CO2 Gas Exchange and Ocean Acidification Studies in the Coastal Gulf of Maine

James Irish, Douglas Vandemark, Shawn Shellito, Joseph Salisbury, Amanda Plagge, Kevin Hanley and Marc Emond

University of New Hampshire Joint Center for Ocean Observing Technology

Durham, NH 03824, USA

Abstract-The University of New Hampshire is studying CO2

gas exchange, ocean acidification, air-sea dynamics, and associated biological processes in the western Gulf of Maine. Three buoys and shipboard cruises have provided data to support these studies. The first, a CO2 monitoring buoy, is deployed jointly with NOAA’s Pacific Marine Environmental Laboratory and has been moored in 70 m of water northeast of the Isles of Shoals off the Maine coast during the last 4 years. The second, Jeffrey’s Ledge Moored Observatory, is a development mooring testing new techniques and is deployed east of Gloucester, MA near our third platform, a dedicated 2D wave measurement buoy. The Jeffrey’s Ledge mooring is testing the direct measurement of the wind stress from a discus buoy using a 3-D sonic anemometer along with a motion package to remove buoy motion effects. A fast-rate atmospheric CO2 sensor is mounted next to the anemometer to evaluate the potential for direct covariance gas flux measurements. Both discus buoys have additional meteorological and oceanographic sensors to provide key supporting measurements and to augment our regional ocean observing system.

Long-term data from the CO2 buoy have helped to quantify the seasonal air-sea flux cycle of CO2 in the Gulf of Maine, and this buoy site is now the central node for near-term Carbon cycle process control experiments and longer term ocean acidification monitoring in this region. Jeffrey’s Ledge buoy momentum flux estimation results indicate reasonable first-order buoy motion corrections, likely flow disturbance leading to a non-zero mean vertical velocity, and possible artifacts due to large azimuthal rotations of the buoy. All aspects are under study. The paired sonic anemometer and open-path CO2 sensor package shows promise for long-term measurements using solar power. Ten-minute resolution vertical water column oxygen profiles from this mooring show phytoplankton bloom signatures and permit robust net community production estimates in summer and fall 2009. Finally, all three buoys are providing information on mooring response, platform survival/servicing, sensor biofouling, and low-power data system and telemetry issues in the Gulf of Maine region.

I. INTRODUCTION

The University of New Hampshire Joint Center for Ocean Observing Technology is developing and testing new technologies for ocean observing systems. The niche for these activities lies between operational deployment of already reliable sensors such as the ADCP and CTD sensors and first-time field-testing of new techniques. A central focus for much This research was supported by the National Oceanic and Atmospheric Administration’s Office of National Ocean Service to the Joint Center for Ocean Observing Technology at the University of New Hampshire under grant numbers= NA05NOS4731206.

of our moored platform activity has been the development of sensor suites for enhanced coastal ecosystem monitoring via measurements of dissolved carbon and oxygen using fairly mature technologies. To this end we were the first coastal continental US test site deploying the NOAA Pacific Marine Environmental Laboratory (PMEL) MAPCO2 automated CO2 observing subsystem. This activity then evolved into one node of a growing Coastal Carbon monitoring network that will now form the marginal shelf backbone for NOAA’s expanding ocean acidification measurement program. Allied with this work has been the deployment and evaluation of seawater oxygen and CO2 observations using spectrophotometric approaches (e.g Aanderaa Optode and Sunburst Instruments SAMI-CO2). Large perturbations of these dissolved gases occur in the surface and subsurface water column in the Gulf of Maine, providing potentially important and robust tracers tied to seasonal and episodic phytoplankton growth and grazing. The two primary platforms discussed in this paper combine these sensors with meteorological, air-sea flux, and bio-optical sensors to define observing system suites intended for extended deployments of six or more months in high seas and coastal waters known for high rates of bio-fouling. Preliminary results derived from these buoys will also be discussed including the ability to monitor air-sea flux of CO2 and to compute net community production (NCP) at daily to seasonal time scales.

