san pedro shelf california: testing the pearson-rosenberg model (prm)

Marine Environmental Research 35 (1993) 303-321

San Pedro Shelf California: Testing the Pearson-Rosenberg Model (PRM)

Don Maurer

Biology Department, California State University Long Beach, Long Beach, California 90840, USA

George Robertson & Thomas Gerlinger

County Sanitation Districts of Orange County, California Fountain Valley, California 92708-7018, USA

(Received 1 July 1991; revised version received 31 December 1991; accepted 7 January 1992)

ABSTRACT

Based on quarterly sampling (n = 260) over four years (1985-89) from California's San Pedro Shelf, the Pearson-Rosenberg Model (PRM) or organic enrichment was tested for the Orange County ocean outfall. The null hypothesis was that test species, abundance, and biomass curves (SAB) from the shelf closely resemble those from the model Principal areas of agreement between test curves and model curves include increased abundance and biomass approaching an ocean outfall. Major departures from the P R M include: (1) no sharp decline in SAB curves to azoic conditions, (2) displacement of SAB curves away from the outfall, and (3) opportunistic species did not exclude or eliminate rare species. Moreover, the role of local dominant species (bivalve--Parvilucina tenuisculpta, ostracod--Euphilomedes carcharodonta polychaetous annelid--Capitella capitata) can greatly influence SAB curves within the model Bioenhance- ment should not necessarily be viewed as a diagnostic feature of a polluted site. Since the P R M was originally developed for semi-enclosed, low energy depositionai habitats with long residence times (j~ords, sea lochs), open ocean, high energy, erosional habitats (coast and shelf) may not be

303 Marine Environ. Res. 0141-1136/93/$06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain

304 D. Maurer, G. Robertson, T. Gerlinger

the most appropriate sites to apply this model Uncritical application of the P R M to the Orange County ocean outfall may lead to unnecessary and costly decisions.

INTRODUCTION

Pearson and Rosenberg (1978) proposed a model (PRM) of the response of soft-bottom infaunal invertebrates to a gradient of organic enrichment. Response was assessed by three basic measures of community structure: number of species (S), abundance (A) and biomass (B). They prepared SAB curves expected to occur along a gradient of organic enrichment (Fig. 1). The PRM has been widely cited and became the dominant paradigm for marine benthic pollution ecology of the 1980s and potentially the 1990s (Gray, 1980; Long & Chapman, 1985; Chapman et al., 1986; Brown et al., 1987; Friligos & Zenetos, 1988; Muhlenhardt- Siegel, 1988).

Environmental stress is implied when test SAB curves conform to the model. Since the PRM depicts well defined patterns (Fig. 1), this model has attracted the attention of environmental managers as a decision- making tool. However, in many instances researchers cite the PRM as a description of the responses of benthos from their study rather than presenting their SAB curves and comparing those to the model. In some studies presenting SAB curves (Stull et aL, 1986; Swartz et al., 1986; Gray, 1989), there was a general conformity with the model. However, in our opinion, the degree of similarity between test curves and the model can be problematical. Accordingly, we are concerned about the unqualified application of the PRM and how this may influence costly managerial decisions.

Based on an extensive monitoring program we tested the applicability of the PRM to an ocean outfall on the San Pedro California continental

< INCREASING ORGANIC INPUT

Fig. 1. SAB curves along a gradient of organic enrichment. S = Number of species; A = abundance; B = biomass. (Modified from Pearson & Rosenberg, 1978).

Testing the Pearson-Rosenberg Model 305

shelf. The alternate hypothesis is that the test curves do not closely resemble the PRM and that general application of this model to the outfall may be premature and should be viewed cautiously. Managerial decisions involving large expenditures of money should not rely heavily on this model without critically examining its applicability to a given study area.

Environmental setting

The County Sanitation Districts of Orange County (CSDOC) ocean outfall discharges 253 million gallons-per-day (MGD) of treated (50% advanced primary and 50% secondary) domestic and industrial effluent on the San Pedro Shelf of southern California (Fig. 2). The pipe is 3 m in diameter and more than 8 km long. The last 1.6 km houses a multi-port (500) diffuser that is designed to ensure that the dispersion of processed wastewater will have a 175:1 dilution immediately at the diffuser ports at a depth of 60 m.

