Abstract
Oyster reef restoration can significantly increase benthic denitrification rates. Methods applied to measure nutrient fluxes and denitrification from oyster reefs in previous studies include incubations of sediment cores collected adjacent to oyster clumps, benthic chambers filled with intact reef segments that have undergone in situ equilibration and ex situ incubation, and cores with single oysters. However, fluxes of nutrients vary by orders of magnitude among oyster reefs and methods. This study compares two methods of measuring nutrient and metabolic fluxes on restored oyster reefs: incubations including intact segments of oyster reef and incubations containing oyster clumps without underlying sediments. Fluxes of oxygen (O2), dissolved inorganic carbon (DIC), ammonium (NH4+), combined nitrate and nitrite (NO2/3-), di-nitrogen (N2), and soluble reactive phosphorus (SRP) were determined in June and August in Harris Creek, a tributary of the Chesapeake Bay, Maryland, USA. Regression of fluxes measured from clumps alone against those measured from intact reef segments showed significant positive relationships for O2, DIC, NH4+, and SRP (R2 = 0.920, 0.61, 0.26, and 0.52, respectively). Regression of clump fluxes against the oyster tissue biomass indicates significant positive relationships for O2 and NH4+, marginally significant and positive relationships for DIC and N2, and no significant relationship for NO2/3- or SRP. Although these results demonstrate that the incubation of oyster clumps without underlying sediments does not accurately represent biogeochemical fluxes measured from the whole oyster and sediment community, this work supports the need to understand the balance between the metabolism of oysters and local sediments to correctly estimate biogeochemical rates.
Highlights
In Chesapeake Bay, populations of the native oyster Crassostrea virginica have experienced substantial decline due to overfishing, disease, and habitat loss [1]
Regression analyses found a significant positive relationship between soluble reactive phosphorus (SRP) fluxes measured from clumps and those measured from reef segments (p = 0.019) with a degree of correlation (R2 = 0.5172) similar to those for dissolved inorganic carbon (DIC) and NH4+ (Fig 3F)
The majority of fluxes measured from clumps fell within the range of values measured for reef segments, which supports that much of the metabolism occurs within the clumps rather than in underlying sediments
Summary
In Chesapeake Bay, populations of the native oyster Crassostrea virginica have experienced substantial decline due to overfishing, disease, and habitat loss [1]. Microbiome structures from live and dead shell, sediment, and oyster digestive glands suggest that the transformation of NO2- to N2 is not controlled by the abundance of complete denitrifiers in the sediment but rather by complex interactions within the microbial community [21] None of these previous studies have directly resolved the relative importance of oyster clumps in determining denitrification rates measured from entire restored reefs. We hypothesized that oysters alone would provide accurate estimates of denitrification compared to those measured from reef segments containing oysters and sediment, whereas oxygen demand and inorganic nitrogen dynamics (i.e. NO2/3- and NH4+) measured from oysters alone may differ from whole reef incubations, due to the exposure of new surface area and absence of sediment microbial communities This prediction was made following knowledge that denitrification was consistently higher from restored reef sites compared to control sediments [19], with the expectation that samples with higher oyster clump biomass densities would have higher biogeochemical rates
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