II. ISLES OF SHOALS CO2 BUOY

A Buoy The Isles of Shoals CO2 buoy was constructed from a

GLOBEC buoy hull [1], and utilizes the NOAA PMEL MAPCO2 system [2], [3]. In the first configuration, the PMEL system consisting of battery and electronics pressure cases, and CO2 reference gas cylinder was mounted inside the GLOBEC buoy well (Fig 1). The MAPCO2 system also includes a floating pumped equilibrator to obtain a small volume of headspace gas that is in equilibrium with the dissolved CO2 in the ocean. This equilibrator was mounted in a PVC tube that penetrated the buoy Surlyn foam flotation collar and was plumbed through the buoy’s bulkhead to the electronics. Also part of the plumbing system is an airblock mounted on the buoy tower that permits sampling of atmospheric gases. These samples are compared with the

reference gas, and the results stored in the electronics pressure case [2], [4]. The PMEL system sends the data back to PMEL via an Iridium link once per day for further processing and display.

Figure 1. A top view of he PMEL pCO2 system mounted in the buoy well. The electronics case is at the lower right, the batteries at the top and the reference gas cylinder at the left in the well. On the left side of the picture is the PVC top to the tube going through the buoy foam flotation collar that houses the CO2 equilibrator. The blue tube coming out the top of this carries the gas into the buoy and PMEL system.

B Buoy Improvements/Upgrades To provide supporting environmental observations for the

CO2 observations, a Gill Windsonic anemometer and KVH C100 compass, a Vaisala air temperature, relative humidity and atmospheric pressure sensor, a Sea-Bird Microcat sea surface temperature and conductivity (salinity) sensor, an Aanderaa Optode dissolved oxygen sensor, and WETLabs chlorophyll and CDOM fluorometers and optical backscattering turbidity sensors were added to the buoy in the last few years.

To control and record data from these sensors, a data system entirely separate from the MAPCO2 system was added. This computer controls ancillary sensor power, calculates and saves ten-minute statistics of all data and telemeters the data to shore.

This data system is a Watchman500 from Axys Technologies, Inc. It is an intelligent data system that has the capability of controlling (switching) the power to 10 sensors (1 amp max per switch), receiving serial data back from up to 10 sensor and 16 analog sensors, parsing this data into separate arrays and further processing them. The system is capable of separate sampling programs for each sensor. The CO2 buoy Watchman500 is programmed to calculate the statistics for all data streams during a 10-minute sampling interval, including the maximum, minimum, mean and standard deviation. These values are stored internally on compact flash media. The system is also capable of controlling several telemetry systems. The CO2 buoy uses a Freewaves 900 MHz spread spectrum radio to send the 10-minute results to shore, where they are again saved, and sent via ftp to UNH for archiving and analysis.

The Watchman500 can be programmed by any terminal emulator, but it is much simpler to program it using the

Axys’s Data Management System, which allows the user to configure each port for a sensor, set the sampling parameters and then the messages sent out. This package simplifies the programming of a system considerably.

A major modification in the buoy configuration in 2009 was the change to solar power. Four BP 40 Watt solar panels now charge four 42 ah glass mat batteries through regulators to power the MAPCO2 system, the Watchman500 data system and all the other sensors and telemetry equipment. The solar charged batteries, regulators, and Watchman500 data system replace the former battery case in the buoy well (Fig. 2). The batteries have the capacity (168 ah) to power the buoy for a month without any solar charging. The new power system will comfortably maintain power through the winter months with short, overcast days. The change to solar power will allow the mooring to remain out until biofouling, loss of reference gas, or other failure requires recovery and repair. Previous dependence on the total battery power in a battery pack limited deployment options – especially for added sensors in the water column and atop the buoy.

The buoy (Fig. 3) is moored with a standard chain catenary mooring made up from ½” chain in the water column and ¾” chain along the bottom with a 5-ton swivel between the two chains and a 1,200 kg dead weight cast iron anchor. The mooring has about a 2:1 scope. This configuration of chain mooring allows the buoy, mooring and anchor to be deployed and recovered from the UNH 50 foot R/V Gulf Challenger. The recovery uses chain grabs and the winch line over a block in the A-frame to pull the chain and anchor aboard after the buoy has been pulled on deck.

In the mooring at 36 m depth, a Sunburst Instruments SAMI CO2 system measures seawater CO2, and also records temperature and dissolved oxygen concentration from an Aanderaa Optode. Finally, Tattletale tidbit temperature sensors are attached to the mooring chain at 3 m intervals down to 40 m to obtain an estimate of the thermocline depth and to record a water column temperature time series.

