Mapping seston depletion over an intertidal eastern oyster (Crassostrea virginica) reef: Implications for restoration of multiple habitats

Abstract

Research on the effects of bivalve filtration emphasizing oysters has mainly involved extrapolations from laboratory based measurements on individual oysters to potential whole-ecosystem impacts, with only a few studies on reef-scale processes and less using direct measurements. This study characterized spatial effects of whole-reef (oysters, Crassostrea virginica, and other filter feeders on the reef) filtration in the water immediately above and adjacent to a small (∼300 m2) intertidal reef in Tarpon Bay, Sanibel, Florida. Changes in water column parameters were measured in 2010 (chlorophyll a only) and 2013 (chlorophyll a and turbidity) by slowly paddling a kayak back-and-forth across the reef while logging position and water data. Although oysters were the dominant filter feeder, mussels, slipper shells, sponges, and a filter-feeding crab also occurred on the reef. Ambient water flow speed and direction were concurrently determined in 2013 by an acoustic-doppler current profiler. Measurements were made on two days (1–2 June) in 2010, and two days (November 15 and December 9) in 2013. ArcGIS software was used to plot the data and construct two-dimensional maps showing changes in chlorophyll a and turbidity, which clearly indicated the spatial extent of decreases in both as water flowed across the reef. Seston decrease (interpreted as depletion) levels were spatially variable, averaging 23–25% but as high as 68% in some areas directly over the reef. The extent of detectable depletion usually extended 10–20 m beyond the edge of the reef, potentially increasing light levels and thereby providing enhanced growth conditions for adjacent seagrasses and algae, suggesting that restoration success of macrophyte habitats could be enhanced by close spatial coupling with oyster reef restoration.