ALBERT

Description: We analysed carbon (δ13C) and nitrogen (δ15N) isotope ratios of organisms and biogenic tissues from Comau Fjord (southern Chile) to characterise benthic food webs and spatial isotope variability in this ecosystem. These values were intended to serve as a baseline for detecting anthropogenic impacts on Patagonian marine fjord ecosystems in later studies. Benthic macro algae and invertebrate suspension feeders were primarily considered, with some supplementary data from cyanobacteria, plankton, fish, and coastal vertebrates. Six depth transects typified the lateral salinity gradients from the innermost part of the fjord to its mouth, as well as the vertical density gradients caused by freshwater inflow. Carbon isotope signatures indicated predominant consumption of either CO2 or HCO3– for benthic macroalgal. All CO2 users belonged to rhodophytes. The δ15N values of benthic macrophytes decreased with decreasing salinity, both vertically and along the fjord axis. This implies the influence of 15N-poor terrestrial dissolved inorganic nitrogen (DIN) at these sites. Enhanced influence of freshwater influx also lowered N contents and increased C/N ratios in algal tissues. Exceptionally high macroalgae δ15N values at the seabird and sealion colony Isla Liliguapi point to animal faeces as an additional source of 15N-enriched DIN. Thus, DIN sources not originating from the open sea are additionally utilised by the benthic macroalgae in the fjord. In contrast, mussel tissue from the same locations was much less influenced by varying DIN sources. Among benthic suspension feeders, mytilids (Mytilus chilensis, Aulacomya ater) had the lowest and scleractinian corals (Desmophyllum dianthus) had the highest δ15N values, and Balanidae (Elminius kingii) and gorgonians (Primnoella sp.) showed values in between. The preference for specific size classes of marine particulate organic matter (seston) as food serves as an explanation for the δ15N variability observed between the different benthic suspension feeders.

Description: Magellania venosa, the largest recent brachiopod, occurs in clusters and banks in high abundances of up to 416 ind m-2 in the fjord Comau, northern Chilean fjord region. At some stations, it numerically dominates the subtidal benthic community below 15 m depth, competing for space with the mytillid Aulacomya atra. To determine the question why Magellania venosa is a successful competitor, the brachiopod’s in situ growth rate was studied and its overall growth performance was compared with that of other brachiopods and mussels. The length growth was measured between Februay 2011 and March 2012 after mechanical tagging and Calcein staining. Settlement and juvenile growth were determined from recruitment tiles installed in 2009 and from subsequent photocensus. Growth of Magellania venosa is best described by the general von Bertalanffy growth function, with a maximum shell length (L∞) of 71.53 mm and a Brody growth constant (K) of 0.336 yr-1. The growth rate is the highest recorded for a rynchonelliform brachiopod. Maximal individual somatic production (PInd) is 0.29 g AFDM ind-1 yr-1 at 42 mm shell length and annual production ranges from 1.28 – 89.25 g AFDM yr-1 m-2. The high shell growth rate of Magellania venosa, together with its high overall growth performance may explain the high abundances of this brachiopod in the fjord Comau. However, the somatic production per biomass of the population ( -ratio) is low (0.535) and Magellania venosa may play a minor role in the food chain. Settling behaviour indicates that Magellania venosa is a pioneer species with low juvenile mortality. The brachiopod-bivalve coexistence suggests that neither the presence of potential brachiopod predators nor space competitors (i.e. mytiilids) affect the survival of the brachiopod population.