Figure 2. Modified MAPCO2 system in the same buoy with new data system and solar power. The subsystem’s usual battery case was replaced with a stack of four 42 ah glass mat batteries and regulators, with a Watchman500 data system on top (seen in the clear plastic splash proof case).

Figure 3. The buoy with MAPCO2and supporting sensors deployed NE of the Isles of Shoals in the Coastal Gulf of Maine. They gray cylinder at the top of the tower is the MAPCO2 Iridium telemetry system. The white PVC tube with gray top on the left of the buoy well is the equilibrator mounting tube.

The data from the MAPCO2 system relayed to PMEL daily via the Iridium link is displayed on their web site http://www.pmel.noaa.gov/co2/coastal/NH/NH_main.htm while final quality data are available at CDIAC, http://cdiac.ornl.gov/. Also, the near real-time 10-minute data from the supporting sensors are relayed to shore every 10-minutes via the radio link and are presented on the Northeast Regional Ocean Observing System (NERACOOS) web site http://www.neracoos.org/.

C. Future Improvements The UNH CO2 buoy is now being upgraded for ocean

acidification studies. A Sunburst Instruments SAMI pH sensing system and a Sea-Bird Seacat with pumped Aanderaa Optode are being added to the base of the buoy. These systems will be self-contained with internal batteries and data storage. The data will also be sent to, recorded by and telemetered to shore by the MAPCO2 data system.

III. JEFFREY’S LEDGE MOORED OBSERVATORY

A. Buoy and Instrument Suite A second UNH Joint Center buoy, similar to the first in size

and solar power, is dedicated to development and testing for new technology and measurement approaches. At present the sensor package is centered on a new gas flux measurement application using a Direct Covariance Flux Sensor (DCFS) along with an open path Licor Inc., Li-7500 IRGA CO2 sensor. The DCFS (Fig. 4) uses a Gill R3-50 3-axis sonic anemometer that samples the wind at 20 Hz for 20 minutes each hour. The DCFS also has a 6-axis Systron-Donner MotionPak II inertial motion sensor and a Precision Navigation TCM-3 3-axis fluxgate compass. A Persistor CF2 microcontroller with a real-time clock powers up the system on the hour, and records the data from the sonic anemometer and motion package for 20 minutes. Then the DCFS powers off the sensors, processes the wind data to correct for buoy motion and calculates the covariance derived momentum and sensible heat fluxes..

The DCFS is mounted on the top of the buoy with the LiCor CO2 sensor within 20 cm (Fig. 5). The buoy (Fig. 6) is designed to be symmetric about its central axis so that there would be no forces to rotate the buoy (more discussion on this later) and no preferred orientation effect on the wind measurements.

The DCFS has logic output lines that control another Persistor that logs the LiCor CO2 data. With the start and stop logic signals from the DCFS, the LiCor data can then be directly correlated with the DCFS wind stress data to estimate CO2 fluxes. This technique is still being evaluated but shows promise.

Figure 4. The Direct Covariance Flux Sensor (DCFS) on its side. The 3-axis sonic anemometer is on the right, and the electronics package with inertial motion unit, fluxgate compass and microcontroller are in the case on the left.

To supply supporting environmental data, the buoy also has

a Gill 2-D Windsonic anemometer, a Rotronics MP101A air temperature and relative humidity sensor, an R.M. Young atmospheric pressure sensor, and a Garmin GPS-17 sensor on the tower (Fig. 6 and 7). The surface water sensors include a Sea-Bird Microcat temperature and conductivity (salinity) sensor, Aaneraa Optode oxygen sensor and, WETLabs chlorophyll and CDOM fluorescence and optical backscattering turbidity sensors.

Figure 5. The top of the Jeffrey’s Ledge Moored Observatory tower showing the DCFS package in the middle with the LiCor CO2 sensor (top) mounted nearby. Toward the front of the picture is the Gill radiation shield for the temperature and relative humidity sensor. The parallel plate pressure port is to the right. On the top right is the GPS sensor. Behind the DCFS is a Windsonic 2-D anemometer. Finally, the Iridium antenna is the white round to the left of the radiation shield.

This buoy’s main data system is also a Watchman500 (Fig.