The San Gabriel Canyon and the Newport Canyon bound the study area on its western and eastern flanks (Fig. 2). Based on the Districts' monitoring program (CSDOC, 1989) percent silt-clay was highest (>80%) in the canyons, particularly along the Newport Canyon (Stations C1-C5). Silt-clay also increases with depth from the outfall downslope to the southeast and in the western portion of the study area. The northwestern and north central nearshore area contains the smallest amount of silt-clay (_<20%) as well as an area around the outfall that extends east and toward Newport Canyon (Fig. 2).

The physical oceanography off southern California is very complex and dynamic with several major current flows coupled with significant seasonal oscillations (Hickey, 1979; Simpson et al., 1984; Lynn & Simp- son, 1987). Seasonal oscillations influence the development of the South- ern California Eddy that has a significant effect on the current variations measured near the outfatl. Ocean currents near the outfall are predomi- nantly northward (upcoast) with the strongest reversals (downcoast) occurring during summer and spring. Peak current speeds reach up to 35 crn/s with long-term mean currents much lower (10-15 cm/s). Currents are strongly influenced by local bathymetry with the flow near the diffuser primarily aligned with the bottom contours (CSDOC, 1989).

Four seasonal oceanographic periods have been recognized involving some stage of mixing or stratification. Depending on the oceanographic period and position relative to the thermocline, the annual range of water quality parameters follows: temperature 9-19°C, salinity 33-2-33.9%, dissolved oxygen 2.9-11.3 mg/litre pH 7.1-8.6, transmissivity 26-89%, total suspended solids 0.1-15-6 mg/litre ammonia 0.02-0.24

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MATERIALS AND METHODS

Thirteen 60 m isobath stations (Fig. 3) were sampled for 16 quarters (winter, spring, summer, fall) from 1985 to 1989 with a modified Van Veen grab (0-1 m2). Samples were collected for analysis of benthic infaunal communities (n = 5) and total organic carbon (n = 3). The latter were sampled at the same stations over 12 quarters from 1987 to 1990. Sam- pling station locations were determined by differential Loran C (+ 15 m). These stations are very important in the Districts' monitoring program because they are situated along the depth of the outfall (Fig. 3). This sampling design reduces the effect of depth on abiotic and biotic vari- ables. These stations include: New Control (CON) 7760 m distance from the zone of initial dilution (ZID-mlefined as within 60 m of the outfall), Old Control (C) 5600 m from ZID, Upcoast Farfield (13) 3830 m from ZID, Upcoast Nearfield (5 and 1) 1570 and 540 m respectively from ZID, within ZID (0 and 4) 60 m each from the outfall, ZID Boundary (ZB2 and ZB) 61 m each form the outfall, Downcoast Nearfield (9 and 12) 420 m and 1030 m respectively from the ZID, Downcoast Farfield (37) 3390 m from the ZID, and Canyon 2 (C2) 6150 m from the ZID.

Benthic infaunal samples were sieved through a 1.0 mm mesh screen. Infauna were sorted under dissection microscopes into five major taxonomic groups: molluscs, echinoderms, polychaetes, crustaceans, and minor phyla. Animals were identified to the lowest practicable taxonomic groups and counted. Wet weight biomass, measured to the nearest 0.01 g was determined for each of the five major taxonomic groups. For this analysis, biomass data were assessed to eliminate single occurrences of large shell-bearing organisms that tended to skew the data and were not consistent with replicate values.

The TOC (Total Organic Carbon) in sediments was quantified using an Oceanography International Model 524-B carbon system analyzer with a Horiba Model PIR 2000 infrared Co2 analyzer and an ampullae sealing unit. The method includes the drying and grinding of samples to ensure complete oxidation of the organic matter (CSDOC, 1989).

