Size and settling velocity of suspended flocs during a Phaeocystis bloom in the tidally stirred Irish Sea, NW European shelf

Abstract

The settling and resuspension of carbon-rich suspended particulate matter (SPM) are important determinants of pelagic and benthic productivity, and the fate of primary production, in tidally stirred shelf seas. The physical and biological properties of SPM were measured over 5 wk of a declining spring plankton bloom, dominated by the flagellate Phaeocystis, in stratified and mixed waters of the Irish Sea, NW European shelf. SPM consisted of slow-settling, small flocs with a modal settling velocity of 2 × 10-3 to 4 × 10-3 mm s-1 in long-term suspension and fast-settling, large flocs with a modal settling velocity of 2 × 10 -2 to 2 mm s-1, deposited as benthic fluff at slack waters and resuspended by tidal currents; flocs reached 500 μm in diameter and settled at up to 20 mm s-1. Larger flocs were composed of living Phaeocystis and other scavenged material. Floc size and settling velocity of SPM scaled on the weight percentage in SPM of arabinose, a photosynthate produced by Phaeocystis. Over a 5-fold range of arabinose content, there was a 3 order of magnitude range of median settling velocity. Larger flocs in mixed waters, with high Phaeocystis content, were stronger than smaller flocs in stratified waters with low Phaeocystis content. Resuspended flocs in mixed waters were stronger than flocs in long-term suspension. SPM concentration in mixed waters halved during the period of observation, with most of this reduction occurring in Week 1. We conclude there was net sedimentation of Phaeocystis-enriched SPM as benthic fluff despite frequent resuspension. © Inter-Research 2007.