7) that is mounted in the buoy well (Fig. 8). Also in the well can be seen the LiCor CO2 sensor electronics in its weatherproof case. This Watchman500 system is similar to that discussed above with the CO2 buoy, except that the Jeffrey’s Ledge buoy uses an Iridium modem to return diagnostics and a subset of measurement data to UNH every hour. This provides a check on the health of the system and a preliminary look at data quality.

Moored below the buoy and connected electrically to the buoy was an instrument package at 5 m depth (Fig. 9). This package contained a second Watchman500 data system, backup batteries and sensors mounted on an in-line instrument frame. Sensors included a Microcat temperature and conductivity (salinity) sensor, Aanderaa Optode dissolved oxygen sensors at the top and bottom of the instrument frame, a Pro-Oceanus Systems, Inc. Gas Tension Device, WETLabs chlorophyll fluorometer and optical backscattering turbidity sensor and a Sunburst Instruments SAMI CO2 sensor. The SAMI had its own batteries and recorded its data internally. The Microcat had its own batteries and recorded the data internally as well as sending the data to the Watchman500. The Watchman500 received 12 V battery power from the buoy’s Watchman, which could control and monitor the battery power going to the 5 m package. The Watchman in the 5 m package switched power to the sensors depending on the sampling program. An alkaline backup battery pack in the Watchman’s pressure case provided several weeks power to the system should the power cable from the buoy be damaged, or the buoy Watchman not provide power to the 5-m instrument package.

Figure 6. The Jeffrey’s Ledge Moored Observatory deployed on Jeffrey’s Ledge in the Gulf of Maine in 2009. The solar panels provide system power, the radar reflector and Coast Guard navigation light are seen above the solar panels. A Surlyn foam flotation collar surrounds the aluminum well and buoy frame. The DCFS wind sensor extends above the buoy in the center of the tower. The LiCor CO2 sensor is seen just to the right of the DCFS. The other meteorological sensors are seen as in Fig. 5.

Figure 7. Watchman500 data system in a splash proof plastic case. The sensor inside the case on the left is a KVH C100 compass. The Watchman500 is the gray package with writing toward the right. The Iridium modem is the black box at the top. The cables to power and receive signals from the sensors can be seen on the right.

Figure 8. The Watchman500 in the buoy well (left) with the LiCor Li7500 electronics on the right. Below the Watchman are the solar panel regulators and diode netword supplying the system power bus. Below the LiCor electronics is another splash proof plastic case with the Persistor microcontroller. Four 42 ah gel cell batteries are located in the bottom of the buoy well.

The 5-m instrument package Watchman500 records its data internally on compact flash media. As the buoy Watchman, it samples the sensors and produces 10-minute statistics for all quantities. It also sends this data up the cable to the buoy where that data stream is again stored by the buoy Watchman and a subset of the 5-m data are telemetered to shore via the Iridium modem.

Figure 9. The 5-m instrument package mounted in an in-line mooring frame. The Watchman500 data system can be seen partially inserted in its pressure case with backup battery. On the upper right leg of the frame is a Microcat temperature and conductivity sensor. In the back can be seen the WETLabs FLNTUS chlorophyll and turbidity sensor. In the middle of the cluster in sensors is a SAMI CO2 sensor. Hidden in the back is a Gas Tension Device. In copper foil in the front of the frame (top) is an Aanderaa Optode. Another is mounted on the back of the frame (bottom).

B. Moored Array Vertical Profile Measurements Directly below the 5-m instrument package is an RD

Instruments 300 kHz Workhorse ADCP with auxiliary battery pack in an in-line frame. This instrument runs autonomously on three internal batteries and stores the data on PCMCIA

media for later retrieval. Initially, there weren’t enough serial ports on the Watchman to interface this instrumentation system to the 5-m instrument package.

Below the 5-m instrument and ADCP packages on the mooring chain is an array of Aanderaa Optodes with sensors at 12, 24 and 36 m depth. The electrical cables carrying power to each of these sensors is a simple 4 wire 18 gauge bundle carrying power down the cable and RS232 signals back up. The Optodes are programmed to wake up every minute, take a sample and send it to the 5-m package and go back to sleep. The cable bundles were then run inside a split garden hose for abrasion protection (Fig.10) and tie-wrapped to the chain with a little “slop” in the cables to allow for mooring chain movement. This array was used in two three-month deployments without modification between deployments, and showed no wear over this period. A similar construction with Kevlar strength member was used in CODE for 11-month deployments without difficulty [5]. This technique of electrical cable in hose was used on the cable between the buoy and 5-m package. However, here the cable-hose assembly was also encased in a section of fire hose for additional protection between the hole in the Syrlyn foam flotation collar and the chain below the shackles attaching it to the buoy.

