Spatial dissimilarity of zooplankton and hydrodynamic conditions in a Patagonian channel used intensely by aquaculture: the influence of a geomorphological constriction

Background Marine aquaculture is a very important economic and food production activity in Patagonian channels. The biophysical mechanisms through which farms interact with surrounding areas is poorly understood. A better understanding of the relationship between zooplankton distribution, hydrodynamics and aquaculture farms in Patagonian channels can contribute to the environmental sustainability of this activity. Methods The study was conducted in winter in the Caucahué Channel (Chiloé Island, southern Chile), which is composed of two asymmetric northern and southern sections separated by a geomorphological constriction (a narrows) and hosts 55 aquaculture farms. Intensive zooplankton and water column sampling (time scale: 12 h) was carried out, together with current measurements as a background of the channel hydrodynamics (time scale: 30 days). Results Spatial dissimilarities in composition and abundances of zooplankton communities and in water column variables were identified between the two sections of the channel in this short-term time scale. In the southern section we found higher abundances of holo- and meroplankton and higher species richness. No differences in zooplankton community were found between sampling sites near and far from aquaculture farms. Southward asymmetrical residual flow and semidiurnal tidal excursion were verified in the central part of the channel during two tidal fortnightly time periods. Conclusions (i) Clear dissimilarity in zooplankton composition were found between the two sections of Caucahué Channel in the time scale studied; and (ii) Quemchi geomorphological constriction and the asymmetrical southward residual flow could act as a physical barrier favoring the spatial dissimilarities found in biotic and abiotic variables between the two sections of the channel. Keywords: Aquaculture zoning, Coastal circulation, Plankton transport, Mussel farms, Salmon farms