Community composition, migration and trophic positions of micronekton in two biogeochemical provinces of the South West Indian Ocean

Inlcudes bibliographical referencesMicronekton fauna was investigated as part of a multi-disciplinary research project carried out in two different bioregions of the South West Indian Ocean: the East African Coastal Province (EAFR) and the Indian South Subtropical Gyre (ISSG). Food web structure was addressed using stable isotopes. Since particulate organic matter had high δ 15 N values in the ISSG province, copepods were chosen as baseline in trophic level estimations. Feeding regime and size were shown to influence the trophic position of micronekon. In the ISSG, carnivores (fishes and squids) and omnivores (crustaceans) had higher δ 15 N values and trophic positions than filter feeder s (gelatinous organisms such as salps and pyrosomes) and detritivores (leptocephali larvae). Fishes and squids encompassed a wide range of overlapping isotopic niches suggesting that organisms across different trophic levels feed on the same resources. Estimated trophic levels ranged from 1.67 to 4.73, showing that micronekton in the ISSG can be tertiary consumers. An average enrichment value of 6.7 ‰ was recorded between the sampled micronekton specimens and swordfish Xiphias gladius in the ISSG. Trawls, being selective in nature, were shown to sa mple smaller - sized micronekton with a lower trophic position than the micronekton being eaten by swordfish. In the EAFR, mean δ 15 N values of micronekton were higher than in the ISSG, exhibiting slightly higher trophic levels. Mesoscale dynamics in the EAFR provide mechanisms that enrich surface layers in nutrients and chlorophyll - a, therefore contributing to a higher abundance and micronekton species richness. In the ISSG, the large - scale wind - driven anticyclonic gyre pushes the nitracline, thermocline and deep chlorophyll maximum deeper in the water column , influencing the diel migration patterns of micronekton , with a significant proportion of micronekton staying in deep layers or slightly above the thermocline at dusk . Regardless of the differences in the ISSG and EAFR in δ 15 N values and trophic positions of micronekton, larger - sized swordfish sampled from these two provinces had similar mean δ 15 N values since swordfish are highly migratory and forage in different p arts of the Indian Ocean. However, smaller - sized swordfish specimens had lower mean δ 15 N values. With a combination of trawl surveys, stable isotope estimates, stomach content and acoustic analyses, this study shed new light on trophic interactions in the oligotrophic ISSG province

Patterns among micronekton communities in relation to environmental conditions at two shallow seamounts in the south-western Indian Ocean

Seamounts are ubiquitous topographic features across all ocean basins. They rise steeply through the water column from abyssal depths. Depending on their size, shape and summit depths, seamounts reportedly have an influence on the physical flow regimes which may promote the aggregation of zooplankton, micronekton, and top predators above or in the immediate vicinity of their summits. Micronekton form a key trophic link between zooplankton and top marine predators, and are divided into the broad categories: crustaceans, cephalopods and mesopelagic fishes. The vertical and horizontal distributions, assemblages and trophic relationships of micronekton were investigated at two shallow seamounts of the south-western Indian Ocean. La Pérouse is a steep bathymetric feature rising from a deep seabed located at 5000 m and with a summit depth at ~60 m below the sea level. This seamount is located at the north-western periphery of the oligotrophic Indian South Subtropical Gyre province. A seamount to the south of Madagascar, named “MAD-Ridge” in this study, has a summit depth at ~240 m below the sea level and rises from a base at ~1600 m. MAD-Ridge is located within an “eddy corridor” within the productive East African Coastal Province. The micronekton acoustic densities were greater at MAD-Ridge relative to La Pérouse, in accordance with the difference in productivity between the two sites. Physical processes within the cyclonic mesoscale eddy sampled during the MAD-Ridge cruise led to enhanced micronekton acoustic densities in the eddy relative to the MAD-Ridge seamount. While the shallow scattering layer (0-200 m) consisted of common oceanic micronekton species, the summits and flanks of La Pérouse and MAD-Ridge both showed presence of resident or seamount-associated fish species during day and night. Micronekton were also shown to exhibit a range of migration strategies such as diel vertical migration, mid-water migration and no diel migration. However, despite the differing productivity between both pinnacles, crustaceans, smaller-sized squids and mesopelagic fishes exhibited trophic levels ranging from 3 to 4 at both seamounts. This thesis highlights important knowledge gaps on seamount ecosystems and ecological patterns associated to shallow pinnacles. It also underlines the importance of studying seamount ecosystems of the south-western Indian Ocean in order to promote management and conservation measures for a sustainable use of such specific environments

The movement of plastics through marine ecosystems and the influences on bioavailability and uptake into marine biota.

Microplastics are a diverse array of contaminants comprising a suite of sizes, shapes, and polymer types. Here I present a body of work investigating the distribution and movement of microplastics through the marine ecosystems via transportation and transformation pathways. First, I look at litter items of beaches of the Cornish coast, demonstrating that 41% of litter was plastics fragments unattributable to source and that this litter was continually re-stocked such that it was always present despite cleaning efforts. Then I took to the seas to conduct sea surface trawls in the North East Atlantic to investigate the floating proportion of marine plastic debris. Microplastics were found in every sample, yet were highly variable in concentration over geographic space ranging from 0.038 to 0.45 particles m-3. Counter to the prevailing trends, plastic fragments (84 μm – 21.8 mm) were the dominant shape (63%), with fewer fibres present. The likelihood of encounter and therefore risk of plastic to plankton was calculated and it was found that for every 1 plastic particle, there were between 500 and 1000 plankton, suggesting very low risk of biological uptake for this region. Plastics are not just found on the sea surface and are increasingly found in benthic sediments and biota. I tested whether marine snows would act as a transport mechanism of plastics from the surface to the seafloor. I demonstrate that under experimental conditions a range of plastic particle sizes, shapes, and polymer types, all readily incorporated into marine snows. This incorporation into marine snows both overcame the buoyancy of floating particles but also increased the sinking rate of dense particles. Buoyant polyethylene went from floating as a free particle to sinking at 818 m day–