To complete the vertical array of sernsors, at about 50 m in the mooring another in-line instrument frame has a Sea-Bird Seacat temperature and conductivity (salinity) sensor with a Sea-Bird dissolved oxygen sensor and pump. Also in the frame are a Seapoint Chlorophyll fluorometer and an optical backscattering turbidity sensor. This instrument is self-powering and recording.

The Jeffrey’s Ledge Moored Observatory was deployed for a 1-month test in 2007 south of the Isles of Shoals for initial tests of the DCFS. With the full configuration as discussed above, it was deployed for two 3-month tests in the spring and late fall of 2009. These last two deployments were on Jeffrey’s Ledge in the Gulf of Maine, east of Gloucester, MA. in 70 m of water. At this site UNH also has deployed a Datawell Wave Rider buoy measuring directional wave spectra that provides additional forcing data for gas flux studies. Some of the preliminary results from these tests are discussed below.

IV. INTERESTING RESULTS

A. MAPCO2 Results The MAPCO2 system measures air and surface water

dissolved oxygen and CO2 concentrations. It was first deployed in May 2006, and is recovered and serviced for about a month in August each year when there is little exciting action in the CO2 field. The data obtained from the 4 plus years of the CO2 mooring (Fig. 11) show that there is a flux of CO2 into the oceans during the spring-to-summer, but a nearly equal flux out of the ocean in the late fall-winter [6]. The CO2 entering the ocean is used during the spring-summer by primary productivity (that then produces the high oxygen

levels seen below in Fig. 15). The deep water CO2 is brought to the surface with fall-winter storms and released into the atmosphere . Also, note obvious bad data points in Fig. 11. This is preliminary data from the NOAA/PMEL web site http://www.pmel.noaa.gov/co2/coastal/NH/NH_main.htm, and has not been edited and quality assurance techniques applied. Overall, the coastal (70 m) Gulf of Maine appears to act as a weak atmospheric source of CO2.

Figure 10. The buoy (background), ¾” mooring chain, 5-m instrument package, ADCP and ½” mooring chain with Optode array spread out on the floor of the UNH Ocean Engineering high bay. The green garden hose abrasion protection can be seen on the chain, and the 12 m Optode is shown on the left the in blue protective foam (for handling before deployment).

Figure 11. The MAPCO2 system results since spring 2006. The blue curve is the seawater CO2 and shows the seasonal change in direction of CO2 gas flux.

B Jeffrey’s Ledge Buoy Motion and Rotation The DCFS has a motion package with 3-axes of acceleration,

3-axes of rotation and a fluxgate compass with pitch, roll and heading that were recorded at 20 Hz. This allowed examination the buoy motion in great detail. There appears to be a broad peak about 0.5 Hz in all variables that is probably related to a normal mode of buoy motion, that is a pitch and roll in the random waves that is modified by the interaction with the mooring chain and the instrumentation on the mooring line (Fig. 12).

Figure 12. Spectral density of a representative pitch and roll time series from 14 Nov 2007 at 14:00 UTC showing the strong 2-second peak in buoy motion.

The buoy motion was quite severe and we are considering moving to a compliant elastic mooring to reduce the pitch and roll of the buoy to obtain a smoother riding platform from which to make measurements. This would provide a platform with smaller corrections to the observed winds. The compliant elastic moorings used in the GLOBEC moorings on Georges Bank [1], [7], [8] and [9]. Observations of buoy motion tend to support this notion of smoother buoy movement, but no high frequency data was recorded with a compliant elastic tether mooring with this buoy to directly address this issue.

Of importance to the wind measurement from the DCFS, was the rotational motion of the buoy. A failure in one of the flux gate elements in 2009 did not provide the detailed buoy orientation needed for the flux measurement correction. Therefore, 2007 data was examined and an unexpected result was observed. The initial thought was that since the buoy was attached to the mooring chain with a shackle into a perpendicular plate on the buoy, the buoy would maintain the same relationship to the mooring chain catenary, and rotate slowly with the tides. However, this was not the case.

