Differential Effects of Bivalves on Sediment Nitrogen Cycling in a Shallow Coastal Bay

Abstract

In coastal ecosystems, suspension-feeding bivalves can remove nitrogen though uptake and assimilation or enhanced denitrification. Bivalves may also retain nitrogen through increased mineralization and dissimilatory nitrate reduction to ammonium (DNRA). This study investigated the effects of oyster reefs and clam aquaculture on denitrification, DNRA, and nutrient fluxes (NOx, NH4 + , O2). Core incubations were conducted seasonally on sediments adjacent to restored oyster reefs (Crassostrea virginica), clam aquaculture beds (Mercenaria mercenaria) which contained live clams, and bare sediments from Smith Island Bay, Virginia, USA. Denitrification was significantly higher at oyster reef sediments and clam aquaculture site than bare sediment in the summer; however DNRA was not enhanced. The clam aquaculture site had the highest ammonium production due to clam excretion. While oyster reef and bare sediments exhibited seasonal differences in rate processes, there was no effect of season on denitrification, DNRA or ammonium flux at the clam aquaculture site. This suggests that farm management practices or bivalve metabolism and excretion may override seasonal differences. When water column nitrate concentration was elevated, denitrification increased in clam aquaculture site and oyster reef sediments but not in bare sediment; DNRA was only stimulated at the clam aquaculture site. This, along with a significant and positive relationship between denitrification and sediment organic matter, suggests that labile carbon limited nitrate reduction at the bare sediment site. Bivalve systems can serve as either net sinks or sources of nitrogen to coastal ecosystems, depending mainly on the type of bivalve, location and management practices. Supplemental Table 1: Number of clams per core by season.

Stoichiometry of regenerated nutrients differs between native and invasive freshwater mussels with implications for algal growth

“…This study contributes to our understanding of how invasions by non‐native mussels may alter benthic biogeochemistry and nutrient availability in the pelagic environment, stimulating primary production (Conroy et al., ; Heath et al., ; Ruginis et al., ; Strayer, ; Zhang et al., ). Results from this study confirm that filter‐feeding mussels increase the net heterotrophy of the benthic system, augmenting oxygen uptake, total DIC, nitrogen and phosphorus release, and nitrogen loss via denitrification (Benelli et al., ; Ruginis et al., ; Smyth, Murphy, Anderson, & Song, ; Welsh, Nizzoli, Fano, & Viaroli, ). Light measurements highlight also the importance of benthic primary production for nutrient cycling in the absence and even more in the presence of filter feeders, with high and comparable rates of gross primary production in all the considered treatments.…”

“…Through filtration, mussels remove particulate materials from the water column. A fraction of such algal material is assimilated by the mussels and stored for their growth, a fraction is excreted to the water column as dissolved inorganic nutrients, and a fraction is egested as biodeposits (faeces and pseudofaeces) to the sediment (Smyth et al., ; Strayer, ; Vaughn, ). In this study, we did not measure the quality and quantity of biodeposits.…”

Seasonal patterns of benthic-pelagic coupling in oyster habitats

“…The N 2 -N flux rates we measured in summer (−2.7 μmol N 2 -N m −2 h −1 ) are much lower than summertime rates measured beneath aquaculture (553.6 μmol N 2 -N m −2 h −1 ; Ray et al 2020) in nearby Ninigret Pond (< 50 km distance), and lower than nearly all other studies that report N 2 -N fluxes in oyster habitats. Our measured rates of denitrification in spring (48.8 μmol N 2 -N m −2 h −1 ), however, are within the range reported in other studies (Higgins et al 2013, Hoellein et al 2015, Onorevole et al 2018, Smyth et al 2018. The high rate of N 2 fixation we found in fall (−44.8 μmol N 2 -N m −2 h −1 ) is the highest mean N 2 fixation rate ever reported for oyster habitats.…”

Comparison of methods for determining biogeochemical fluxes from a restored oyster reef

“…In situ approaches for measuring oyster denitrification and other biogeochemical fluxes are logistically challenging [20, 28], but incubations must include oysters if the goal of the research is to accurately quantify oyster reef biogeochemical fluxes. The observation that the incubations of oysters alone generally fall within the range of measured reef segment rates suggest that oyster clumps are driving reef-associated biogeochemistry.…”

“…Although observations show that denitrification associated with oysters effectively removes nitrogen [18–21], methodologies and results vary widely. Methods used to estimate denitrification and nutrient cycling associated with oysters include: benthic tunnels [14], sediment core incubations with the addition of oyster biodeposits [22, 23], incubations of sediment cores collected adjacent to oyster reefs [18, 24–28], incubations of live oysters without substrate [15], in situ experimental chambers that encompass intact reef segments [20], and in situ equilibration of intact reef segments with ex situ incubation and measurement [19]. Many of these studies do not include oysters or the highly abundant reef-associated organisms that alter biogeochemical cycles [18, 29].…”