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were calculated to determine the effect of season and station on the number of species, abundance, biomass and TOC. Some comparisons were made with and without a few select species. Initial analyses were made with raw and transformed (log~x) data. Since the results were similar with both treatments, raw data were used for further analysis.

RESULTS

Total organic carbon

For purposes of testing the PRM which focuses on a gradient of organic enrichment, data on total organic carbon were collated (Table 1). Mean TOC was always highest at C2 and 0, respectively. When these data were pooled over all replicates and quarters, the same relationship emerged (Fig. 4). Exclusive of C2 there is an indication of a gradient of TOC toward the outfall (0). Mean TOC is slightly higher upcoast with a more gradual slope from the south (Fig. 4). Two-way ANOVA and Tukey HSD indicated that there was a significant difference in TOC across stations and time. Tukey HSD analysis revealed TOC concentrations at Sta- tion 0 were higher than all other stations except for the Newport Canyon station, C2. Station C2 had higher TOC values than all other stations. There is evidence to suggest that sedimentary processes at C2 in Newport Canyon differ considerably from those on the adjacent shelf yielding

TABLE 1 Total Sediment Organic Carbon (% dry wt) 1987-90 for San Pedro Shelf Stations

Station 87.3 87.4 88.1 88.2 88.3 88.4 89.1 89.2 89.3 89.4 90.1 90.2

CON 0.42 0.43 0.37 0.42 0.31 0-38 0.38 0-40 0.44 0.36 0.38 0.38 C 0.48 0.54 0~46 0.43 0.44 0.39 0.40 0.38 0.49 0.37 0.43 0.39 13 0.46 0.62 0.50 0-51 0.40 0.45 0.44 0-43 0.49 0-40 0.47 0.42 5 0.46 0.65 0-48 0.52 0-46 0.48 0.44 0.45 0.55 0.46 0-42 0.44 1 0.45 0.56 0.45 0.55 0-37 0.45 0.47 0.47 0.49 0.37 0-43 0-40 0 1.06 0-87 0.70 0.62 0.50 0.80 0.73 0.60 0.93 0-52 0-59 0.57 ZB2 0.48 0.77 0.54 0.65 0.47 0.46 0.42 0.43 0.46 0-39 0.45 0.55 ZB 0-34 0.46 0.41 0.45 0.33 0.39 0-35 0-42 0-47 0.34 0.47 0.54 4 0.38 0.46 0.36 0-37 0.28 0-35 0-35 0.39 0.38 0.34 0.37 0.39 9 0.32 0.44 0-36 0.35 0-28 0.34 0.37 0.32 0.37 0.31 0.31 0.35 12 0-29 0.39 0.34 0.32 0.24 0-31 0.30 0.29 0.34 0.26 0.33 0.30 37 0.29 0-35 0-27 0-54 0.24 0.28 0.30 0.26 0.30 0.25 0.29 0-26 C2 1.48 1 . 3 7 1 . 4 5 1 -69 1 . 1 8 1 -17 1.22 1 . 6 2 1 -79 1 -36 1 -50 1.27

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markedly different estimates of TOC for the canyon. Regardless, CSDOC's outfall is one source of organic enrichment on the San Pedro Shelf.

General biota

The macrobenthic, soft-bottom infauna of the California shelf is very rich (~4500 species). From the inception of this sampling program in 1965 to the present, approximately 1200 species have been recognized. For example, during the 1988-89 sampling period alone, 606 infaunal species were identified (CSDOC, 1989). Taxonomic listings can be found in the Districts' Annual Reports.

SAB curves

The number of species (S), abundance (No./0.1 m 2) (A) and biomass (g/0.1 m 2) (B) and total organic carbon (% dry wt) were pooled and com- piled (Fig. 4). The mean number of species ranged from 46 at C2 to 106 at Station 5. During this period the mean number of species was always lowest at C2 with some lower numbers at ZB. The mean number of species at the outfall (Station 0) was somewhat high compared to mean TOC whereas there was an inverse relationship at C2 (Fig. 4).

