Nutrient Dynamics Under The Impact Of Eco-aquaculture

The structure and function of coastal ecosystems have changed greatly under theenvironmental pressors in recent decades, such as over-exploitation, climate change,and environmental pollution. Mari-aquaculture has developed rapidly due to humanneeds. As one of the typical bay for aquaculture, water quality in Sanggou bay is stillin a good condition under the impact of integrated multi-trophic aquaculture (IMTA).However, for economic benefit in recent years, the culture area and the culture densityin Sanggou Bay have been enlarged and raised. Consequently, the tidal dynamicsfeatures, nutrients composition and cycle have been affected. Six cruises were takenout during2012-2014in Sanggou Bay, and mesocosm experiments were conductedduring2012-2013, to understand the level, distribution and seasonal variations ofnutrients, and to study biogeochemical processes of nutrients under the impact ofIMTA. Some important conclusions are drawn as follows:(1) Nutrient concentrations in rivers and groundwater around Sanggou Bay showwide range of seasonal variations. Dissolved inorganic nitrogen was dominated bynitrate both in rivers and groundwater. DIN levels in rivers are generally at the levelsbetween polluted to serious polluted waters, and were higher than some temperate andsub-tropical rivers in China. The annual average concentration of DSi in rivers iscomparable with average level of fresh water system in Asia. Guhe enriched with DINand depleted in PO43-with the DIN:PO43-ratios were higher than Redfield ratio.Groundwater was enriched in dissolved inorganic nitrogen and dissolved silicate, anddepleted in PO43-. As a consequence the DIN:PO43-ratios were higher than Redfieldratio.(2) Nutrient distribution has seasonal variation in Sanggou Bay. DIN, PO43-, andDSi concentrations are the highest in autumn due to the influence of aquaculture, andDIN and PO43-are the lowest in summer due to utilization of kelp and phytoplankton. The levels of DON are high due to the impact of aquaculture activities. Phosphorus isthe limit element for phytoplankton growth with high DIN:PO43-atom ratio both inspring and summer, and nitrogen is the potential limit element in summer. Comparedwith other bay for aquaculture, nutrient levels are still low in Sanggou Bay, whilenutrients composition and phytoplankton community have been changed.(3) Nutrient budgets demonstrate that Sanggou Bay behaves as a source of PO43-and a sink of DIN and DSi during winter and spring, and behaves as a sink of PO43-,DIN, and DSi during summer and autumn. Sediment-water interaction is an importantsource of nutrients in the bay. Large quantities of DIN and PO43-were removed fromthe bay through kelp cultivation during winter and spring, while DIN and PO43-wereremoved through bivalve cultivation and water exchange with coastal water.(4) The results of mesocosm deal with the accumulation of nutrients in differentsize components of the community. Nitrate may be limit element for phytoplanktongrowth in the surface water. Plankton responded significantly to nitrate additions inspring2013and autumn2012by increasing their biomass, more than10-fold thatinitially present in the enclosed water, and by changing community structure, and leadto red tide. Under the integrated farming of bivalve and algae, the dominance of thecommunity is picoplankton, and diatom is dominance of algae. Picoplankton is sink ofnitrogen in autumn2012. From an initial dominance of nanoplankton to increaseddominance of0.2-2μm size component of the community in2013spring due to theinfluence of bivalve (which contributes to more than80%of the accumulated N).Microbial loop may play an important role in nutrient cycle of Sanggou Bay.(5) The growth rate of phytoplankton in surface water in autumn was0.41d-1.Microzooplankton grazing rates on phytoplankton was0.79d-1. Microzooplanktongrazing pressure on phytoplankton standing stock and primary production were54.6%and162%per day, respectively.