Drivers of the winter–spring phytoplankton bloom in a pristine NW Mediterranean site, the Bay of Calvi (Corsica): A long-term study (1979–2011)

Abstract

a b s t r a c tThis work is based on a long time series of data collected in the well-preserved Bay of Calvi (Corsica island, Ligurian Sea, NW Mediterranean) between 1979 and 2011, which include physical characteristics (31 years), chlorophyll a (chl a, 15 years), and inorganic nutrients (13 years). Because samples were collected at relatively high frequencies, which ranged from daily to biweekly during the winter-spring period, it was possible to (1) evidence the key role of two interacting physical variables, i.e. water temperature and wind intensity, on nutrient replenishment and phytoplankton dynamics during the winter-spring period, (2) determine critical values of physical factors that explained interannual variability in the replenishment of surface nutrients and the winter-spring phytoplankton bloom, and (3) identify previously unrecognised characteristics of the planktonic ecosystem. Over the >30 year observation period, the main driver of nutrient replenishment and phytoplankton (chl a) development was the number of wind events (mean daily wind speed >5 m s À1) during the cold-water period (subsurface water 613.5°C). According to winter intensity, there were strong differences in both the duration and intensity of nutrient fertilisation and phytoplankton blooms (chl a). The trophic character of the Bay of Calvi changed according to years, and ranged from very oligotrophic (i.e. subtropical regime, characterised by low seasonal variability) to mesotrophic (i.e. temperate regime, with a well-marked increase in nutrient concentrations and chl a during the winter-spring period) during mild and moderate winters, respectively. A third regime occurred during severe winters characterised by specific wind conditions (i.e. high frequency of northeasterly winds), when Mediterranean ''high nutrient -low chlorophyll'' conditions occurred as a result of enhanced crossshore exchanges and associated offshore export of the nutrient-rich water. There was no long-term trend (e.g. climatic) in either nutrient replenishment or the winter-spring phytoplankton bloom between 1979 and 2011, but both nutrients and chl a reflected interannual and decadal changes in winter intensity.

Inter‐annual variations over a decade of primary production of the seagrass Posidonia oceanica

“…Alternatively, a common process could enhance both benthic and planktonic GPP leading to an indirect positive relation between GPP’ and Chl a’. In the Bay of Revellata, inter‐annual variations of phytoplankton growth are driven by inter‐annual changes in dissolved inorganic nutrients, mainly due to weather conditions driving vertical mixing (Goffart et al ). Enhanced dissolved inorganic nutrients inputs should lead to an increase of both planktonic and benthic GPP, and we hypothesize it could explain the GPP’ and Chl a ’ relation.…”

“…We also hypothesize that an indirect relation to nutrient inputs might also explain the negative relation between GPP’ and SST’ and GPP’ and SR’ in April (second period, Eq. 4), as warm and sunny conditions during this month might indicate early stratification and less stormy weather leading to less nutrient inputs by mixing (Goffart et al ). This might also explain why the month of April stood out in the analysis as a specific period, as this corresponds to transition period from a well‐mixed winter regime to a more stratified spring regime (Bay ).…”

Decadal shifts of coastal microphytoplankton communities in a semi-enclosed bay of NW Mediterranean Sea subjected to multiple stresses

“…3c). Changes in water temperature has been also suspected as the main hydrologic feature related to climate change and large scale processes in NW Mediterranean Sea (Goffart et al, 2015(Goffart et al, , 2002Trigo and Davies, 2000;Vargas-Yáñez et al, 2010. We assumed that the absence of significant variability of water temperature in Toulon Bay during wintering deep mixing events (i.e.…”

Why the damselfishChromis chromisis a key species in theMediterranean rocky littoral – a quantitative perspective

“…The bay has a total area of c . 2200 ha and many aspects of ecology and biology (including seagrass Posidonia oceanica communities, benthic invertebrates, fish assemblages) have been studied over the past 50 or so years, largely by researchers from the University of Liège, Belgium (Spyker & van den Berghe, ; Goffart et al., ). Between 1997 and 2000 the present author conducted research on planktivorous fishes at this research station, but there have subsequently been many developments elsewhere in the Mediterranean Sea that are reviewed here.…”

“…Between 1997 and 2000 the present author conducted research on planktivorous fishes at this research station, but there have subsequently been many developments elsewhere in the Mediterranean Sea that are reviewed here. Planktonic communities of the Bay of Calvi have been widely studied (Dauby, ; Dauby & Hecq, ; Brohee et al., ) and these appear to be greatly influenced by the upwelling of nutrients in the spring (Goffart et al., ). The contention promoted here is that C. chromis is a key fish in the Mediterranean rocky littoral, by virtue of its great abundance and the important role that planktivorous species play in channelling nutrients from the pelagic‐zooplankton system to the rocky littoral as allochthonous detritus but also when they are themselves eaten by littoral predators.…”