Mean abundance ranged from 306/0.1 m 2 at C2 to 1270/0.1 m 2 at Sta- tion 0. During this period mean abundance was generally lowest at C2 and occasionally at CON and C (Fig. 4). High abundance values were commonly recorded from 0, ZB2 and ZB. High abundance at 0 also coincided with high TOC. There was an inverse relationship between abundance and TOC at C2 (Fig. 4).

Mean biomass ranged from 4.3g/0.1 m 2 at Station 12 to 8.9 g/0.1 m 2 at C. Station C2 commonly produced low estimates of biomass, but occasional echinoderms and molluscs contributed to the variability at this station. Mean biomass was also generally low at Stations 5, 9 and 12. Mean biomass was somewhat high at Stations 0, ZB and ZB2, which reflects the high TOC concentration, particularly at Station 0. Biomass was generally inversely related to TOC at C2 with some exceptions due to molluscs and echinoderms mentioned above. Results of two-way ANOVA for 1985-3 to 1989.2 indicated that mean number of species, abundance and biomass were significantly (8=0-05) affected by season and station.

The bivalve Parvilucina tenuisculpta, the ostracod Euphilomedes carcharodonta and the polychaete Capitella capitata have been recognized as dominant (abundance and biomass) species. In some samples this


triumvirate comprises 25-50% of the abundance and biomass. Thus it might be expected that SAB curves could be affected by these species.

Since only three species are involved, the configuration of the S curve without these dominants would not be significantly altered. For this rea- son the S curve can be dismissed from consideration. Since, biomass data are only available for major taxa, the relative effect of the three dominants on the B curve cannot be presently assessed. However, abundance was recorded for all species providing the basis to examine the effect of the triumvirate on A curves.

Mean abundance without the three dominants ranged from 296/0.1 m 2 at C2 to 843/0.1 m 2 at Station 0. With a few exceptions, abundance was generally lowest at Station C2 (Fig. 4). However, the difference in abundance between Station C2 and all other stations was not nearly as marked as it was with samples containing the three dominants. Although highest mean abundance was reported for the within-ZID station 0, deletion of the thee dominants yielded considerable variation in individual A curves per quarter. Some relatively low abundance values were also recorded for the within-ZID stations. Results of the two-way ANOVA indicated that mean abundance without the three dominants was significantly affected by season and station.

Pearson (1987) plotted a composite statistic, the abundance species ratio (A- S), through an ocean dump site. According, to him, the A : S will peak through the center of organic enrichment. The A : S ratio was plotted herein for the entire fauna and without the three dominants (Fig. 5). Maximum values with and without the entire fauna were recorded for the within-ZID stations, particularly Stations 0 and ZB2, which may reflect their response to TOC.

DISCUSSION

The Pearson-Rosenberg Model (PRM)

Characteristic biotic measures of the PRM (Pearson & Rosenberg, 1978; Gray, 1989) were summarized and compared with trends from this study (Table 2). The PRM predicts that there is a slight increase in number of species toward the organic source and a rapid increase in biomass. This is followed by a decline in species number and biomass (Fig. 1). In the present study the number of species fluctuated along the 60 m stations with a slight reduction near the outfall (Fig. 4). According to the PRM, abundance increases as the number of species starts to decline and maximal abundance coincides with a second peak of biomass (Fig. 1).


TABLE 2 Characteristic Biotic Measures

PRM Ocean outfall

Slight initial increase in number of species towards organic source

Abundance rapidly increases with coincidental large reduction in number of species

Rapid increase in biomass towards organic source

SAB curves decline sharply to azoic conditions near source

Stages: 1. Rare species eliminated early,

moderately common species

2. Many rare species eventually eliminated, a few opportunists assume dominance

3. Mean size of organisms decreases along gradient of organic

Number of species varies towards organic source

Abundance increases with slight reduction in number of species

Progressive increase in biomass, then rapid increase near organic source

SAB curves do not decline sharply and alterations are displaced from organic source

No evidence for elimination of rare species become abundant

No evidence of increased elimination of rare species, three species become dominant

Some evidence of reduction in mean size; mean size may increase enrichment for some species

