Impact of clam and mussel farming on benthic metabolism and nitrogen cycling, with emphasis on nitrate reduction pathways

Abstract

The influences of suspended mussel and infaunal clam cultivation on benthic metabolism and nutrient cycling were compared in Goro lagoon, Italy. Both aquaculture types stimulated benthic metabolism, with sediment oxygen demand (SOD), CO 2 and ammonium effluxes of up to 14, 16 and 1.2 mmol m -2 h -1. However, whilst mussel farming preferentially stimulated anaerobic metabolism and sediment reduction, clam farming did not. The mussel ropes were also large oxygen sinks and ammonium sources, with oxygen consumption and ammonium production rates of 1.4 to 1.5 and 0.18 to 0.43 mmol kg -1 h -1. Consequently, the overall impacts of mussel farming on oxygen and nutrient dynamics were much greater than those of clam farming. There were also differences in nitrate-reduction processes and the nitrate sources that fuelled them. In winter, at high water column nitrate concentrations, highest nitrate reduction rates (~320 µmol m -2 h -1 ) occurred at the mussel farm. Nitrate reduction was driven predominantly by water column nitrate and ~30% of nitrate reduced was recycled to ammonium via dissimilatory nitrate reduction to ammonium (DNRA). At the control and clam farm sites, nitrate reduction rates were lower (~180 µmol m -2 h -1 ), nitrification supplied ~30% of nitrate and denitrification was dominant. In summer under low nitrate conditions, nitrate reduction was highest (~130 µmol m -2 h -1 ) at the mussel farm site, but this activity was completely dependent upon water column nitrate and 95% of nitrate was reduced via DNRA. In contrast, at the clam farm station, DNRA was unimportant and nitrification was the major nitrate source for denitrification. Consequently, whilst nitrate reduction processes eliminated fixed N from the clam farm sediments via coupled nitrificationdenitrification, the dominance of DNRA at the mussel farm site resulted in a net N input to the sediment compartment. These large differences in the impacts of clam and mussel farming can be explained by the fact that infaunal clams stimulate transfer of both organic matter and oxygen to the sediment, whereas suspended mussels enhance only organic matter inputs.

Seasonal oxygen, nitrogen and phosphorus benthic cycling along an impacted Baltic Sea estuary: regulation and spatial patterns

“…DNRA was not a relevant process at stations B1, H2 and H4, but at the impacted station H6 in October DNRA rates were significantly higher than rates of denitrification, a typical characteristic of sulfidic, organic-rich sediments (Christensen et al 2000;Nizzoli et al 2006). Here, bottom water oxygen concentrations played a major role in the regulation of NO 3 -reduction pathways: when oxygen was abundant, the black, sulfidic sediment surface was replaced by a thin layer of light-brown oxidized sediment in which nitrification could take place.…”

Effects of a commercial, suspended eastern oyster nursery upon nutrient and sediment chemistry in a temperate, coastal embayment

“…Most research on bivalve mariculture has been focused on how the production and sedimentation of feces and pseudofeces affects the benthic and pelagic communities (reviews by Cranford et al 2008, McKindsey et al 2011, Shumway 2011. A few studies have examined the potential effects on nutrient cycling, fluxes and retention at the coastal ecosystem scale (Newell 2004, Nizzoli et al 2006, Cranford et al 2007. Most of these studies have been before-after-control-impact (BACI) studies, but there are a few problems with a BACI design: (1) the response variable that is being measured varies naturally over time, therefore any changes observed over time may not be related to the before and after impact but rather to natural environmental changes; (2) there will always be natural differences between the control and the impact site (i.e.…”

An ecosystem-based approach and management framework for the integrated evaluation of bivalve aquaculture impacts

“…Potential effects on nutrient cycling, fluxes, and retention may also be expressed at the coastal ecosystem scale (e.g. Newell 2004, Nizzoli et al 2006, Cranford 2007 such that the boundary of the aquaculture system to be managed must include both benthic and pelagic components and extend beyond the footprint of the farm.…”