Reconstructing Historical Oyster Filtration in the Guana River Marsh Aquatic Preserve

Methods

Summary

The Nature Conservancy’s Oyster Calculator uses the ecosystem service of water filtration to inform restoration objectives and answer a common critical management question of "How much shellfish restoration is enough?"

Quantifying the filtration capacity, or the amount of water an oyster population is capable of filtering, helps to inform oyster habitat management and restoration decisions. See Setting Objectives for Oyster Habitat Restoration Using Ecosystem Services, A Manager’s Guide, for a more thorough explanation of the science, applied equations, example case studies, and broader research applications!

Since oysters are harvested for food and industry, monitoring information often includes data on sub-market (<75mm) and market-sized (>75mm) oysters. Both average oyster size and density are important to know for both size classes to inform the amount of filtration an oyster population can provide. The amount of water a single oyster can filter is also limited by their size and water temperature.

FR  x  N  x  A = FC

FR = Individual oyster filtration rate (*informed by size and temperature)
N = Oyster density
A = Reef area
FC = Filtration capacity

Average oyster size and water temperature are applied to a predetermined equation that accounts for their physiological relationship to estimate the average filtration rate of an individual oyster for each size class. This is then multiplied by average density and reef area to determine the average filtration rate that the oyster population of each size-class can provide. These size-class estimates are added together to provide an estimate of the volume of water the entire oyster population is capable of filtering. An estuary’s volume and residence time, the average amount of time water flows through the estuary, are used to estimate how much filtration the oyster population is capable of over time.

The challenge is estimating oyster size, density, and filtration from the past. I’ve already collected 5-10ft (152-305cm) long vibracores from living oyster reefs in Guana River, Florida. I subsample 5cm sections throughout each core, wash them over stacked sieves, and measure the body size of each oyster with calipers. The size of an oyster is related to the length of its hinge (also known as the resilifer), so I can measure the hinge of fragments to estimate size as well. The number of preserved individual oysters is counted in each core subsection and extrapolated to a standardized quadrat area to provide a density estimate. Temperature records and reef area are accessible from public sources like the Florida Department of Environmental Protection and NOAA. Radiocarbon analyses are important to date the shells in each core subsection to know how old my samples are and provide accurate estimates over time.

Oyster metrics of size and density are applied to each size class and combined with temperature and reef area within a Bayesian modelling framework to provide estimates that incorporate the variation between samples and uncertainties associated with historical or paleontological data.