For the Orange County outfall abundance increases progressively toward the outfall peaking at 0. This peak coincides with a high T OC concentration (Fig. 4). The P R M predicts a rapid decline in SAB curves with increasing stressor load and recognizes an azoic zone where SAB values decline to almost zero (Fig. 1). In this test case the SAB curves do not decline sharply toward the outfall, and are not displaced as predicted by the P R M . Azoic conditions do not occur at or immediately adjacent to the outfall. The most dramatic decline in the S curve was recorded downcoas t from the outfall between farfield Stations 37 and C2 (Fig. 4). According to the P R M , SAB values at Station C2 would suggest moving toward azoic conditions presumably associated with organic enrichment. Based on preliminary data we assert, that sediment t ransport processes produce a sedimentary regime and geochemistry in the canyon that differs markedly from the remaining 60 m shelf stations.

Detect ion o f the first stages of stress or impact on benthic assemblages is a major goal o f marine pollution research (Gray, 1989). The first phase of response is that the rarer species decrease in abundance and are


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Fig. 5. Abundance-species ratio (A:S) per station for e.ntire fauna and without the three dominant species for the Orange County outfall. San Pedro Shelf, California (Districts 1985-89). ( ) A:S ratio---all species; ( . . . . ) A:S ratio---without three

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eliminated from the community, and moderately common species increase in abundance (Table 2). At the outfall there is evidence for enhancement in abundance and biomass but no evidence for elimination of the rarer species (Fig. 4 and 5).

In the next phase the PRM calls for the elimination of many rare species and dominance by a few opportunistic species (Table 2). The present study shows numerical dominance by three species (Fig. 4 and 5) but there is no concomitant reduction in rare species. In fact, the annual summer survey of 1989 records one of the highest numbers of species (n=5, 90-100) at the within-ZID Station 0 since the inception of the monitoring program in 1965. In addition, the ophiuroid Amphiodia urtica which occurred widely throughout the San Pedro Shelf in the early 1960s before the operation of the deep water outfall, is beginning to return to within 60 m of the outfall, where it was rarely seen from the 1970s through the early 1980s. These biotic responses are not consistent with distressed azoic conditions.

The PRM also predicts that the mean size of individual species should decrease along increase organic enrichment gradients (Table 2). For this outfall size reduction involves a population shift from a mollusc, Parvilucina tenuisculpta to a polychaete Capitella capitata between 1988 and 1989. At present C. capitata is numerically dominant at Station 0. Before 1989 numerical dominance was shared by Capitalla, Parvilucina


and the ostracod Euphilomedes carcharodonta whose abundances fluctuated annually. A change in rank order abundance from the bivalve to the polychaete carries with it an obvious reduction in mean size approaching the outfall. What is more relevant is whether there is intraspecific reduction in size that might suggest considerable physiological stress. How- ever, capitellid are noticeably larger near the Los Angeles County outfall on the Palos Verdes California Shelf and show no diminution in size near the Orange County outfall.

In regards to Parvilucina tenuisculpta, Fabrikant (1984) reported that average clam size increased as sediment organic nitrogen concentration increased to certain concentrations off the coast of Palos Verdes, Califor- nia. Organic nitrogen has been associated with an ocean outfall. The relationship between sediment organic nitrogen and P. tenuisculpta off the Orange county outfall also deserves attention in view of the PRM assertion concerning size reduction in an organically enriched area.

In summary, there were similarities between the test SAB curves (Fig. 4) and those of the PRM (Fig. 1). The major ones involve enhancement in abundance and biomass approaching the outfall. These patterns conform to general predictions of the PRM. Coincidental with these biological responses there were also some departures from the PRM. The latter included: (1) no sharp decline in SAB curves to azoic conditions, (2) displacement of SAB curves away from the source of organic enrichment rather than occurrence in the immediate vicinity, (3) dominant (abundant) species did not exclude or eliminate rare species. These differences underscore our concerns expressed at the outset; that the PRM should be examined case by case, with actual test data before unqualified acceptance. These concerns agree with others who have reported departures from the PRM. Reduction in the number of species in more organic sites is not always the rule (Dauer & Conner, 1980). Higher biomass at these sites is not always represented by a deposit feeding pollution indicator, but may involve predaceous polychaetes (Oyarzun et al., 1987).

