Impacts of longline mussel farming on oxygen and nitrogen dynamics and biological communities of coastal sediments

Abstract

Benthic communities and benthic mineralization were studied in two shallow coastal regions of New Zealand: Tasman Bay, a possible future site for mussel farm development, and Beatrix Bay, which already hosts several longline mussel farms.

In Tasman Bay, microphytobenthic (MPB) production added significantly to the total primary production of the bay. The activity of benthic microalgae had a pronounced effect on oxic conditions, solute exchange and denitrification rates. Benthic mineralization, quantified as the dark oxygen uptake, was in the range of 675±11 μmol m⁻² h⁻¹. Denitrification rates were high and fueled entirely by nitrate produced by the nitrifying community within the sediment. Competition for inorganic nitrogen between benthic microalgae and nitrifiers/denitrifiers resulted in diel variation in nitrogen cycling and reduced the inorganic nitrogen efflux and denitrification activity in the light. Calculated in electron equivalents, denitrification accounted for 11–20% of the total carbon mineralization—one of the highest numbers reported for coastal sediments.

Reduced sediments, containing low MPB biomass and few subsurface macroinvertebrate species, were observed below a mussel farm in Beatrix Bay, presumably due to the intensified sedimentation of organic matter. Oxygen consumption increased in the organic-rich sediments, and ammonium effluxes were up to 14 times higher than those of unaffected sediments 250 m away from the farm. Denitrification rates below the farm were low as the coupled nitrification–denitrification was inhibited by the presence of sulfide. The dissimilative reduction of nitrate to ammonium (DRNA) was, however, stimulated in the reduced sediment. The enhanced benthic mineralization was associated with sulfidic sediments and a lower nitrogen removal rate due to impeded benthic photosynthesis and denitrification activity. The described local conditions associated with mussel farming should be taken into account when new areas are considered for development.

Introduction

Annual world harvest of wild mussel stocks has increased during the past 20 years and has led to overexploitation of natural beds. Mussel aquaculture is consequently an expanding industry. New Zealand is among the five most important countries in terms of mussel production and international trade. While a number of species of mussels are farmed worldwide, the Greenshell™ mussel (Perna canaliculus) has been developed commercially in New Zealand, exclusively, with a current annual production of approximately 78,000 t wet weight (personal communication, Clarkson, Mussel Industry Council, New Zealand). Greenshell™ mussels are produced offshore on longline cultures in a number of locations in New Zealand. The most important growing area is within the Marlborough Sounds on the northern part of the South Island. A major expansion in production is planned for other areas of New Zealand, however, including the more exposed waters of Tasman Bay to the west.

Longline production in New Zealand generally places the mussels in the upper 10–15 m of the water column. Mussel filtration enhances the grazing pressure on the phytoplankton community and harvesting of the mussels represents an export of both carbon and nutrients. Perhaps more importantly, however, mussel farms also result in a concentration and redistribution of nutrients. Conditions of localised enrichment can arise through excretion of dissolved inorganic nutrients into the water column and increased sedimentation of organic material below the farms in the form of faecal and pseudofaecal materials, dead mussels and associated epibiota. Sedimentation rates have been reported to be two to three times higher underneath the mussel farms compared to ambient rates outside the farms Dahlback and Gunnarson, 1981, Grant et al., 1995. If we also consider mussel and epibiota drop-off during harvesting, much higher rates would be expected and longline mussel production may thereby dramatically alter the benthic environment.

Marine aquaculture of fish is known to have a severe local environmental impact due to the release of residual fish food and excreta to both the aquatic and benthic environments Kaspar et al., 1988, Hall et al., 1992, Christensen et al., 2000. In response, oxygen becomes depleted and sulfate reduction is stimulated leading to increased H₂S production and subsequent displacement of fauna and benthic algae. The benthic effects of mussel farming, where no excess food is supplied, are supposed to be much less than those of caged fish farming. However, relatively little is known of the effects of longline mussel farming on benthic ecology, microbial mineralization and nutrient dynamics.

A major part of the carbon reaching the sea floor becomes mineralized mainly by different populations of microorganisms Berner, 1980, Canfield, 1989. In sediments that are exposed to light, benthic microalgae may add significantly to the total primary production of the system Sloth et al., 1996, Cahoon, 1999, Gillespie et al., 2000, Glud et al., 2002. Additionally, photosynthesis changes oxygen conditions and influences mineralization processes within the sediments Rysgaard et al., 1994, Fenchel and Glud, 2000. Expansion of the oxic zone can stimulate the prerequisite conditions for nitrification Risgaard-Petersen et al., 1994, Lorenzen et al., 1998, An and Joye, 2001. However, both nitrification and denitrification may be simultaneously inhibited by enhanced competition for inorganic nitrogen in the presence of benthic algae (Rysgaard et al., 1995). Burrowing sediment infauna can also greatly influence the oxic conditions of surface sediments, especially by transporting oxygen down into otherwise anoxic zones of the sediment. Such activities have been shown to stimulate nitrogen removal due to an enhanced, coupled nitrification–denitrification activity (Pelegri et al., 1994).

In the present study, we assessed benthic communities, oxygen dynamics, nutrient fluxes and denitrification rates in sediments from two coastal locations in New Zealand. The study included soft-bottom habitats of stations in Tasman Bay, which is currently not farmed but proposed for a major mussel farm development. Sediments from the same water depth beneath and outside the influence of a mussel farm in Beatrix Bay, Marlborough Sounds, were also studied.