Physical constraints on phytoplankton in estuaries and shallow coastal waters

Abstract

Highly dynamic aquatic systems have often been reported to support actively growing populations of phytoplankton. The partially-mixed, macrotidal, temperate estuary Southampton Water is no exception, with reports of frequent temporal patterns of spring and summer maxima in algal biomass. During this study, the close coupling between the estuarine hydrology and the phytoplankton community was confirmed by the results from an intensive sampling strategy, spanning various temporal scales. Using high frequency data, collected from acoustic Doppler current profilers (ADCP) and CTDs, the physical mixing processes within the estuary were determined and combined with high resolution phytoplankton species data to assess the physical pressures on the microalgal community. Initial surveys were conducted to provide snapshots of the estuary during productive periods, followed by more intensive, longer term monitoring to observe population growth and succession. Through seasonal investigations, the aggregation of algal biomass (quantified by chlorophyll a concentration) at differing vertical heights in the water column was realised. On closer examination using microscopic identification, the different vertical profiles were shown to be due to phytoplankton succession from the spring to the summer months. Diatoms (Rhizosolenia delicatula) proliferated in the spring, where the population was localised in the near-bottom layers, whilst the summer bloom was dominated by autotrophic dinoflagellates (Prorocentrum micans and Peridifiium trochoideum), manifesting in a sub-surface chlorophyll a maximum. The vertical position of diatom species, bothpelagic and benthic, suggested no dependence on incident irradiance, but seemed solely governed by current velocities, shear and wind mixing events. Other passive constituents of the water column, such as suspended particulate matter, were also closely coupled with boundary shear and followed regular patterns of re-suspension similar to those shown by the diatom community. In the summer, apparent active vertical migration was observed for several species of dinoflagellate. Whilst this apparent migration was closely linked to the incident irradiance, the extent and timing of migration was highly dependent on the tidal state and the water column stability. During one 25 hour Eulerian investigation, apparent positive vertical migration was observed in several dinoflagellate species, where the controlling factor was incident irradiance. Dinoflagellates were observed to descend during the dark periods when the water column was stable. However, the vertical distribution of the autotrophic ciliate Mesodinium rubrum suggested that migrations into the surface waters were linked with periods of water column stability and not triggered by surface irradiance.The unique tidal regime that governs the physical mixing processes in Southampton Water translates into periods of stability separated in time by intermittent turbulence. This periodic stability within the water column during reduced tidal forcings permitted the surface aggregation of dinoflagellates, which became homogeneously distributed when turbulence intensified during the ebb and flood currents. Diatoms, conversely, relied on vertical mixing to enter the surface layers of the water column, and aggregated in the lower layers during times of water column stability. Data from the seasonal surveys suggested that diatoms and dinoflagellates were able to co-exist during the summer by utilising contrasting properties of tidal mixing to develop and reside within this partially mixed environment. The segregation of these two phytoplankton groups was not apparent from the chlorophyll concentrations alone, and was only made evident through the high resolution phytoplankton sampling through both time and space. The close coupling between the phytoplankton community and physical forcings were also investigated in the usually well-mixed southern North Sea. During a 12 hour Lagrangian survey, the stabilising effect of the Rhine region of freshwater influence (ROFI) was recognised and provided the temporary stability necessary for apparent dinoflagellate (Prorocentrum micans and Gonyaulax sp) migration. Associated solely with this lower salinity plume was the diatom Rhizosolenia stvliformis, which was not detected during other times of the survey. Very small changes in total algal biomass were detected through the use of chlorophyll a determinations (chlorophyll a < 2 mg m^-3). The intermittency of the mixing forces proved to be an important physical characteristic which defines the species and distribution of the phytoplankton community.

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