| Abstract |
The habitat inhabited by and in turn modified by the benthos is the result of many interacting factors, of regimes which cover scales from the global to the local, and of the intimate linkages between the water column and its factors and the substratum and its factors.
Human impacts have major effects in the coastal benthic systems. In the case of Mediterranean fish farms it seems that they often are settled in the important habitat of Posidonia oceanica fields because the environmental characteristics of these areas (strong currents, good water quality, coarse sediment) fit perfectly with the special requirements of the fish farms.
To study the impact of fish farming in different benthic Mediterranean habitats it was examined the pattern of organically enriched sediments by fish farming activities in a gradient of different sediment types. As testing grounds, muddy and seagrass habitats were chosen because of the different biogeochemical characteristics and functions of these areas. Our results showed the relationship of the biogeochemical variables with bathymetry and sediment composition. Although the spatial extent of the organic enrichment due to fish farming was the same, faunal organisms and geochemical variables showed different patterns according to the functions of their habitat.
The sediment fluxes of nutrients and oxygen were used as proxies for the study of the relationship between macrobenthic diversity and ecosystem function in the gradients of organically enriched sediments. The relationship between benthic diversity and TOC concentration followed the pattern proposed by (Hyland et al. 2005). Our results showed that fauna, abiotic factors, oxygen fluxes and nutrient exchange determined the function of the benthic ecosystem. The shifts in
biogeochemical processes occurred as a function of diversity and depended on the nature of the sediments.
In order to remove the effect of sediment functioning variability, we repeated the analyses separately for the two major habitat types (bare sediment of muddy and seagrass habitats). It seems that the overall response of biological and geochemical variables to the organic enrichment varied considerably among habitat types and the effects on the benthic environment are more difficult to detect in coarse sediments through standard monitoring.
The gradient change of organic enrichment can induce changes in the nutrient and oxygen fluxes because of the different rate of microbial mineralization. The analyses were repeated after splitting the data sets into fine-coarse sediments and high-low TOC samples in order to isolate the effect of diversity while keeping the variability of other factors stable. In coarse, organically poor sediments, high macrobenthic diversity leads to higher fluxes from the sediment to the water column and implies an increase of the nitrification process. Also at fine organically enriched sediments high diversity causes an increase of the nitrate sediment uptake (negative flux) indicating enhancement of coupled nitrification/denitrification. These results imply that effects on the biogeochemical conditions and the nutrient exchange differ and depend on the sediment characteristics.
Within-functional trait diversity was also used to examine the functional redundancy of specific functional traits. Characteristics considered to respond in different geochemical conditions of the sediment were chosen for the traits list of that organism and were classified into ecological and morphological traits. Each trait was split up into several categories chosen to encompass the range of possible attributes of all species. The abundance of species possessing the same code was summed for each sample. Functional codes analyses differentiated the
sediment types and the TOC states, confirming the sensitivity of functional characteristics to the geochemistry of the sediment.
Changes in species number and abundance in the compared sites were the ones that determined the shifts of the functional processes and not the presence/absence of individual traits. The important functional codes for the homogeneity within the examined habitats were fewer in the disturbed areas indicating that the functioning of these ecosystems is significantly reduced. This means that in disturbed habitats, the functional attributes of the organisms are restrained and the ecosystem processes accomplished by macrofauna are reduced. The ecosystem loses complexity and shifts towards a more microbial driven functioning. Thus, the increasing sources of human induced soft-sediment alteration can cause habitat homogenization and lead to a loss of functional diversity.
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Μελέτη των επιπτώσεων των ιχθυοκαλλιεργειών στην βενθική πανίδα και τις οικοσυστημικές λειτουργίες
