AME 10:149-163 (1996)  -  doi:10.3354/ame010149

Dynamics of size-fractionated phytoplankton and trophic pathways on the Scotian Shelf and at the shelf break, Northwest Atlantic

Mousseau L, Legendre L, Fortier L

Composition of ultraplankton (<5 um) and of biomass and primary production of size-fractionated phytoplankton (<5 or >5 um) were determined during 1 yr at 2 stations off Nova Scotia, eastern Canada. Water temperature was a major factor regulating the abundance of ultraplankton cells, whereas nutrient concentrations and irradiance mainly controlled the relative proportions of procaryotes to eucaryotes. Procaryotes (mainly phycoerythrin-rich cyanobacteria) clearly dominated the ultraplankton throughout the year. Eucaryotes were more severely affected by low nutrient concentrations and high irradiances than procaryotes. The study indicates that ultraplankton assemblages were highly dynamic, with succession among groups in response to seasonal variations in environmental conditions. The spring bloom was dominated by large phytoplankton and ended upon exhaustion of nitrate and silicate. During the remainder of the year, nutrient concentrations were low (although non-limiting) and ultraplankton were the main primary producers. In summer, sporadically enhanced production of large phytoplankton was likely caused by inputs of nutrients and seeding of the euphotic zone by local vertical mixing. This mechanism was effective at the shallow shelf station only. The size structure of the grazer assemblage controlled the fate of primary production, i.e. sinking to depth or export to higher trophic levels. Hence, primary production and standing stock were controlled by bottom-up (nutrient input and seeding of the euphotic zone) and top-down (grazing pressure by micro- or mesozooplankton) processes, respectively. Interactions between these processes determined the nature of the trophic web in the euphotic zone. Differences in hydrodynamics and grazer communities between the 2 sampling sites led to the following model: (1) at the 2 sites, year round, there was a microbial food web, in quasi-steady state; (2) on the shelf, year round, herbivorous and microbial components co-existed, leading to efficient transfer from large and small phytoplankton to large metazoans; and (3) at the shelf break, because of scarcity of mesozooplankton, the microbial food web dominated year round and most of the large spring-bloom phytoplankton could be lost to sedimentation.

Ultraplankton composition . Size-fractionation . Chl a biomass . Primary production . Trophic pathways

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