TROBAT · TROPHIC RESOURCE DYNAMICS IN BATHYAL DEMERSAL COMMUNITIES

Final Activity Report Summary - TROBAT (Trophic resource dynamics in bathyal demersal communities)

The deep-sea suprabenthos (hyperbenthos or benthic boundary layer (BBL) mesozooplankton) is a complex and very rich compartment of the deep-sea fauna, composed mainly of peracarid crustaceans such as mysids and amphipods. Species of these taxa are part of the permanent suprabenthos, but other taxa, sich as euphausiids and natantian decapods, have a more temporary relation with the water-sediment interface through daily and seasonal ontogenetic migrations. The suprabenthos make a significant contribution to the diets of deep organisms, namely fish and decapods, mainly for juveniles and recruits. The primary aim of TROBAT was to improve our knowledge of the functioning of the bathyal communities, that are being increasingly subject to human impact, by studying the role played by suprabenthic fauna in the bathyal ecosystem functioning, through their importance as secondary producers. To preserve exploited deep-sea ecosystems it is necessary to understand their structure and functioning, particularly their trophic relationships and energy flow patterns. Secondary production (P) is an important factor for establishing the energy flow, particularly in deep-sea environments where food is considered the main limiting factor. The specific objectives of TROBAT were to study the population structure and production of the suprabenthos in order to quantify the food sources that were exploited by deep-sea demersal fish and crustacean species and to produce predictive models of P and P/B ratios (biomass turnover rate). The study was based on a bathyal BBL community in the western Mediterranean that was sampled throughout a seasonal cycle at depths ranging from circa 150 to 750m. The analysis of the population structure of suprabenthos showed that the BBL community had seasonal fluctuations in abundance and biomass, being higher in spring-summer. These fluctuations were linked to the increased organic matter in the sediments and ultimately to the surface primary production. For many suprabenthic species there was little information available on their feeding ecology and trophic niches; we therefore used the potential of stable isotopes and fatty acids to reveal trophic relationships. Isotopic analysis of d13C and d15N for 24 species suggested that there were up to three trophic levels within the suprabenthic fauna studied, with only two trophic levels being apparent in the deepest assemblage. Changes in the seasonal fatty composition of two dominant components of the BBL (the mysid boreomysis arctica and the euphausiid nematoscelis megalops) showed that both species were strongly linked to changes in primary productivity within the euphotic zone. To estimate production (P) and production-biomass ratios (P/B) by Hynes-Hamilton method of suprabenthos we established the population size, size-frequency distribution and length of developmental time for the 48 species. The estimates of secondary production showed a wide range of production (P) between 0.00001 to 0.01643 mg WW/m2/year, and P/B ratios from 2.93 to 11.60. The high variability which was showed by P and P/B estimations for suprabenthos fauna is often ignored in studies on energy flux in trophic webs and general balances of carbon cycles. Modelling of the biomass and production of suprabenthos was important to assess their significance as trophic sources of exploited fish species. We integrated all data obtained, including isotopic data, to produce predictive models of secondary production and P/B for bathyal ecosystems. Preliminary analyses on the predictive multilinear regression (MLR) models showed how P/B was higher for species with more than one generation per year, i.e. bivoltine and trivoltine species, among dominant species in density and among species with a higher trophic level. MLR models for P showed that production depended on biomass among smaller species and bi/trivoltine species. Estimates of energy flow based on P and P/B for suprabenthos had not been attempted previously in ecological studies at bathyal depths.

Data: CORDIS, © European Union