-term average of 9.5 inches for Junein south Florida; however, total rainfall over the en-tire period of study was below average due to scantrainfall in April, May, and July (MacVicar 1983).Similar observations of decreased salinity at severaloffshore bank reef sites in the Florida Keys weremade during July 1993 and were speculated to havebeen caused by flood waters from the MississippiRiver impinging on the Florida Keys region (Ogdenet al. 1994); however, no proof of that phenomenonwas provided. As shown in our study, such decreasesin salinity can occur throughout nearshore watersand bank reefs of the Keys from local storrnwaterdischarges, particularly when rainfall is near orabove average during the wet season.Dramatic salinity reductions from freshwater' dis-

charges similar to those in the present study can be

Fig. 15. Chlorophyll a concentrations (fLg 1-') at three sta-tions (Port Pine, Pine Channel, Looe Key) between May 29 andJuly 9, 1992

432 B. E. Lapointe and W. R. Matzie

of western Florida Bay (Boesch et al. 1993), result-ing in significant regionwide nitrogen enrichment.In addition to the regional background sources

of nutrients and chi a advected into coastal watersof the Florida Keys, local rainfall and potable watersupplies (5.5 billion gallons per year) pumped intothe Florida Keys result in additional wastewater nu-trient burdens via submarine groundwater dis-charge. These inputs result in local increases in phy-toplankton biomass as observed in the presentstudy. Rainfall itself contains significant concentra-tions of NH4 + and N03-, which enhance primaryproduction in coastal waters adjacent to urbanizedareas (Paerl 1985). Rainfall in the Florida Keys dur-ing our study had an average DIN concentration of15 f-lM, including f-lM of ammonium. Menzeland Spaeth (1962) reported similar ammonia con-centrations (4.97 p.M) for rainfall at Bermuda. Lowto undetectable concentrations of TP and SRP werepresent in the rainfall, pointing to the importanceof wastewater sources of phosphorus. Potable waterdeliveries to the Florida Keys have resulted in highhuman population densities and nutrient-enrichedgroundwaters from the widespread use of septictanks, cesspits, and injection wells. Episodic rainfallevents are important meteorological forcing mech-anisms that increase submarine groundwater dis-charge and nutrient loads to coastal. waters.

STORMWATER DISCHARGES AND DECREASED DO

Our study showed that rainfall events are fol-lowed by periods of critically low DO in sensitiveseagrass and coral reef communities in the FloridaKeys. Predawn DO at all stations dropped to hyp-oxic levels «2.5 mg 1-1) within days after the initialstormwater discharges in June. The impacts of re-duced DO were most dramatic at the inshore PPstation, where anoxia developed immediately fol-lowing the first heavy rain event and persisted forseveral days. The anoxia at PP was caused by en-hanced submarine discharge of groundwater con-taminated by septic tank effluent. Untreated or par-tially treated domestic wastewaters, including septictank effluent, have high BOD concentrations thatcan reduce DO in surface receiving waters (Mitchell1974; Bicki et al. 1984). Lapointe et al. (1990), us-ing heat-pulsing groundwater flow meters, showedthat rainfall events greatly enhance lateral ground-water flow and submarine groundwater dischargeinto canals and nearshore waters of the FloridaKeys. Hence, heavy rainfall events would transportunusually high BOD and nutrient burdens into sur-face waters, resulting in severe oxygen depletion asobserved in this study. Additional factors contrib-uting to anoxia and hypoxia include cascading ef-fects of organism dic-offs that add to the BOD andexacerbate environmental impacts. Nutrient enrich-

ment increases water-eolumn light attenuation dueto increased phytoplankton biomass, suspended ma-terials, and dissolved organic matter, resulting inlight limitation of phytoplankton growth (Marsh1977; Laws and Redalje 1979) and reduced oxygenconcentrations. Over time, increased sediment ox-ygen demand (SOD) resulting from the bacterialmineralization of accumulated organic matter leadsto cumulative reduction of DO (Mee 1988) and hy-pereutrophication in wastewater-impacted waters(Lapointe et al. 1994).The DO record from LK following the stormwa-

