Seasonal variability in nutrient regeneration by mussel Mytilus edulis rope culture in oligotrophic systems

Abstract

Blue mussel Mytilus edulis cultures contribute to nutrient cycling in coastal ecosystems. Mussel populations filter particulate nutrients from the water column and inorganic nutrients are regenerated by excretion of metabolic wastes and decomposition of (pseudo-)faeces. The objective of this study was to determine the intra-annual variability in nutrient regeneration by mussel rope cultures in oligotrophic fjord systems. In situ respiration and nutrient uptake and release rates of 1 m mussel ropes were measured using 250 l pelagic chambers. There was a 20-fold difference between winter and summer respiration and nutrient release rates. Inorganic nitrogen release ranged from 50 to 1000 µmol h -1 per meter rope. These variations were mainly related to mussel growth but were also related to changes in water temperature and biofouling biomass (organisms that colonized the mussel ropes). In total 24 genera of fouling organisms were observed, diversity increased over time (ranging from 2 to 12 genera m -1 ), and fouling biomass was mainly characterised by ascidians (max. 37 ± 14 g m -1). However, mussels dominated the culture ropes, representing > 90% of total faunal biomass. The amount of organic material associated with the ropes was stable (6.9 ± 0.3 g m -1). At the scale of one mussel farm, nutrient regeneration by mussel rope cultures increased inorganic nitrogen concentrations by 20% and inorganic phosphorus concentrations by 5% during summer conditions. During winter, there was no significant effect of mussel cultures on the inorganic nutrient pools. Nutrient regeneration by mussel cultures also affected stoichiometry as nutrients were excreted in dissimilar proportions (nitrogen > phosphate > silicate). The increased nutrient availability may contribute to primary production, especially in nutrient-limited (oligotrophic) fjord ecosystems. However, fjordscale effects are largely dependent on hydrographic conditions of the fjord system.

Influence of farmed pearl oysters and associated biofouling communities on nutrient regeneration in lagoons of French Polynesia

“…Over the last decade, particular attention has focused on the effect that biofouling communities may exert on ecosystem processes (McKindsey et al 2009, Jansen et al 2011, Woods et al 2012. Results of the present study at 2 sites exposed to a high level of biofouling confirmed the significant role these communities can play regarding nutrient regeneration in the water column around pearl oyster culture.…”

“…As this increase of fluxes is not attributable to pearl oysters, whose activity remains stable, it could originate from the activity of biofouling communities accumulated on ropes (Mazouni et al 2001, Richard et al 2006, Jansen et al 2011. Furthermore, it is now well established that in suspended culture, ropes are not only composed of the cultivated species and biofouling communities but also of detritus and organic material (Mazouni et al 2001, Nizzoli et al 2005, Richard et al 2006, Jansen et al 2011). In our study, a notable accumulation of organic material along the pearl oyster ropes started from November 2011, after heavy rainfall.…”

“…The influence of bivalve cultivation on nutrient cycling is highly variable depending on the commercial species and/or cultivation systems considered. It may also vary seasonally (Picot et al 1990, Bacher et al 1995, Newell 2004 or according to the composition of the whole filter feeder community, including biofouling organisms which develop on infrastructures and bivalve shells (Mazouni et al 2001, Richard et al 2006, Jansen et al 2011. Whereas the influence of bivalve aquaculture on ecosystem processes has been widely investigated for temperate ecosystems (see the reviews of Prins et al 1997, Cranford et al 2003, Newell 2004, Dumbauld et al 2009, Forrest et al 2009), similar studies on tropical ecosystems remain very scarce.…”

The use of shellfish for eutrophication control

“…Mazouni et al 1998Mazouni et al , 2001Souchu et al 2001), leading to significant contributions to the ambient pool of dissolved organic and inorganic nutrients (Nizzoli Jansen et al 2011). The nutrients excreted in the water column by mussel longline cultures are highly variable depending on season, temperature and size of the mussels (Richard et al 2006;Cranford et al 2007;Jansen et al 2011;Nizzoli et al 2011;Jansen et al 2012) with 20-fold differences between summer and winter (Jansen et al 2011). Increased water column nutrient concentrations in mussel culture operating under eutrophic conditions have been more difficult to detect.…”

Using a weight-structured oyster population dynamic model to explore top-down control of coastal water quality in a subtropical embayment

“…While they have been shown to be effective in removing particulate biogenic material from the water column (Jackson et al, 2001;Newell et al, 2009) and their reefs influential in attenuating local wave climates (Smith et al, 2009), they can also contribute to increased dissolved inorganic nutrients in the water column and increased organic particulates in the underlying sediment through excretory and egestive processes, respectively (Chapelle et al, 2000). Paradoxically, the former can itself lead to eutrophication under nutrient-limiting conditions (Jansen et al, 2011), while the latter can potentially result in anoxic conditions in the underlying sediment (Dahlbäck and Gunnarsson, 1981). Furthermore, filter-feeders have been implicated in shifting the size-structure of phytoplankton communities away from nanoplankton and towards picophytoplankton (Fulford et al, 2007(Fulford et al, , 2010Newell et al, 2009), decreasing the particle size of prey available to grazers (including other filter-feeders) and potentially influencing the quality, quantity, and availability of food for higher trophic levels.…”