Seasonal nitrogen and phosphorus dynamics during benthic clam and suspended mussel cultivation Daniele Nizzoli a,⇑ , David Thomas Welsh b , Pierluigi Viaroli a a Department of Environmental Sciences, Parma University, Parco Area delle Scienze 33/A, 43100 Parma, Italy b School of Environment and Environmental Futures Centre, Griffith University, Gold Coast Campus, PMB 50 GC Mail Centre, Bundall, 9726 Qld, Australia article info Keywords: Ruditapes philippinarum Mytilus galloprovincialis Nitrogen Phosphorous Coastal lagoon Eutrophication abstract Effects of suspended mussel and infaunal clam cultivation on sediment characteristics, and benthic organic and inorganic nitrogen and phosphorus fluxes were compared in a shallow coastal lagoon. The two species had different impacts on sediment features, but both created ‘‘hotspots’’ of nutrient fluxes with annual N and P regeneration rates being 4.9 and 13.5 (mussel) and 4.5 and 14.9 (clams) fold greater than those of unfarmed control sediments. Mussel farming also caused considerable nutrient regenera- tion within the water column with the mussel ropes contributing 25% of total inorganic N and P pro- duction and at times dominating the sediments (e.g. 95% of SRP production in summer and 45% of DIN production in winter). Such nutrient regeneration rates seriously question the proposal that suspen- sion-feeding bivalves act as a eutrophication buffer, especially during summer when nutrient regenera- tion rates are maximal, but other nutrient sources (freshwater run-off and unfarmed sediments) are at their lowest. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Aquaculture and in particular the cultivation of filter-feeding bivalves are rapidly growing worldwide (Lovatelli, 2006). Bivalve farming occurs mostly in coastal areas and often in coastal bays and lagoons as they provide ease of access and exhibit high rates of primary production which sustains both high densities of molluscs and rapid growth rates. The most commonly cultivated species are oysters, mussels and clams (Dumbauld et al., 2009; Lovatelli, 2006). Whilst, oysters and mussels have traditionally being cultivated in Mediterranean lagoons and coastal waters clams, and particularly the exotic Manila clam (Ruditapes philippinarum), were introduced only recently but their cultivation is becoming increasingly wide spread, especially in the Northern Adriatic (Viaroli et al., 2006, and references therein). Increasing anthropogenic pressures on coastal lagoons have raised concerns about the environmental impacts of aquaculture activities on these valuable and fragile ecosystems. Environmental impacts from bivalve aquaculture are generally considered less se- vere compared to other types of aquaculture, such as fish farming, because filter feeders do not require external inputs of feed as they harvest particulate suspended solids directly from the water col- umn (Dumbauld et al., 2009; Newell, 2004). However, bivalve farming is increasingly recognised to strongly modify nutrient dynamics and the functioning of shallow coastal ecosystems as fil- tration, coupled to biodeposition of faeces and pseudofaeces cre- ates ‘‘hot spots’’ of organic matter enrichment in the same way as sedimentation of feeds and faeces do below fish cages. Biodepo- sition can result in organic matter loads to the underlying sedi- ments of up to 100s of g DW m 2 d 1 (Callier et al., 2006; Graf and Rosenberg, 1997), values similar to those reported below fish cages (Hall et al., 1990; Holmer et al., 2002). Sediment organic matter enrichment stimulates heterotrophic metabolism increasing sediment oxygen demand, benthic minerali- sation rates, and the release of the regenerated nutrients from the sediment to the overlying water (Giles et al., 2006; Kaiser et al., 1998; Mazouni et al., 1996; Richard et al., 2007). In addition, enhanced organic matter loads modify biogeochemical processes such as nitrification and nitrate reduction (Christensen et al., 2003; Nizzoli et al., 2006) and induce shifts in benthic macrofauna community structure toward the dominance of small sized opportu- nistic species (Christensen et al., 2003; Kaiser et al., 1998; Pearson and Rosenberg, 1978) which in turn influences benthic biogeochem- istry via changes in bioturbation activities (Dunn et al., 2009; Jordan et al., 2009; Nizzoli et al., 2007). To date the vast majority of studies on bivalve farming have fo- cussed on the effect of suspended mussel and oyster cultures which influence benthic metabolism and nutrient recycling exclu- sively through increased biodeposition of organic matter, but very few studies have addressed the effects of cultivation of infaunal species such as clams. Despite the fact that these effects are likely 0025-326X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2011.03.009 ⇑ Corresponding author. Address: Department of Environmental Sciences, Parma University, Viale GP Usberti 11/A, 43124 Parma, Italy. Tel.: +39 0521 905976; fax: +39 0521 905402. E-mail address: daniele.nizzoli@nemo.unipr.it (D. Nizzoli). Marine Pollution Bulletin 62 (2011) 1276–1287 Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul