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Commercial dredging for blue mussels (Mytilus edulis) and oysters (Ostrea edulis, Crassostrea gigas) constitute the main bivalve fisheries in Denmark. These activities predominantly take place in Limfjorden, a large microtidal sound, and in the Inner Danish waters. Both areas are shallow, estuarine, receive high nutrient inputs from agriculture, and are of nature conservation interest (Natura 2000 sites), thus presenting challenges for an ecosystem approach to fisheries management. Using high-resolution fisheries data (~10 m), we investigated the effects of bivalve dredging on benthic communities at both local (Natura 2000 site) and regional (fishery-wide) scales. Regionally, our results showed that dredging intensity correlated with shifts in species composition and reduced community biomass. We were, however, unable to detect an effect of dredging on community density, trait richness, and trait composition. These metrics were significantly related to other environmental drivers, such as sediment organic content (negative) and mussel bed biomass (positive). At the local scale, the observed relationships between dredging, biomass, and species composition varied significantly. This occurred as dredging impacts were greater in areas that contained suitable reference conditions and experienced relatively low levels of disturbance. By contrast, communities which experienced high nutrient loading, regular anoxic events, and high natural variability were relatively unaffected by dredging. Our results therefore highlight the importance of spatial scales in fishing impact estimations. Furthermore, we demonstrate how targeted sampling, high-resolution fisheries data, and suitable reference areas can be used to detect fishery effects in coastal areas that are highly stressed by eutrophication.

The accumulation of carbon within the Weddell Gyre and its exchanges across the gyre boundaries are investigated with three recent full-depth oceanographic sections enclosing this climatically important region. The combination of carbon measurements with ocean circulation transport estimates from a box inverse analysis reveals that deepwater transports associated with Warm Deep Water (WDW) and Weddell Sea Deep Water dominate the gyre's carbon budget, while a dual-cell vertical overturning circulation leads to both upwelling and the delivery of large quantities of carbon to the deep ocean. Historical sea surface pCO2 observations, interpolated using a neural network technique, confirm the net summertime sink of 0.044 to 0.058?±?0.010?Pg?C?yr-1 derived from the inversion. However, a wintertime outgassing signal similar in size results in a statistically insignificant annual air-to-sea CO2 flux of 0.002?±?0.007?Pg?C?yr-1 (mean 1998–2011) to 0.012?±?0.024?Pg?C?yr-1 (mean 2008–2010) to be diagnosed for the Weddell Gyre. A surface layer carbon balance, independently derived from in situ biogeochemical measurements, reveals that freshwater inputs and biological drawdown decrease surface ocean inorganic carbon levels more than they are increased by WDW entrainment, resulting in an estimated annual carbon sink of 0.033?±?0.021?Pg?C?yr-1. Although relatively less efficient for carbon uptake than the global oceans, the summertime Weddell Gyre suppresses the winter outgassing signal, while its biological pump and deepwater formation act as key conduits for transporting natural and anthropogenic carbon to the deep ocean where they can reside for long time scales.

Mussels within the Baltic Mytilus edulis × M. trossulus hybrid zone have adapted to the low salinities in the Baltic Sea which, however, results in slow-growing dwarfed mussels. To get a better understanding of the nature of dwarfism, we studied the ability of M. trossulus to feed and grow at low salinity (7 psu) compared with its performance at relatively high-salinity (20 psu) in controlled laboratory experiments, supplemented with field (Great Belt) growth experiments with M. trossulus and M. edulis in net-bags. Subsequently, the growth of M. trossulus transplanted in cages to various localities in the northern Baltic Sea was used to evaluate the effect of very low salinities, down to 3.4 psu. The laboratory feeding experiments with M. trossulus at 7 psu showed that the growth in shell length was negligible, whereas the body dry weight nearly doubled during the 15 d experiment, with a weight-specific growth rate of 3.7% d -1. The same parameters measured at 20 psu showed a pronounced growth in both shell length and body dry weight, with a weight-specific growth rate of 2.2% d -1. The growth rates of M. trossulus and M. edulis in suspended net-bags in the Great Belt (22 psu) were similar: 5.6 and 6.8% d -1, respectively. M. trossulus in cage experiments had positive growth rates at locations with salinities above 4.5 psu, up to 2.60% d -1, but negligible increase in the shell length, and at sites with salinities below about 4.5 psu, the somatic growth was negative, around -0.3% d -1, which indicates valve closure and respiratory weight loss. A trend line in a plot of all available growth data for both mussel species as a function of salinity indicates that the growth of mussels is steadily hampered by reduced salinities from 30 psu down to about 10 psu, below which the growth is rapidly reduced to become negative below 4.5 psu. We suggest that reduced ability to produce shell material at extremely low salinity may explain dwarfism of mussels in the Baltic Sea. Reduced bio-calcification at low salinity, however, may impede shell growth, but not somatic growth, and this may at first result in an increased condition index, as seen in the benthic Baltic Sea mussels transferred to cages suspended in the water column.