Bioenhancement

Addition of nutrients into coastal and estuarine water may have a biostimulatory effect (Dauer & Conner, 1980). Nutrient addition to the open portion of the relatively nutrient poor Mediterranean Sea may bene- ficially stimulate primary productivity and thereby enhance fisheries pro- duction (Bossini, 1975). Nutrient addition to some Mediterranean lagoons with low flushing rates produce eutrophic conditions (Friligos & Zenetos, 1988). The great increase in shell-fisheries yield in the Baltic Sea in depths above the halocline over the last 25 years has been attributed


to eutrophication (Rosenberg et al., 1990; Cederwell & Elmgren, 1990). Between 1923 and 1976-77 estimates of mean benthic biomass increased 4.3-6.5 times (Cederwell & Elmgren, 1980).

If semi-enclosed bodies of water are vulnerable to eutrophication from natural or human activities, are there better physical analogues to the open San Pedro Shelf?. Since 1961 sewage sludge has been dumped off the mouth of the Elbe River in the North Sea (Caspers, 1980). At the outset of dumping a large population of the bivalve Abra alba occurred in the dumping area. This species ranges widely throughout northwestern European waters. Population densities reached 17 500/m 2 at the central dumping area. Caspers (1980) concluded that this species was excellently suited to exploit the greatly increased supply of nutrients. The wide geo- graphic range of A. alba and its feeding mode (surface deposit feeder) provides an interesting analogue to P. tenuisculpta. The fact that the widely distributed P. tenuisculpta, living abundantly in unimpacted areas throughout the Southern California Bight, has not only recruited to, but has thrived at the CSDOC's ocean outfall indicates that wastewater discharge here is not lethal to or chronically affecting this bivalve.

Increased densities of deposit feeding benthos are not unexpected at an ocean outfall. The latter provides a source of nutrient for bioenhancement. Large populations of bivalves from organically enriched areas have provided important resources to commercial fisheries (Caspers, 1980; Madsen & Jensen, 1987). Macrobenthos preadapted to process sewage particles provide valuable biomass for marine food webs, when these abundant food organisms are eaten (Anger, 1975). Bioenhancement is not inherently deleterious to the environment. How bioenhancement may disrupt a local food web may be very diagnostic. If increased populations of deposit feeding infauna are not consumed by demersal fish, then their potential to positively influence local food webs is strongly nullified. This does not appear to be the case at the Orange County ocean outfall, where the hornyhead turbot is abundant and is known to feed predomi- nantly on spionid and ampharetid polychaetes (Allen, 1982).

To our knowledge there are no data relating to how E. carcharodonta might contribute to local food webs. It might be anticipated that deposit feeding benthos and demersal fin fish would ingest this species co-occurring with high densities of C. capitata and P. tenisculpta (Kleppel et al., 1980).

Becker (1988) described how a group of demersal fish in Puget Sound, Washington may adapt its feeding patterns to a habitat dominated by opportunistic benthic invertebrates. According to his study Capitella spp. were somewhat more abundant in the diets of Dover sole, English sole and Rex sole (local CSDOC species) than in the benthos, whereas most


other taxa were relatively less abundant in the benthos. Presence of opportunistic prey in disturbed areas may enhanc~ the food value of such habitats to certain demersal fish. However, if these food-rich areas attract and retain fish, trophic transfer of sediment contaminants may be accelerated together with susceptibility to diseases and toxic effect of contaminants (Becker, 1988). Baltic marine scientists also-recognize the potential of biomagnification of toxics through top carnivores, but they do not always agree as to causes (Cederwell & Elmgren, 1980; Persson, 1987; Reise & Schubert, 1987). The potential for these latter responses should be closely monitored.

If it can be demonstrated that infauna are concentrating priority pollutants and that these pollutants are being enhanced at higher trophic levels, showing evidence of biomagnification and increased disease (Becker, 1988), then bioenhancement must be considered a potential environmental problem. At present significant bioconcentration of pollutants or biomagnification in higher trophic levels has not been demonstrated for the Orange County outfall. Organic enrichment of the San Pedro Shelf appears to have had a biostimulatory effect on benthic infauna without concomitant adverse effects.