ter discharges in early June shows that peak middayDO levels can increase following such events but theminimal predawn DO levels actually decrease.These results support our previous conclusions thatpredawn DO measurements are a necessity for mon-itoring programs because they represent critical pe-riods affecting organism die-offs in coastal waters;daytime values can be misleading as they are higherthan pre-dawn values and highly dependent onmany factors, including time of measurement,weather conditions, etc. (Lapointe and Clark 1992).The diel variability in DO at LK illustrates a classiceutrophication response, whereby nutrient enrich-ment from stormwater discharges enhances phyto-plankton biomass and leads to high DO concentra-tions during midday hours when maximum photo-synthesis is occuning. However, the increased phy-toplankton biomass also increases communityrespiration, leading to decreased DO at predawnperiods. In addition, light attenuation by the in-creased phytoplankton biomass (as well as suspend-ed solids and dissolved organic matte," from thestormwater discharges) would reduce downwellingirradiance, leading to decreased oxygen productionby corals, seagrasses, and benthic macroalgae.The ecological effects of chronic stress due to low

DO in nearshore waters of the Florida Keys havebeen apparent for many years. Anoxic and hypoxicDO levels have long been known to be associatedwith sewage nutrient inputs to coral reef (johannes1975) and seagrass ecosystems (Odum and Wilson1962). Low DO levels not only stress marine organ-isms but can also become lethal through bacterialcontamination, toxicity, and respiratory dysfunction(Pastorok and Bilyard 1985). Residents of the Flor-ida Keys have witnessed die-offs of marine organ-isms in canal systems following heavy stormwaterevents, which have led to dramatic ecologicalchange in these communities over time. For ex-ample, cumulative wastewater impacts have resultedin the long-term loss of turtle grass (77wlassia tes-tudinum) and replacement by phytoplankton, mac-roalgae, Cuban shoalweed (Halodule wrightii) andthe jellyfish Cassiopeia in wastewater-impacted canalsand nearshore waters (Tomasko and Lapointe 1991;

Lapointe et al. 1994). The 44% loss of coral coverat LK between 1984 and 1989 (Porter and Meier1992) may also be related to DO stress as illustratedin our study; reef corals are sensitive to water-col-umn DO concentrations as they are oxygen con-formers (J. Porter personal communication).

DYNAMICS OF NUTRIENT ENHANCEDPHYrOPlANKTON BLOOMS

The different patterns we observed for the de-velopment of phytoplankton biomass (chi a) alongthe eutrophication gradient following stormwaterinputs reflect the predominant physical transportmechanisms and the varying spatial scales of phy-toplankton growth following nutrient enrichment.Elevated nutrient concentrations associated withstorrnwater discharges at the inshore PP stationwere followed by increased phytoplankton biomass.The magnitude of the time delay at the more off-shore stations was directly related to the distancefrom shore. For example, a l-wk delay was observedat the nearshore PC station compared to a 3-wk de-lay at the more offshore LK station. This finding isconsistent with physical transport studies in near-shore surface waters that show net cumulative flowsin the lower Keys from the Gulf of Mexico and Flor-ida Bay toward the offshore bank reefs and AtlanticOcean (Smith 1994). This large-scale pattern ofphytoplankton growth is also related to biologicaland chemical factors. The doubling time of phyto-plankton biomass from inshore to offshore waterswould require greater time delays, on a scale of daysto weeks. That phenomenon, combined with cu-mulative nutrient inputs via submarine groundwaterdischarge and offshore spatial dispersion by tidalcurrents, would result in additional time delays ofweeks to months for larger scale nutrient enrich-ment and development of phytoplankton biomass.Our July 24 observation of reduced salinity and DOin surface water overlying oceanic water of highersalinity and DO at the most offshore station (BW)illustrates the time delay needed for Widespread spa-tial spreading to offshore waters.Changes in the concentrations ofNH4 + and TDP

