Water Quality Monitoring
Our water quality monitoring program was developed to monitor the university water quality metrics for oyster restoration sites by targeting the parameters that influence oyster growth and filtration. The monitoring program has three main components:
Monthly water column profiles are taken from approximately May - October using YSI multiparameter instruments. Profiles are taken in areas around the reef and a bare-bottom control site. This allows us to ground-truth the continuous loggers and allows for greater spatial coverage compared to the continuous measurements.
2. Continuous Loggers
Continuous loggers are placed throughout the Tuckerton Reef during the growing season and allow for high-frequency monitoring. A YSI sonde recording continuously on a buoy can monitor things like temperature, salinity, conductivity, dissolved oxygen, pH, turbidity and Chlorophyll. We also use HOBO data loggers (Onset Corporation) which log temperature, salinity, conductivity, and dissolved oxygen only.
3. In-situ samples
Water samples are collected at depth using a Van Dorn sampler. These samples are brought back to the lab and filtered. This measures Total Suspended Solids, or TSS, which is a measure of the turbidity of the water column and can affect oyster filtration.
Filtration Model Development
It is difficult to measure oyster filtration capacity in the field, so we apply a model, or a series of equations, which are used to estimate oyster filtration rates based on biological and environmental parameters. These equations are based on oysters filtering in laboratory environments with changing temperature, salinity, etc. We use the continuous and discrete measurements from our water monitoring as well as the oyster monitoring data to estimate oyster filtration. We can then further extend this model to look at nitrogen removal through burial and denitrification, another important service of oyster reefs.
Below is a chart of how we use our data collected to parameterize the model:
Temperature-Dependent Filtration Rate
The first step of the model is to calculate filtration rate. Filtration rate is dependent on two main factors: temperature and oyster biomass. Temperature increases oyster filtration rate, with maximum filtration at 27 C. Below are the two published equations we used for filtration rate. These equations were chosen based on their frequent use in the literature as well as their use of oysters from the Chesapeake Bay, which is geographically similar to New Jersey.
1. Cerco & Noel (2007): clearance rate (units: meters cubed per gram per day). This equation is based on a 1 g oyster (dry-weight) and needs to be adjusted for biomass.
2. zu Ermgassen et al. (2013): biomass-dependent filtration rates (units: liters per hour). This equation takes into account the average dry weight of oysters (W).
Since we collect temperature data from both discrete and continuous monitoring, both of these estimates are used to estimate filtration rate. Temperature from continuous loggers are averaged monthly and compared to the monthly profile data.
We use biomass calculated for oysters sub-sampled from our oyster surveys to create a shell length:biomass relationship to enable us to estimate the average biomass for each sample. This is more accurate than using published values from other sites since these are typically based on natural reefs or for single oysters. Our oysters tend to grow long and thin (see photo below) and using equations designed for singles might overestimate filtration. Each planted area is treated separately due to different planting conditions and substrates used. A linear relationship showed the best fit for our site.
Adjustments for Environmental Variables
The next step of the model is to adjust the filtration rates for other environmental parameters that affect oyster filtration such as seston (TSS), Salinity and Dissolved Oxygen (DO). These adjustments were based on Ehrich & Harris (2015).
Seston (TSS, mg/L): oyster filtration decreases with increased suspended solids. Filtration is maximum between 4 and 25 mg/L of suspended solids. A step function is used to adjust for turbidity.
Salinity: filtration increases with salinity. Oyster filtration decreases below 7.5 (Cerco & Noel 2005) or 12 (Fulford et al. 2007). Since our site never experiences salinities below 20, salinity adjustments were not added.
Dissolved Oxygen (DO, mg/L): higher filtration is observed with higher D.O. Maximum filtration occurs when DO is greater than 2-3 mg/L.
Scale to Reef Size
To understand filtration capacity for an entire reef, the footprint of the oysters in the reef area is needed. We estimate this from side-scan sonar images of the planted areas and drawing polygons around each planted area to estimate the reef size. The brighter areas represent areas planted with shell. This method is subject to error based on how the polygons are drawn, but gives us the best estimate of the reef footprint after planting.
Estimating Nitrogen Removal
Oyster filtration and nitrogen removal are related through: (1) clearance rates of phytoplankton, (2) assimilation of nitrogen into oyster biomass, and (3) biodeposit burial and denitrification. estimates can be used to estimate the clearance rate of phytoplankton to get at Nitrogen removal from filtration.
Clearance rates can be converted to a nitrogen uptake rate based on ambient Chlorophyll a measurements and Chl:N ratios in the food (Grizzle et al. 2008). Nitrogen removed from filtration can be further broken down as follows (Beseres Pollack et al. 2013):
50%: Assimilated in body or biodeposit production
20%: Denitrification of biodeposits
10%: Burial of biodeposits
Another method of estimating nitrogen removal based on Newell et al. (2004) assumes a denitrification and burial rate of 0.75 mg N per gram oyster per year and can be estimated from oyster biomass density.
Reef-wide nitrogen removal can be calculated using average oyster density and reef size to estimate the total oysters on the reef. How much nitrogen removed in a year can be found by converting monthly filtration rates to yearly filtration rates (focusing on the growing season when temperatures are above 15 C). Nitrogen removal can be standardized to kg N Km-2 yr-1 to compare to other sites as well as scaling up for Barnegat Bay to determine the size of oyster reefs needed to filter a portion of the bay volume or remove a portion of the TMDL.
Another useful online tool is the Oyster Calculator from the Nature Conservancy which uses equation (2) and bases filtration capacity on mean size and density of oysters (zu Ermgassen et al. 2016).
Oysters growing out from a single recycled oyster shell. This cluster is about one year old.
Shell length to biomass relationship for oysters planted in 2019.
Polygons drawn around each planting area for the reef in 2020. (Image credit: Nathan Robinson)
Beseres Pollack J., Yoskowitz D., Kim H.C., Montagna P.A. 2013. Role and value of nitrogen regulation provided by oysters (Crassostrea virginica) in the Mission-Aransas Estuary, Texas, USA. PLoS ONE 8(6): e65314.
Cerco C.F. and Noel M.R. 2007. Can oyster restoration reverse cultural eutrophication in Chesapeake Bay? Estuaries and Coasts 30(2): 331-343.
Ehrich, M.K. and Harris L.A. 2015. A review of existing eastern oyster filtration models. Ecological Modeling 297: 201- 212.
Fulford R.S., Breitburg, Newell R.I.E., Kemp W.M., Luckenbach M. 2007. Effects of oyster population restoration strategies on phytoplankton biomass in Chesapeake Bay: a flexible modeling approach. Marine Ecol. Prog. Ser. 336: 43-61.
Grizzle RE, Greene JK, and Coen LD. 2008. Seston removal by natural and constructed intertidal eastern oyster (Crassostrea virginica) reefs: a comparison with previous laboratory studies, and the value of in situ methods. Estuaries and Coasts 31: 1208-1220.
Newell R.I.E. 2004. Ecosystem influences of natural and cultivated populations of suspension-feeding bivalve molluscs: a review. J Shellfish Res 23:51−61.
zu Ermgassen, P.S.E., M.D. Spalding, R. Grizzle, and R. Brumbaugh. 2013. Quantifying the loss of a marine ecosystem service: filtration by the eastern oyster in U.S. estuaries. Estuaries and Coasts 36: 36-43.
zu Ermgassen, P., Hancock, B., DeAngelis, B., Greene, J., Schuster, E., Spalding, M., Brumbaugh, R. 2016. Setting objectives for oyster habitat restoration using ecosystem services: A manager’s guide. The Nature Conservancy, Arlington VA. 76pp.