The buoy often appeared to have a general preferred orientation during a 20 minute burst, and rotate fairly rapidly (order of minutes) back and forth around this (Fig. 13). An average time to rotate back and forth is about 2 minutes. Sometimes it rotates 180º in about a minute, than back in another minute. Another condition appears to be a continuous oscillation of 60º to 90º back and forth around its mean

position, again with a period of something like 2 minutes. Also, at time

s the mean position, will change for a period of time, then change back. Finally, 360º rotations in about 2 minutes are occasionally observed. These are typical of the results observed in all deployments. The buoy does not maintain a relatively constant orientation, but has a continuous, sometimes rapid angular motion.

Figure 13. A typical 20 minute DCFS compass burst. The data was sampled at 20 Hz, and shows a typical change of about 180º in 1 minute as well as shorter rotational motion and even a 360º turn around in about 2 minutes.

Therefore, for best wind directional measurements from the 2-D Windsonic anemometer on the CO2 and Jeffrey’s Ledge buoys, the KVH compass was kept powered continuously, and read with each wind measurement (1 Hz) to provide good wind directional measurements without buoy motion effects. The high frequency 2-second buoy motion discussed above, is also seen as the in the DCFS compass (Fig. 13) and may be reduced with a compliant elastic mooring. The wind speed and direction from the same record (Fig. 14) show that we can probably correct for this buoy rotational motion to obtain reasonable corrected wind speeds and velocities. This study is ongoing.

Figure 14. The wind speed and direction from the DCFS with the first order correction for buoy rotation. The correction algorithm is being improved.

B Oxygen Depth-Time Section The vertical array of oxygen sensors on the Jeffrey’s Ledge

mooring provided oxygen data at 1-m on the buoy, 4 m on the 5-m instrument package, 12, 24 and 36 m on the mooring chain and 50 m on the near-bottom Sea-Bird package. These data show the time and depth varying oxygen structure in the water column. The summer oxygen profile time series (Fig. 15) shows the bottom of the high productivity regions (red) is about 25 m depth where the oxygen concentration is about 100% saturated. This is about the depth of the photic zone where light penetrates and drives high productivity. The deeper waters show a steadily decreasing oxygen concentration with time. This is in good agreement with data taken closer to Portsmouth, NH at the UNH aquaculture site [10]. Biological drawdown of oxygen at 50 m is observed and typically continues until the fall overturning reoxygenates the water column.

Figure 15. An oxygen section time series from the summer 2009 showing the high productivity creating supersaturated surface waters, and the slow decrease in saturation in deeper waters.

The red region above 25 m is largely dominated by primary

productivity. The dark red in the upper 5 meters around July 10 to 14 is actually a red tide bloom (Alexandrium fundyense). The rest of the red areas, are typical summer primary biological activity. There are tidal fluctuations that are barely seen, and other advective events of water brought down the Maine and New Hampshire coast that contribute to the several-day variability seen.

The fall-winter deployment (Fig. 16) shows less primary productivity (less red). Unfortunately the 50 m oxygen sensor (as well as the chlorophyll fluorometer) were damaged (maybe by fishing activity) so there is not deep data after 6 November. The surface waters are less than 100% saturation, and appear to decrease to about 95% in late fall. This is again in agreement with observations at the aquaculture site [10] to the west. This decrease in oxygen concentration occurs during the

time that the storms are mixing the surface waters with lower concentration deeper waters, causing the reduction in oxygen levels.

C Oxygen Sensor fouling To obtain near surface dissolved oxygen time series on the

MAPCO2 buoy during its 11 month deployment, an Aanderaa Optode was mounted on the base of the buoy (Fig. 17). This sensor was wrapped in copper foil, and a Sea-Bird poison plug was tied near the sensing membrane to slow biofouling. However, autotrophs on the buoy-mounted Optode oxygen sensing membrane have repeatedly created a biofouling microclimate where oxygen concentrations at the sensing membrane can reach 140% saturation during high sunlight periods. This biofouling can be identified by looking at the statistics of the oxygen concentration during a 10-minute sample. The Optide woke up and took a reading every minute, so the 10-minute sample consisted of 10 data points. The Watchman500 data system calculated the maximum, minimum, mean and standard deviation of these data. When the maximum and minimum began to show differences, and as the standard deviation grows, it can readily indicate when the biofouling (Fig, 18) effect is significant. At the start of the time series in Fig. 18 the high standard deviation is seen because the sunlight hitting the autotrophs varied with the buoy’s motion, so the autotrophs produce more or less oxygen in the microclimate around the sensing membrane. Divers observed that the fouled membrane had a “brown-green” cast to it.

