Microphytic standing stocks at Kerguelen Islands (Subantarctic, Indian Ocean), annual variations in relation to environmental factors: II, intertidal and subtidal microphytobenthos

Abstract

The microphytobenthic standing crop in marine coastal sediments from Kerguelen main island (from surface to 8 cm deep) varied in relation to the tidal position, grain size, and shelter of sediments. A time-series analysis was performed at three distinct sites: a sheltered fjord (PRAY), a moderately exposed embayment (PAF) and a deep fjord with higher open-ocean influence (PN). Temporal variations on all studied parameters were observed on intertidal, as well as on subtidal sediments, but with a great range in variations and pattern. Compared to phytoplankton blooms at the same location and stations, the microphytobenthos productive periods were not so marked, especially on intertidal sediments. Nevertheless, high productivity periods were in most cases linked to austral spring and summer (up to 30–50 μg Chl a g−1 dw, in intertidal sheltered sands; >170 μg Chl a g−1 dw, in sheltered subtidal muds). On subtidal muddy sites, the high phaeopigments concentrations (up to 195 μg Phaeo a g−1 dw at PRAY site) were attributed to kelp and epiphyte degradation and sedimentation, and also to macrofauna trophic influence, whereas at Portes-Noires fjord (PN) a Phaeophytine a late spring increase in 1991 was suspected to be of plankton origin. The PN site exhibited a lower productivity than at the other two sites, regardless of the tidal position of the sediment, due to a delayed annual cycle in surficial temperature and standing crop. Compared to the phytoplankton blooms, the microphytobenthos productivity at Kerguelen main island was less restricted in time and may occur throughout the year, not just to the benefit of the benthos food-chain, but also to the pelagic one via resuspension, as well as to filter feeders (i.e. mussels).