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Oysters have been lauded for being “ecosystem engineers” as well as providing a variety of ecosystem services including storm protection, habitat stabilization, provision of microhabitat, and carbon sequestration. Other services like improved water quality and control of harmful algal blooms are closely linked to oyster filtration. Frequently, filtration rates attributed to oysters (and bivalves in general) have been the result of laboratory studies. These observations may then be incorporated into models meant to simulate the system-wide level impact of oyster reefs. There is controversy as to how well lab data can be applied to systems of interest, and the studies that have extended this exploration to in situ, field-based measurements are limited.
This part of my dissertation work was focused on characterizing in-field oyster filtration rates in the Guana Tolomato Matanzas National Estuarine Research Reserve (GTM NERR) in the St. Augustine region of Florida. Along with this objective, I explored two sub-questions to support better filtration estimates: 1) How do filtration rates differ on higher and lower points on oyster reefs? Prior studies suggest reef elevation may affect certain characteristics of oyster success such as growth and mortality, so filtration rates may show similar patterns. 2) How do filtration rates attenuate as oysters are aggregated? As oysters become clustered on reefs, re-filtration occurs and should be taken into account when upscaling study results to reef-scale estimates.
Biodeposition methods employing sediment traps were based on and adapted from those in Yu and Culver (1999) and Sroczyńska et al (2012). Pairs of control and experimental traps were placed on high and low points on 9 reefs within the reserve for a two week period. Control traps collected background sediments, while experimental traps additionally collected faeces and pseudofaeces (rejected food particles) from 10 live filtering oysters. Oyster-produced biodeposits were ashed to determine inorganic content, and then paired with data on inorganic matter in the water column, number of oysters in each experimental trap and total hours of trap deployment to generate an integrated effective clearance rate (Yu and Culver 1999) which was then standardized to gram dry weight.
Smaller experimental plots explored effect of oyster aggregation on filtration rates. Plots contained replicates of controls and experimental traps containing 1, 5, and 10 oysters. A re-filtration curve was generated as a tool to up-scale results to the reef level. Further lab studies characterizing how the effect of aggregation as well as size class were completed to provide supplemental data and context for field results. Preliminary data and conclusions from these experiments are presented. |