Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> The spring phytoplankton bloom is a key event in temperate and polar seas, yet the mechanisms that trigger it remain under debate. Some hypotheses claim that the spring bloom onset occurs when light is no longer limiting, allowing phytoplankton division rates to surpass a critical threshold. In contrast, the Disturbance Recovery Hypothesis (DRH) proposes that the onset responds to an imbalance between phytoplankton growth and loss processes, allowing phytoplankton biomass to start accumulating, and this can occur even when light is still limiting. Although several studies have shown that the DRH can explain the spring bloom onset in oceanic waters, it is less certain whether and how it also applies to coastal areas. To address this question at a coastal location in the Scottish North Sea, we combined 21 years (1997â2017) of weekly in situ chlorophyll and environmental data with meteorological information. Additionally, we also analyzed phytoplankton cell counts estimated using microscopy (2000â2017) and flow cytometry (2015â2017). The onset of phytoplankton biomass accumulation occurred around the same date each year, 16â<span class="inline-formula">±</span>â11âd (meanâ<span class="inline-formula">±</span>âSD) after the winter solstice, when light limitation for growth was strongest. Also, negative and positive biomass accumulation rates (<span class="inline-formula"><i>r</i></span>) occurred respectively before and after the winter solstice at similar light levels. The seasonal change from negative to positive <span class="inline-formula"><i>r</i></span> was mainly driven by the rate of change in light availability rather than light itself. Our results support the validity of the DRH for the studied coastal region and suggest its applicability to other coastal areas.
Highlights
IntroductionThe spring bloom is a major seasonal feature of temperate and polar seas and plays significant ecological and biogeochemical roles (Townsend et al, 1994)
Some hypotheses claim that the spring bloom onset occurs when light is no longer limiting, allowing 10 phytoplankton division rates to surpass a critical threshold
Many studies have shown that the Disturbance Recovery Hypothesis (DRH) explains the spring bloom onset in oceanic waters, it is less certain whether and how it applies to coastal areas
Summary
The spring bloom is a major seasonal feature of temperate and polar seas and plays significant ecological and biogeochemical roles (Townsend et al, 1994). These areas cover a small percentage of the ocean surface, they are among the most productive in the world (Mann, 2009) and 45 provide important ecosystem services (Barbier, 2017). According to the DRH, an early μ acceleration driven for example by a seasonal improvement in light conditions (i.e., by an accelerating increase in light availability) could 50 still trigger a phytoplankton biomass accumulation in winter This is plausible considering that coastal waters usually have high nutrient and turbidity levels during winter and spring (Mann, 2009), making light the main limiting factor for phytoplankton growth, especially at high latitudes with low surface light intensities and stormy weather. We addressed the questions: 1) Does the 55 spring bloom start in winter in the absence of a deepening in the mixed layer? 2) Is light availability a main driver of the process? And 3) Does the DRH hold true?
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