In the Wadden Sea, annual recruitmentin the major bivalve species is mostly successful insummers following severe winters, whereas it usuallyfails after mild winters. On the basis of a 40 yr data series,we corroborated this relationship between wintertemperatures and subsequent summer abundance ofrecruits in 3 bivalves: Macoma balthica, Cerastodermaedule, and Mytilus edulis. A 30 yr data series showedthat annual abundance of their main epibenthic predators(0-group shrimp Crangon crangon and 0- and 1-group shore crabs Carcinus maenas) was higher insprings after mild winters compared to cold ones. Summerrecruit numbers in 4 bivalve species (the aboveand Mya arenaria) were higher when predator biomasshad been lower in the preceding spring. We arguethat the latter relationship was probably of a causalnature and might explain not only the relationship betweenwinter temperatures and bivalve recruitmentsuccess, but also the observed synchronies of annualrecruitment success between bivalve species and betweendistant parts of the Wadden Sea. Shrimp abundancein May showed a significantly increasing longtermtrend that might partly explain the decliningtrends in recruitment success, biomass, and annualproduction observed in the western Wadden Sea inM. balthica and M. edulis.We conclude that top-downeffects by predation on early bottom stages of bivalvesstrongly affect biomass and production of some of themost important members of the benthic community ofthe Wadden Sea, keeping this system well below itscarrying capacity for bivalves in nearly all years

In the shallow micro-tidal cove Kertinge Nor, Denmark, a series of field campaigns were conducted from April 1995 to September 1996. During these campaigns, the effect of benthic grazing on phytoplankton concentrations was studied using a dual approach. In the first approach, the density, size distribution and in situ growth of 3 dominant benthic grazers were determined to assess grazing potential and its realisation. In the second approach, the realised grazing potential was estimated from the decline in area-specific chlorophyll a (chl a) concentrations after breakdown of stratification. Forcing functions were monitored continuously during the field campaigns using meteorological data, current meters, temperature loggers and CTD point measurements. In addition, measurements from a local monitoring program in Kertinge Nor were included in the data analysis. Stratification of the water column in the cove was mainly governed by wind speed and solar radiation and occurred 50 to 75% of the time. The potential grazing pressure of the benthic suspension feeders varied but was always greater than that required to graze the entire volume of the cove per day. Using both approaches, it was estimated that realised grazing was ~50% of the potential. The lack of realisation of the grazing potential could be attributed to a lack of mixing of the water column, which resulted in strong vertical gradients in concentrations of chl a. The primary mode of decoupling between benthic suspension feeders and phytoplankton was the stratification of the water column, which created refuges for the phytoplankton.

Suspension-feeding mussels exert top-down grazing control on primary producers, and provide bottom-up feedback of regenerated nutrients. Besides direct excretion, an important pathway of nutrient regeneration is through the decomposition of feces and pseudofeces, of which mussels can produce large quantities. Information on mussel feces and pseudofeces quality and nutrient regeneration rates is scarce. Feces and pseudofeces, produced in varying proportions, are commonly treated as one pool. We determined nutrient regeneration rates of feces and pseudo feces decomposition in incubations using natural seawater and juvenile Mytilus edulis from spat collectors. Apart from one 1993 trial, our results are the first to present nutrient regeneration dynamics of feces and pseudofeces separately. Dissolved inorganic nitrogen (DIN) and phosphate regeneration continued at stable rates for approximately 3 wk, after which 13.1 and 12.4% of the available N and 8.7 and 7.9% of the available P was regenerated from feces and pseudofeces, respectively. Rates of silicate regeneration declined continuously, which we attribute to its accumulation in the experimental setup. Coinciding environmental levels of DIN and silicate at growth-limiting levels indicate the potential ecological relevance of biodeposit decomposition. Overall DIN regeneration rates were similar between feces and pseudofeces, but depletion of ammonia was initially more rapid for pseudofeces due to stronger nitrification. Phosphate and silicate regeneration rates were 1.1 and 1.4 times greater from feces than pseudofeces, respectively. Future research should clarify the role of bivalve suspension feeders in controlling Si and P availability in coastal ecosystems in relationship to the proportion of pseudofeces generated, which depends on food concentration.

Multixenobiotic resistance (MXR) is a membrane-transport mechanism that allows organisms to exclude many compounds from their cells and tissues. It is thus a first line of defence against a variety of toxic compounds. Since mussels and oysters possess MXR proteins, an analysis of the expression level of these membrane-transporters has been conducted in relation to their body burden of some major environmental contaminants. Mussels Mytilus edulis and M. galloprovincialis and the oyster Crassostrea gigas were sampled from a total of 43 sites along the French coasts. High expression levels were found in animals from the major French estuaries (Seine, Loire and Gironde), at a few sites in Brittany and in nearly all sites from the Mediterranean mainland coasts. Multivariate analysis of the data for both species of blue mussel did show significant differences between groups of samples. Results indicated that expression of MXR protein was strongly associated with contaminant concentrations in mussels, and that polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) were directly correlated with MXR protein concentration. However, multivariate analysis of oysters, which were collected in less-contaminated sites, at least for organic pollutants, did not show any significant differences between MXR protein expression and contaminants. Although the results do not infer a causal linkage between mussel MXR protein, PAHs and PCBs, since many other chemical contaminants are also present at some sites, they do show clearly that MXR protein expression can be used as an indicator of pollutant exposure in blue mussels. The findings also highlight the need to use alternative analytical methods for the interpretation of complex environmental data, and that non-parametric multivariate statistical methods are appropriate for this task.