Assessing the micro-phytoplankton response to nitrate in Comau Fjord (42°S) in Patagonia (Chile), using a microcosms approach

Abstract

Anthropogenic (aquaculture) changes in environment nutrient concentrations may affect phytoplankton (biomass and taxa composition) in marine coastal waters off the Chilean Patagonia. The effects of adding nitrate (NO₃(-)) to natural phytoplankton assemblages were evaluated considering biomass, cell abundance, and taxonomic composition. Microcosm experiments were performed in the spring, summer, and winter in the Comau Fjord located in Subantarctic Patagonia. At the end of the experiments, NO₃(-) decreased rapidly and was undetectable in treatments, indicating a strong NO₃(-) deficiency associated with an exponential increase in Chl-a concentrations, particulate organic nitrogen, and carbon in these treatments. Moreover, given the depleted nitrate concentrations of the spring and summer experiments, the micro-phytoplankton taxa structure shifted from mixed diatom and dinoflagellate assemblages (Ceratium spp., Dinophysis spp., Coscinodiscus sp., Rhizosolenia pungens) to assemblages dominated by blooms of the classic chain-forming diatoms found in temperate and cold waters such as Chaetoceros spp., Skeletonema spp., and Thalassiosira spp. Thus, nitrogen sources (i.e., nitrate, ammonia) may influence phytoplankton abundance and biomass accumulation dynamics in the northern section of Patagonia. It also emphasizes the importance of diatom taxa in regards to the short-term response of phytoplankton to changing environmental nutrient conditions due to natural (decreasing freshwater stream flow) and anthropogenic (aquaculture) events. This situation may be one of the future scenarios in the Patagonian fjords, thus stressing the needs for active environmental monitoring and impact assessment.

Diatom blooms and primary production in a channel ecosystem of central Patagonia

“…In Chilean fjords ecosystems, diatoms are known to represent a significant component of the overall phytoplankton biomass (Iriarte et al, 2001;Cassis et al, 2002;Alves-de-Souza et al, 2008;Iriarte & González, 2008). In addition to efficiently transferring energy to higher trophic levels, diatoms make a significant contribution to the downward transfer of organic carbon (Iriarte et al, 2007;González et al, 2010;Montero et al, 2011;Iriarte et al 2013). Diatoms occur in highest abundance during the Patagonian fjord spring bloom, and are generally dominated by Skeletonema spp.…”

Marine cage culture and the environment: effects on water quality and primary production

“…A mesocosm study examined short-term nutrient and phytoplankton dynamics in a Chilean fjord to assess the potential impacts of nutrient loading from marine aquaculture (Iriarte et al 2013). Phytoplankton response in terms of chl a, abundance and species composition to nitrogen loading was evident.…”

“…Similar to other algae species, the nutrient fluxes that influence HAB population dynamics are complex and vary for different species (Anderson et al 2002, 2008, Vargo 2009, Lewitus et al 2012, Iriarte et al 2013, Davidson et al 2014. Environmental factors other than nutrient loading also contribute to the formation of HABs.…”

Responses in bacterial community structure to waste nutrients from aquaculture: an in situ microcosm experiment in a Chilean fjord

“…The Comau Fjord (42.2° S, northern Patagonia, Chile) is naturally prone to N-limitation of the primary production in surface water (Iriarte et al 2007(Iriarte et al , 2013 and has cage aquaculture activity along the shores. In an in situ microcosm experiment, L. studied how the microbial food web in the Comau Fjord responded to increased inorganic nutrient loading with a nitrogen to phosphorus ratio of 28 (mol:mol), typical for aquaculture (Olsen & Olsen 2008, Wang et al 2012.…”