No abstract

Mariculture (marine aquaculture) generates nutrient waste either through the excretion by the reared organisms, or through direct enrichment by, or remineralization of, externally applied feed inputs. Importantly, the waste from fish or shellfish cannot easily be managed, as most is in dissolved form and released directly to the aquatic environment. The release of dissolved and particulate nutrients by intensive mariculture results in increasing nutrient loads (finfish and crustaceans), and changes in nutrient stoichiometry (all mariculture types). Based on different scenarios, we project that nutrients from mariculture will increase up to six fold by 2050 with exceedance of the nutrient assimilative capacity in parts of the world where mariculture growth is already rapid. Increasing nutrient loads and altered nutrient forms (increased availability of reduced relative to oxidized forms of nitrogen) and/or stoichiometric proportions (altered nitrogen:phosphorus ratios) may promote an increase in harmful algal blooms (HABs) either directly or via stimulation of algae on which mixotrophic HABs may feed. HABs can kill or intoxicate the mariculture product with severe economic losses, and can increase risks to human health.

A model was developed to estimate nitrogen and phosphorus budgets for aquaculture production of crustaceans, bivalves, gastropods, and seaweed, using country production data for the 1970-2006 period from the Food and Agriculture Organization and scenarios based on the Millenium Assessment for 2006-2050. Global production of crustaceans (18% yr −1 ), molluscs (7.4%), and seaweed (8%) increased rapidly during the 1970-2006 period. Scenarios indicate that annual nutrient release from all shellfish (crustaceans, bivalves, and gastropods) aquaculture will rapidly grow from 0.4 to up to 1.7 million tonnes of nitrogen and from 0.01 to 0.3 million tonnes of phosphorus between 2006 and 2050. The nitrogen and phosphorus releases from global freshwater shellfish aquaculture will increase from 1% of river export in 2006 to up to 6% in 2050. Marine shellfish production is an important contributor to nutrient loading of coastal seas, particularly in Eastern Asia. Nitrogen (7% of marine aquaculture + river export in 2006 and up to 19% in 2050) and phosphorus (12% in 2006 and up to 30% in 2050) releases from Chinese marine shellfish aquaculture are important and growing contributors to total nutrient inputs to coastal seas. Production of crustaceans and bivalves causes changes in nutrient stoichiometry and increasing reduced and organic nitrogen forms, which are of concern because of their preferential use by some harmful algae. Nutrient withdrawal by seaweed is projected to increase rapidly over the coming decades. To overcome effects of increasing nutrient release from shellfish production, integrated systems that include seaweed may play an important role in reducing this nutrient load.

Vegetated ditches and wetlands are important sites for nutrient removal in agricultural catchments. About half of the influx of inorganic nitrogen can be removed from these ecosystems by denitrification. Previous studies have shown that denitrification in aquatic ecosystems is strongly temperature dependent, resulting from temperature-dependent oxygen availability. Here, we study short-term temperature effects on sediment oxygen demand (SOD) and the maximum depth of oxygen penetration into the sediment (Z), in relation to overall denitrification rates. We set up sixteen wetland microcosms at four different temperatures (11-25°C), in which we determined SOD and Z from sediment oxygen microprofiles. Denitrification rates were measured using 1 5 N-labeling, analysed by membrane inlet mass spectrometry. Temperature strongly affected sediment oxygen dynamics. SOD exponentially rose with temperature, ranging from 0.37 to 1.53 g m −2 d −1 (Q 10 = 2.4).Correspondingly, warming led to shallower oxygen penetration into the sediment, ranging from 4.12 to 2.08 mm. Denitrification rates increased with warming (Q 10 = 2.6), ranging from 8.4 to 86 μmol N m −2 h −1 . The results of this short-term experiment confirm the potential increase of denitrification with rising temperature, promoted by lower oxygen availability in the top layer of the sediment, which supports the understanding of denitrification variability in freshwaters.

Artificial wetlands are constructed around the globe for a variety of services, including wastewater treatment and carbon storage. To become a carbon sink, a newly constructed wetland must have a fully developed vegetation, consisting of species that can produce more organic matter than is being lost through decomposition. However, the effects of environmental conditions on the overall balance between production and decomposition might be complex. In this study, two large-scale field litterbag experiments were performed in a three-year old constructed wetland in the Netherlands, to separate the effects of litter characteristics and environmental conditions on decomposition rates of aquatic pioneer vegetation. Dimension reduction by principal component analysis was used to limit the number of variables for subsequent analyses in linear models. When transplanted to one common environment, litter characteristics alone could explain 52% and 26% of the variation in decomposition after 6 and 12 months, respectively. When both litter characteristics and environmental conditions were tested simultaneously and litter was decomposed in its original environment, 37% and 23% of the variation could be explained after 6 and 12 months, respectively. Both experiments showed two phases of decomposition: the initial leaching phase with an important role for litter characteristics and microbial communities in the model, and the second, slower phase, which is predominantly determined by litter characteristics and environmental conditions such as water quality. Model results could not be extrapolated to a fully developed reference area. Optimization of conditions in order to limit decomposition rates seems difficult and therefore we suggest using management options to influence biomass production and thereby fully exploit the use of newly constructed wetlands for carbon storage.