and in the DIN:SRP ratio over time following theinitial rainfall event reflect increased P limitationof phytoplankton biomass. In contrast to TDP con-centrations that were at maximum levels immedi-ately following the initial rain event at all stations,chi a and NH4 + concentrations at PC, PR, and LKreached maximum values 1-3 wk following the ini-tial rainfall. This pattern results from watershednutrient inputs from enriched groundwaters in theFlorida Keys that have high N:P ratios 00: 1, La-pointe et al. 1990) so that phytoplankton biomassfollowing stormwater discharges becomes increas-ingly limited by phosphorus compared to nitrogen.

Effects of Stormwater Nutrient Discharges 433

Over time this results in the accumulation of NH4+and chI a as DOP, the major dissolved phosphoruspool, becomes depleted. This conclusion is sup-ported by parallel changes in the DIN:SRP ratiosover time, from relatively low values before the on-set of rainfall to higher ratios 1-3 wk later, de-pending on distance from shore. Net flows fromthe Gulf of Mexico to the Atlantic Ocean facilitatespatial spreading and transport of lower salinityand NII4 +-enriched nearshore water offshore, aphenomenon that extended to 12 km offshoreduring the present study (see Fig. I I). This "am-monium wake" downstream of the Florida Keys issimilar to the NH4 +-enriched surface waters ob-served downstream from island masses in the east-ern Caribbean (Corredor et al. 1984).

NUTRIENT THRESHOLDS FOR EUTROPHICATION

Our results showed that nutrient concentrationsover broad areas of the Florida Keys were abovenutrient threshold concentrations noted for thedemise of coral reefs from eutrophication. The de-tailed studies of Tomascik and Sander (1985)showed decreased coral growth rates and die-off ofreef corals occurred on fringing reefs impacted bytourist development in Barbados when annualmean chi a levels averaged 004 fLg I-lor less; thecorresponding annual mean DIN and SRP concen-trations were 1.0 fLM and 0.1 fLM, respectively. InKaneohe Bay, Hawaii, Smith et al. (1981) reportedannual mean chI a of 0.G8 fLg I-I, DIN concentra-tion of 1.1 fLM, and SRP concentration of I J.LMfor the least polluted area of the bay that was con-sidered eutrophic by Laws and Redalje (1979). Thechi a and nutrient concentrations for onset of eu-trophication on these two polluted reefs indicatethat mean values of chi a of J.Lg I-I, DIN of1.0 J.LM, and SRP of 0.1 fLM represent thresholdsabove whieh coral reefs decline (Bell 1992). Re-cent studies of natural eutrophication from seabirdguano enrichment on the Belize Barrier Reef haveshown similar nutrient threshold concentrationsfor macroalgal overgrowth of seagrass and reef cor-al communities (Lapointe et al. 1994).Comparison of nutrient and chi a concentra-

tions in our study with the threshold values aboveindicate that levels were relatively high at all thestations in our study. All four Hydrolab stations hadNH4+ concentrations averaging well over the DINthreshold of 1.0 J.LM; over the course of the study,NH4 + averaged 5.48 J.LM, 2.25 J.LM, 1.86 fLM, and2.03 J.LM at PP, PC, PR, and LK, respectively. TDPconcentrations were also elevated, averaging 0049J.LM, 0.17 J.LM, 0.17 J.LM, and 0.14 fLM at PP, PC, PR,and LK, respectively. The average TDP concentra-tion on nearby coral reefs in the western Carib-bean was J.LM in 1990, about half the average

