Picophytoplankton : ecological and physiological studies in culture and in natural coastal and estuarine waters

Iriarte, Arantza (1991) Picophytoplankton : ecological and physiological studies in culture and in natural coastal and estuarine waters. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The distribution and abundance of picophytoplankton and its impact in overall pelagic community chlorophyll a biomass and productivity in two temperate regions, the coastal waters of the southern and central North Sea and the estuarine waters of Southampton Water have been investigated. The study in the North Sea was conducted during research cruises in March and June/July 1989. The study in Southampton Water involved a ten month seasonal survey during 1990 and included investigation of both temporal and spatial trends of variation. In North Sea water phycoerythrin (PE)-containing picocyanobacteria cell numbes ranged between 6x10⁵ and 4.4x10⁷ cells ml^-1. Vertical distribution profiles showed no evidence of a preferential accumulation deep in the euphotic zone, thus calling into question the possible genotypic definition of previous findings of picophytoplankton as shade adapted phytoplankton. In summer, the smaller than 3μm size fraction was estimated to contribute to a mean of 20% of the plankton community chlorophyll a biomass and to more than 80% of the plankton community respiration rate. In Southampton Water two stations were routinely sampled (i.e. mid and outer estuary). The cycle of total chlorophyll a biomass in mid-estuary was characterized by both a spring and a more intense summer peak. The timing of the spring phytoplankton bloom is suggested to be primarily controlled by the estuarine flushing rate. In the outer estuary, chlorophyll a maximum occurred in spring and large phytoplankton are hypothesized to be phosphate limited in summer at this station. PE-containing picocyanobacteria and eukaryotic picophytoplankton cell numbers, and chlorophyll a biomass and carbon fixation rate by the < 1μm size-fraction, all showed a positive correlation with temperature, and peaked during the summer at both stations. Possible factors leading to an imbalance between growth rates and grazing rates of picophytoplankton during summer are discussed. Along the longitudinal axis of the estuary, highest numbers of PE-containing picocyanobacteria were recorded at the seward end, and an exponential relationship with salinity was shown. The 1-3(5)μm and 1< μm size fractions contributed to around 14 and 6% of the estimated annual rate of depth integrated plankton community primary production respectively. These results suggest the impact of the picophytoplankton in the overall phytoplankton community diminishes from offshore (> 50%) to coastal and estuarine waters (< 20%). It is suggested that factors limiting growth and accumulation of larger phytoplankton are primarily responsible for an increase in the relative significance of small phytoplankton. The growth physiology of the oceanic chlorophycean picophytoplankter Pycnococcus provasolii (clone Ω 48-23) was also investigated in response to varying conditions of temperature, irradiance and photoperiod. Results suggested this organism to be well suited to growth at low irradiance, and it may well prove a significant member of the phytoplankton assemblages deep in the euphotic zone of oceanic stratified waters.