The PRM provides a useful model to explain the effects of organic enrichment on benthic environments. This model served as a stimulus for many pollution studies testing its predictions. The present account acknowledges this influence. At the outset the null hypothesis focused on the viability of the PRM as accurately depicting conditions among the benthos on the San Pedro Shelf. Although the PRM has been invoked in southern California (Stull et al., 1986; Swartz et al., 1986), we are concerned that general acceptance of the model's predictions for the San Pedro Shelf may mislead environmental managers. In addition to several major departures to test SAB curves from those of the model, population dynamics of a few species should not necessarily convey an impression of a stressed environment. Also, our analysis has quantified the concentration of TOC that was not included in the PRM. Bioenhancement of opportunistic species without coincidental disruption of community structure and function should certainly not qualify as an inherent diagnostic character of a polluted site.

Physical oceanography

A final reservation about unqualifiedly applying the PRM involves physical oceanography. The PRM was initially conceived and originally developed for conditions in fjords and sea lochs. Depending on water depth and its relation to sill depth, flushing rates in fjords and sea lochs may be


very slow resulting in net deposition. Moreover, during the summer, natural oxygen depletion frequently occurs in fjords, lakes and some estuaries throughout the world (Taft et al., 1980). These oxygen depletion zones may become anoxic which can severely stress the benthos. Natural oxygen depletion zones are exacerbated by anthropogenic activities (thermal effluents, ocean dumping, ocean outfalls). Even though natural depletion of dissolved oxygen on exposed continental shelves (Gulf of Mexico) occur (Pavela et al., 1983), large scale coastal deoxygenation, such as observed in the New York Bight during the summer of 1976, is not common (Taft et al., 1980). Conditions in and around the Orange County (60 m) outfall from at least 1976 through the present, have not produced azoic reaches. Dissolved oxygen and H2S have never seriously threatened the benthos as it does naturally in the Gulf of Mexico and as it did in the New York Bight.

We submit that conditions predicted by the PRM more accurately reflect those of semi-enclosed waters with reduced flushing rates and net increase in sedimentation than open shelf areas. In addition to long shore currents and currents from mixed, semi-diurnal tides, the San Pedro Shelf experiences internal waves, frontal systems, upwellings, storms and El Niflos. Unusual strong winter storms were associated with the 1982-84 E1 Nifio (Dayton & Tegner, 1984). TheSe processes systematically and stochastically serve to flush organic deposits from the outfall area regularly reducing contaminant levels. These reductions may be somewhat balanced by riverine discharge dredge disposal and atmosphere fallout. Regardless, oceanographic processes on the San Pedro Shelf provide considerable physical energy to the system impeding the development of conditions represented by the PRM. In contrast to many fjords and sea lochs the San Pedro Shelf is an area of erosion and active sediment trans- port. Recognition of the physical and geological setting of an ocean outfall deserves serious consideration before applying the PRM. Although committed to the PRM, Pearson (1987) appears to interpret the predictions of the model somewhat broader than earlier conceived. In a more energetic physical setting than a fjord or loch, but one with a sill, Pearson (1987) concluded that the benthic ecosystem in the Firth of Clyde, Scotland has fully adapted to long-term, continuously high organic input.

CONCLUSION

The present study area is not a disturbed environment according to the PRM. The organic gradient that occurs is very limited in area and presently has enriched the bottom. Since this outfall has been in opera-


tion from 1971, this bodes well for its management provided that toxic output can be consistently minimized.

A C K N O W L E D G E M E N T S

We are pleased to acknowledge our colleague Dr Irwin Haydock who has encouraged and supported us in this account. Special thanks are extended to Dr Rutger Rosenberg who graciously reviewed a draft and to two anonymous reviewers who helped improve the paper.

R E F E R E N C E S

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san pedro shelf california: testing the pearson-rosenberg model (prm)

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