Figure 16. An oxygen section from the fall 2009 into winter. The bottom oxygen sensor was damaged in early Nov. The biological activity is much lower and the below saturation surface waters indicate mixing with deeper lower oxygen level waters. Cleaning the membrane removed this affect for a while.

Anti-fouling methods applied to date do not appear to prevent this fouling from reoccurring over long durations. In

relatively inactive fouling periods, a cleaning can last for several months. During very active biological times, the fouling effects can be seen after 50 days. Figure 18 shows a biologically active time in the spring. The sensor was cleaned by divers on year day 101, and the increase in max-min difference and standard deviation can be seen to increase around day 150, indication that 2 months without cleaning can be too long in this environment.

Figure 17. The Aanderaa Optode oxygen sensor on the bottom of the MAPCO2 buoy with Sea-Bird Microcat at the right after cleaning after recovery. Some remaining copper foil can be seen on the left of the sensor by the mounting block. The white poison plug appears displaced above the sensing membrane. The blue everything else is antifouling paint on the buoy.

Figure 18. A record of Aanderaa Optode statistics – top maximum minimum difference, and bottom standard deviation. An increase in either signal indicated biofouling by oxygen producing organisms. The sensor was cleaned on day 101, and shows fouling effects around day 150.

D DCFS and LiCor results The DCFS and LiCor data from the Jeffrey’s Ledge buoy

show promise of this technology for measuring wind velocities, friction velocity, atmospheric CO2 time series, and direct covariance gas flux products. Analysis of the DCFS data shows reasonable friction velocity when compared with a simple bulk model (Fig. 19). The deviations between the observations and bulk model in Fig. 19 are probably due to the simple model, rather than the DCFS data. This implies that

we can correct for the buoy motion to obtain good wind stress data. However, one problem with the velocity observations is a small average positive-going vertical wind velocity over the 20 minute record. This cannot be real and is possibly due to a flow disturbance by the buoy (see below). The solar panels are less than three diameters from the sonic anemometer (Fig. 3 and 6), and might deflect the winds up and over the buoy. This effect is being studied further.

The LiCor-7500 CO2 sensor is not designed for deployment on a buoy, or more specifically in a salt-water environment where the windows might become fouled in some way. The effects of salt water on the windows and exposure to the marine environment could affect the data by partially blocking the window. However, a simple plot of the CO2 (Fig. 20) shows results that look similar to the CO2 buoy. There are some outliers, but the data do not show any slope or trend which would be expected if the windows were becoming fouled. The results of this test do seem promising with numerous clean multi day measurement periods observed.

Figure 19. A DCFS record of friction velocity (solid triangles) and the estimated friction velocity from a simple bulk model indicating that the DCFS data can be corrected for buoy motion to the first order.

Figure 20. A LiCor record from the Jeffrey’s Ledge buoy showing a two month record of CO2. There are no general trends or large gaps in the data. Spikes are generally associated with rain events.

E Buoy Influence on Wind Measurements Whenever a sensor or sensing system is put into the

environment, a concern is its disturbance of the quantity being measured. A major concern with the DCFS, or any buoy based wind measurement, is the effect that buoy superstructure may have on the local wind field. Modified flow might affect the wind velocity and direction, and affect the direct measurement of wind stress on the surface of the water. The average of the vertical wind velocity over 20 minutes should be near zero. However, we did consistently see a positive upward wind velocity leading us to assume that it is due to deflection of the winds by the solar panels.

