Abstract
From June 2004 to December 2007, samples were weekly collected at a fixed station located at the mouth of Ria de Aveiro (West Iberian Margin). We examined the seasonal and inter-annual fluctuations in composition and community structure of the phytoplankton in relation to the main environmental drivers and assessed the influence of the oceanographic regime, namely changes in frequency and intensity of upwelling events, over the dynamics of the phytoplankton assemblage. The samples were consistently handled and a final subset of 136 OTUs (taxa with relative abundance > 0.01%) was subsequently submitted to various multivariate analyses. The phytoplankton assemblage showed significant changes at all temporal scales but with an overriding importance of seasonality over longer- (inter-annual) or shorter-term fluctuations (upwelling-related). Sea-surface temperature, salinity and maximum upwelling index were retrieved as the main driver of seasonal change. Seasonal signal was most evident in the fluctuations of chlorophyll a concentration and in the high turnover from the winter to spring phytoplankton assemblage. The seasonal cycle of production and succession was disturbed by upwelling events known to disrupt thermal stratification and induce changes in the phytoplankton assemblage. Our results indicate that both the frequency and intensity of physical forcing were important drivers of such variability, but the outcome in terms of species composition was highly dependent on the available local pool of species and the timing of those events in relation to the seasonal cycle. We conclude that duration, frequency and intensity of upwelling events, which vary seasonally and inter-annually, are paramount for maintaining long-term phytoplankton diversity likely by allowing unstable coexistence and incorporating species turnover at different scales. Our results contribute to the understanding of the complex mechanisms of coastal phytoplankton dynamics in relation to changing physical forcing which is fundamental to improve predictability of future prospects under climate change.
Highlights
Coastal upwelling occurs at localized regions of eastern ocean margins under the forcing of along-shore equatorward winds [1]
The overall chlorophyll a concentration showed inter-annual variation: it was somewhat lower in 2005, a year during which the difference between spring, summer and autumn was less marked; in contrast, higher concentrations of chlorophyll a were recorded in spring-summer 2007; the frequency of shortterm variations in chlorophyll concentration was highest during spring-summer 2006, when the contribution of the phytoplankton fraction smaller than 20 μm was higher (Fig 1; note the different vertical scale between the two chlorophyll graphics)
The seasonal cycle is driven by sea-surface temperature and the onset of the thermocline leading to phytoplankton blooms during spring, the prevalence of thermal stratification leading to exhaustion of nutrients and subsequent demise of phytoplankton during summer-autumn, and remixing and regeneration of nutrients during winter
Summary
Coastal upwelling occurs at localized regions of eastern ocean margins under the forcing of along-shore equatorward winds [1]. It involves the offshore displacement of usually nutrientdepleted surface waters and subsequent rise of cold and nutrient-rich deep waters into the coastal euphotic layer. These high nutrient pulses from deep waters are rapidly transformed in high amounts of biomass [2] and trigger phytoplankton succession [3,4]. There is recent evidence [11], that the global decrease in frequency and intensity of coastal upwelling events may have important socio-economic consequences both by the decline of fisheries and by increasing the frequency of harmful algal blooms (HAB)
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