434 B. E. Lapointe and W. R. Matzie

concentration we measured at LK in this study.Levels of chi a were also 200% higher on bankreefs of the Florida Keys National Marine Sanctu-ary compared to more oligotrophic reefs in thewestern Caribbean (Lapointe et al. 1993).Comparison of our NH4+ data for LK to data

collected at the same site in 1985 and 1989 indi-cates that NH4 + concentrations have increased sig-nificantly at this offshore location over the past de-cade. The NH4 + concentration measured at weeklyintervals between April and August 1985 averaged0.26 ± 0.12 Il-M; N03 - and N02 - during the sameperiod averaged 0.19 ± 0.11 Il-M (Lapointe andSmith 1987). Accordingly, DIN concentrations av-eraged jLM, below the threshold of 1.0 Il-Mfor eutrophication and the demise of coral reefs(Bell 1992). The NH4 + and NO:,- plus N02 - dataof Lapointe and Clark (1992) measured in August1989 also indicated DIN concentrations at LK of

Il-M, below the threshold nutrient concen-trations noted for decline of coral reefs. In thepresent study, LK had a mean DIN concentrationof 2.9 ± 1.1 jLM, well above the threshold values.This apparent increase in DIN concentrations overthe past decade correlated with a 44% loss of coralcover and decreased water clarity, as well as in-creased macroalgal dominance, coral bleaching,and black-band disease (Porter and Meier 1992).These symptoms are consistent with our evidenceof anthropogenic nutrient enrichment and nitro-gen build-up fueling the eutrophication process.Several important conclusions can be drawn from

this study to help guide future monitoring and wa-ter quality restoration efforts. First, the importanceof diel variability in DO must be recognized andwater quality standards and monitoring protocolsshould be developed to address this and the impactof low DO on primary and secondary productionin these sensitive coral reef communities. Second,high frequency sampling with diel studies are need-ed to resolve episodic stormwater nutrient inputsand their ecological effects. Sampling at monthlyintervals, or longer, will miss the important episodicevents observed in the present study. Third, our re-sults show that concentrations of nutrients and chia in our study area are now above critical thresholdlevels known to mark the decline of coral reefs; DOconcentrations also fell below the State of Florida'sminimum standards for marine water quality (4.0mg I-I) at all stations. The high concentrations ofnutrients and chi a and low concentrations of DOin coastal waters of the Florida Keys demonstratethat special precautions should be exercised in thetreatment and discharge of wastewaters and land-based runoff. Advanced wastewater treatment (nu-trient removal) is necessary to protect sensitive coralreef communities from deterioration via nutrient

overenrichment. Nutrient reduction strategies mustbe implemented at local, regional, national, and in-ternational levels if nutrient and chi a concentra-tions are to be maintained below threshold levelsfor coral reefs. Until these recommendations areimplemented, the future of coral reefs in the Flor-ida Keys, and similar coral reef systems worldwide,is threatened.

A<:KNOvV].EDGMENTS

The authors thank the Board of County Commissioners in theCounty of Monroe for their unanimous support of our researchon nutrient pollution of coral reefs. George Schmahl (Looc KeyNational Marine Sanctuary), George Garrett (Monroe County),and Virginia Barker (Monroe County) helped with station selec-tion and permitting. Chris Humphries and Dave Forcucci (Flor-ida Keys Marine Laboratory) assisted during many of our sam-plings and provided salinity standards and Seabird ClD data.Mark Clark (Florida Keys Land & Sea Trust) and Larry Brandhelped with chi a analysis. Carl Zimmerman at the ChesapeakeBiological Laboratory provided excellent nutrient analyses. Wethank our volunteers Patricia C. Matzie, Christine Urnezis, Mag-gie Vogelsang, Doug Nowak, and Dave Martin for their manyhours of hard work. The comments of Don Axelrad and twoanonymous reviewers improved the manuscript. Our research wasconducted at Looe Key National Marine Sanctuary under re-search permit #LKNMS-04-92. This is contribution #lO91 of theHarbor Branch Oceanographic Institution, Inc.

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Effects of Stormwater Nutrient Discharges 435

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Receioed [or consideration, December I { 199·/Accepted for publication, August I I, 19(1)