To understand this better a study of the potential flow of the Jeffrey’s Ledge Moored observatory was undertaken using a computational fluid dynamics simulation. The buoy was measured and drawn, and a uniform wind force applied from the left. The output of a model run is shown in Fig. 21. The pressure field shows the positive pressure (orange and red) on the upwind side of the buoy, and the negative pressure (greens) on the downwind side. The streamlines shown are only for a vertical plane through the center of the buoy. The streamlines on the right that are missing went around the buoy and didn’t end up back in the plane. The vertical distortion of the wind field due to the solar panels is clearly seen. Less clearly seen, is the slight rise in the streamline at the level of the DCFS anemometer.

The model results show acceleration in the flow of about 2% in the vertical component at the DCFS, which is in rough agreement with our observations. It also shows an acceleration of around 6% in the horizontal wind component.(along the flow). This implies that our wind measurements will be a few percent faster than free stream winds. It should be noted that these results are preliminary and studies of these effects are continuing. The effect of the buoy on the wind field is measurable and needs to be considered.

Figure 21. Wind flow disturbance by the buoy from a uniform wind from the left. The pressure field shows areas of increased pressure (orange and red) upwind, and areas of decreased pressure (greens) downwind. The flow streamlines show the large disturbance of the flow due to the solar panels. Some of this disturbance extends to the DCFS on top of the buoy.

ACKNOWLEDGMENTS

We would like to thank Chris Sabine, Stacy Maenner, Sylvia Musielewicz, and technicians at PMEL for providing and servicing the PMEL MAPCO2.system for the UNH CO2 buoy. Successful and safe deployment, servicing and recovery operations from the UNH R/V Gulf Challenger were accomplished with the help of Paul Pelletier, Bryan Soars and Debra Brewitt. The Direct Covariance Flux Sensor was developed by Jim Edson (Univ. of Connecticut) and Jonathan Ware (WHOI). The UNH Center for Ocean Observation and Analysis provided supporting CTD, bio-optical profile and

bottle data to help understand sensor behavior and improve data quality.

REFERENCES [1] Irish, J.D., S. Kerry, P. Fucile, R.C. Beardsley, J. Lord, and K.H. Brink,

“U.S. GLOBEC Long-Term Moored Program: Part 1 – Mooring Configuration,” WHOI Tech. Rept., WHOI-2005-11, December 2005.

[2] Shalleto, S., J. Irish, D. Vandemark, S. Maenner, N. Lawerence-Slavas and C. Sabine, 2008. “Time-Series Measurements of Atmospheric and Oceanic CO2 and O2 in the Western Gulf of Maine,” Proc. Oceans08, Quebec City, Canada, Sept.. 2008

[3] Irish, J.D., D. Vandemark, and S. Shellito, “Moored Observatories for the Study of Gas Exchange in the Coastal Ocean,” Proc. ONR/MTS Buoy Workshop, Monterey, CA, March 2010.

[4] Friederich, G.E., P.M.Walz, M.G. Burczynski, and F.P. Chaviz, “Inorganic carbon in the central California upwelling system during the 1997-1999 El Nino-Lina event,” Progress in Oceanography, 54, 185-203, 2002.

[5] Brown, W.S., J.D. Irish and A. Bratkovich, "CODE-1: Moored Temperature and Conductivity Observations," WHOI Tech. Report No. 83-23, CODE Tech. Report No. 21, 81-116, 1983.

[6] Vandemark, D., J. E. Salisbury, C. W. Hunt, S. Shellito, J. Irish, C. L. Sabine, S. Maenner, W. R. McGillis, “Air-sea CO2 flux in the Gulf of Maine,” J. Geophys. Res., in review, 2010.

[7] Irish, J.D., R.C. Beardsley, W.J.Williams and K.H. Brink, "Long-Term Moored Observations on Georges Bank as Part of the U.S. GLOBEC Northwest Atlantic/Georges Bank Program," Proc. Oceans'99, 273-278, 1999.

[8] Paul, W., J. Irish, J. Gobat and M. Grosenbaugh, "Taut Elastomeric and Chain Catenary Surface Buoy Moorings," Proc. Oceans'99, 418-426, 1999. .

[9] Irish, J.D., “On Compliance in Coastal Moorings,” Oceans2005, Wash. D.C., Sept. 2005.

[10] Irish, J.D., L.G. Ward and S.J. Boduch, “Correcting and Validating Moored Oxygen Time-Series Observations in the Gulf of Maine,” Proc. Oceans08, Quebec City, Canada, Sept. 2008.