Abstract
Spring phytoplankton blooms are important events in Shelf Sea pelagic systems as the increase in carbon production results in increased food availability for higher trophic levels and the export of carbon to deeper waters and the sea-floor. It is usually accepted that the increase in phytoplankton abundance and production is followed by an increase in plankton respiration. However, this expectation is derived from field studies with a low temporal sampling resolution (5–15 days). In this study we have measured the time course of plankton abundance, gross primary production, plankton community respiration, respiration of the plankton size classes (>0.8 µm and 0.2–0.8 µm) and bacterial production at ≤5 day intervals during April 2015 in order to examine the phasing of plankton autotrophic and heterotrophic processes. Euphotic depth-integrated plankton community respiration increased five-fold (from 22 ± 4 mmol O2 m−2 d−1 on 4th April to 119 ± 4 mmol O2 m−2 d−1 on 15th April) at the same time as gross primary production also increased five-fold, (from 114 ± 5 to 613 ± 28 mmol C m−2 d−1). Bacterial production began to increase during the development of the bloom, but did not reach its maximum until 5 days after the peak in primary production and plankton respiration. The increase in plankton community respiration was driven by an increase in the respiration attributable to the >0.8 µm size fraction of the plankton community (which would include phytoplankton, microzooplankton and particle attached bacteria). Euphotic depth-integrated respiration of the 0.2–0.8 µm size fraction (predominantly free living bacteria) decreased and then remained relatively constant (16 ± 3 – 11 ± 1 mmol O2 m−2 d−1) between the first day of sampling (4th April) and the days following the peak in chlorophyll-a (20th and 25th April). Recent locally synthesized organic carbon was more than sufficient to fulfil the bacterial carbon requirement in the euphotic zone during this productive period. Changes in bacterial growth efficiencies (BGE, the ratio of bacterial production to bacterial carbon demand) were driven by changes in bacterial production rates increasing from <30 ± 14% on 4th April to 51 ± 11% on 25th of April. This study therefore shows a concurrent rather than a phased increase in primary production and community respiration attributable to cells >0.8 µm during the development of the spring bloom, followed 5 days later by a peak in bacterial production. In addition, the size fractionated respiration rates and high growth efficiencies suggest that free living bacteria are not the major producers of CO2 before, during and a few days after this shelf sea spring phytoplankton bloom.
Highlights
Reduced water column turbulence is one of the principal factors governing the rapid increase in plankton abundance in westernEuropean shelf seas, such as the Celtic Sea, during spring (Pingree et al, 1976; Fasham et al, 1983; Taylor et al, 1997; Smyth et al, 2014)
April 2015 in the central Celtic Sea was characterised by a reduction in vertical mixing and the formation of a well-stratified upper layer, typical for the spring period in shelf seas (Pingree et al, 1976; Fasham et al, 1983)
The change in the water column structure allowed the initiation of a spring bloom (Henson et al, 2006; Wihsgott et al, this issue), seen as a sharp increase in Chl-a concentration, which lasted less than 15 days (Fig. 3A)
Summary
Reduced water column turbulence is one of the principal factors governing the rapid increase in plankton abundance in westernEuropean shelf seas, such as the Celtic Sea, during spring (Pingree et al, 1976; Fasham et al, 1983; Taylor et al, 1997; Smyth et al, 2014). Two studies have measured both plankton production and plankton respiration during a spring bloom with the temporal resolution (≤1 week) required to discern the short term phasing between primary production and respiration (Blight et al, 1995; Caffrey et al, 1998). These studies in coastal waters of the UK (Blight et al, 1995) and the USA (Caffrey et al, 1998) showed a time lag of 5–15 days between the maximum rate of primary production and the maximum rate of community respiration. These studies in coastal waters of the UK (Blight et al, 1995) and the USA (Caffrey et al, 1998) showed a time lag of 5–15 days between the maximum rate of primary production and the maximum rate of community respiration. Blight et al (1995) explained the delay in the respiration response to be due to the time required for phytoplankton derived dissolved organic matter (DOM) to become available to the bacteria
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