engleski

Impact of the sediment characteristics on the organic matter content under the various types of fish farms

Aquaculture is today the fastest growing food-producing sector currently providing about 50 % of the world's fish foods (FAO, 2019). However, its rapid development and intensity revealed various environmental issues. In particular, fish farming affects benthic environments by organic matter (OM) accumulation. As a result, hypoxic and anoxic conditions may occur, leading to toxic gas production and remobilization of trace metals. The progressively changed sedimentary environment may thus act as a source of contaminants, further reducing water quality and affecting the food web. In order to prevent environmental deterioration, a proper site selection is a prerequisite for sustainable aquaculture practice. One of the key physical factors when it comes to fish farm site selection is sediment structure: sandy and gravelly sediment is preferred, suggesting appropriate lateral water transport. In order to examine unknown relationships between sediment characteristics that may affect the proportion and the retention of the OM in the bottom, sediment from three different locations of various aquaculture practices were examined. Sediment was sampled quarterly for one year. The grain size was determined by the combined methods of wet sieving and sedigraph. OM content was calculated by the loss of ignition (LOI). Bulk sediment mineral composition was determined by the x-ray diffraction, while carbonate content was determined by CO2 volumetry. Obtained results were compared with sediment sampled in referent locations paired with each farm site. The sediment samples under marine cage-type fish farms in the Mali Ston Bay were in general slightly finer (gravelly sands, sandy gravels, and muddy gravel) compared to the referent sampling location (gravelly sands and sandy gravel). The mud content was consistently low (< 5%) in both locations, except below the cages after autumn sampling (~ 50%). Carbonate content was consistently high (> 90%) in both locations, with the same exception below cages after autumn sampling (> 50%). In accordance with the grain size distribution and the carbonate content, sediment was dominated by calcite, Mg-calcite, and aragonite, typical for the eastern Adriatic shelf (PIKELJ, 2010). OM content in sediment in both locations was equal and varied between 2.9-8.2 %, which is comparable with other results from the middle and southern Adriatic (MATIJEVIĆ et al., 2008a ; b). The exception is 11.5% of OM below cages found during spring sampling. This result may be the consequence of enhanced input of the Neretva River carrying OM and nutrients from its highly anthropized delta. Suspiciously muddy sediment sampled under the cage in autumn and with only 50% of carbonates is being considered to reflect local variations in seabed morphology (eg. depression), which may affect grain size distribution (PIKELJ, 2010). This conclusion is underpinned by the fact that despite the high share of mud (~ 50%) the OM content in this particular sediment was relatively low (4.7%). Fish farm on the Krka River is pool type with flowing water. Its sediment was classified mostly as sandy gravels as well as the river sediment on the referent location. Small changes in grain size together with carbonate content on both locations were ascribed to the sediment flow within the river system. Dominating minerals were calcite and dolomite with quartz to a lesser extent, reflecting the surrounding lithology, dominated by Mesozoic dolomites and limestones with Holocene proluvial and alluvial deposits (CGS, 2009 ; VELIĆ & VLAHOVIĆ, 2009). OM content in sediment from the farm was constant (~ 4%) during the monitored period, as well as on the referent location (~1.5%). Bearing in mind the fact that the control point is situated immediately downstream of the fish pools, it is likely that the impact of the fish farm was localized to the farm pools. Fish farm on the Ilova River includes ponds separated by natural or constructed barriers, while the chosen referent location is within the Ilova River. Sediment from the pond were determined as gravelly muds and muddy gravels, with average mud content of 35%. A much higher share of mud (~90%) was found in sediment sampled on the referent location and characterized as mud or sandy mud. Carbonates were almost absent from the river sediment, while pond sediment revealed about 30% of carbonates. Based on the grain size and the carbonate share a higher percentage of the OM was expected. However, its consistent percentage of 4% in ponds and of 6 % in river was unexpectedly low. The explanation of obtained results was found in the mineral composition. Quartz is the dominant mineral phase in both locations. Plagioclase and feldspars are less abundant, as well as muscovite. Clay minerals found on both locations are scarce and belong to the kaolinite group, while relatively high carbonate content in ponds may be the result of accumulation and/or local biogenic production (eg. bivalves). In general, mineral composition reflects the surrounding lithology dominated by loess (CGS, 2009 ; VELIĆ & VLAHOVIĆ, 2009). As shown in the above-described case studies of three different fish farm practices performed in various climatic and environmental conditions, grain size is proven as one of the key factors defining the fate of the OM in the bottom sediment. However, as shown in case of muddy marine sediment and the muds in the Ilova River, mineral composition is another important sediment characteristic to be considered when choosing a location for a fish farm. In particular, the mineral composition of fine-grained fractions has to be known when it comes to environmental studies, as already emphasized by PIKELJ et al. (2016).

fish farm ; sediment ; environmental health ; mineral composition ; organic matter

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