Wastes produced by aquaculture are diverse, ranging from particulate organic and dissolved inorganic wastes to chemicals and drugs used to control pathogens. Aquaculture wastes vary in quantity and quality depending principally on the species being cultured, the culture system, and the technology in use. They also depend on feed quality and management practices at the aquaculture sites. The effects and ecological consequences of the massive introduction of organic and inorganic material into the environments depend on the hydrodynamic and abiotic conditions and the overall resilience capacity of the host ecosystem. Intensive mariculture, principally those that require an exogenous input of energy, produce uneaten feed and feces, dissolved nutrients as products of excretion, parti-culate organic compounds, and several types of chemicals. Pathogens and escapees should also be included as waste components. In this article, we will concentrate on parti-culate organic and dissolved inorganic wastes produced by cultured organisms. These wastes are either deposited in the sediments or introduced into the water column.
Of the total feed introduced into the environment, only 25-30%, and even less in the case of shrimps, are removed from the environment by the harvested organisms. Feces, excretion products, and uneaten feed remain in the environment altering energy and mass flows, therefore modifying the carrying capacity of the environment. The fraction of these wastes that stays as particles is deposited in the sediments beneath the culture area, and the dissolved portion is diluted and transported by the prevailing currents. Up to 60-65% of the nitrogen (N) input through feed is dissolved in the water column. This N is immediately available to primary producers, mainly phytoplankton, and increases the risk of harmful algal blooms (HABs) in the area. However, this topic remains controversial, as only a few studies have directly linked eutrophication by aquaculture practices with HABs events. Finfish culture can increase nutrient concentrations and modify nitrogen/phosphorus (N/P) ratios around fish cages even in tidally energetic fjords and channels. The altered nutrient concentrations have been linked to the increase of dinoflagellate abundance and to toxic plankton organisms such as Prymnesium parvum and Pfiesteria piscida. Also, increased filamentous algal growth around fish cages in the Baltic Sea, and increased red and green seaweed abundance in bays with salmon aquaculture in the south of Chile, have been reported. It is also important to mention that the seaweeds cultivated near salmon farms increase their nitrogen tissue content even in situations of rapid N dilution. The main explanation is that these seaweeds behave in fact like environmental sentinels, integrating over time nutrient variations.
The rest of the N and over 70% of the P will be deposited into the sediment. The process takes place mainly in the area surrounding the culture installations, where the organic matter content of the sediment increases by up to 7-8% (dry weight). Under this organic-enrichment condition, high benthic microbial respiration can increase oxygen consumption in the upper layers of sediments and completely depletes oxygen levels below the first 1-2 cm resulting in anoxic sediments. This process results in emission of environmentally undesirable chemical compounds such as methane (CH4) and hydrogen sulfide (H2S). Additionally, remineralized nutrients (N and P) exacerbate marine eutro-phication, being especially relevant for N since, in oxygen-depleted environments, it is mostly released as bioavailable ammonia without undergoing aerobic denitrification to N2. Sulfide enrichment and oxygen depletion significantly reduce biodiversity of marine invertebrates (see Figure 2 for details).
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