Coastal Phytoplankton Do Not Rest in Winter

Abstract

The climatology and interannual variability of winter phytoplankton was analyzed at the Long Term Ecological Research Station MareChiara (LTER-MC, Gulf of Naples, Mediterranean Sea) using data collected from 1985 to 2006. Background winter chlorophyll values (0.2-0.5 μg chl a dm −3 ) were associated with the dominance of flagellates, dinoflagellates, and coccolithophores. Winter biomass increases (<5.47 μg chl a dm −3 ) were often recorded until 2000, generally in association with lowsalinity surface waters (37.3-37.9). These blooms were most often caused by colonial diatoms such as Chaetoceros spp., Thalassiosira spp., and Leptocylindrus danicus. In recent years, we observed more modest and sporadic winter biomass increases, mainly caused by small flagellates and small non-colonial diatoms. The resulting negative chl a trend over the time series was associated with positive surface salinity and negative nutrient trends. Physical and meteorological conditions apparently exert a strict control on winter blooms, hence significant changes in winter productivity can be foreseen under different climatic scenarios.

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

“…Nevertheless, the observation that, in the Bay of Calvi, mesozooplankton biomass collected weekly between January and April was 2-fold lower in 2006 and 2008 than in 2010 (Collignon, 2014) suggests that phytoplankton was not strongly controlled by grazers during the cold-water periods. This is in agreement with several studies reporting that grazing is low in the Western Mediterranean in winter in both coastal areas (Duarte et al, 1999;Agustí and Duarte, 2000;Zingone et al, 2010) and the open sea (Auger et al, 2014). Moreover, Behrenfeld (2010) showed that vertical mixing, which is essential for phytoplankton bloom formation, dilutes both predators and preys, whereas stratified conditions concentrate them.…”

“…However, our hypothesis is grounded in both the robustness of the very detailed 1999 data series, and the fact that, in the available 15 Tchl a time series at station PHYTOCLY, the phytoplankton bloom always occurred within the cold-water period. A few field studies have also reported the regular occurrence of winter blooms in vertically mixed coastal Mediterranean waters (Ribera d 'Alcalà et al, 2004;Goffart and Hecq, 2005;Zingone et al, 2010). According to the latter study, those blooms are characterised by moderate chl a concentrations vertically distributed down to the bottom, which result in winter depth-integrated biomass values that are comparable to those found in spring at the same station.…”

“…Few studies only have focused on changes in winter temperature in the shallow and coastal waters of the western basin. Zingone et al (2010) and Vandromme et al (2011) reported the occurrence of cold and warm winters, but no increasing or decreasing trend with time was detected in mean surface temperature from December to March over the 1985-2006 period (Nykjaer, 2009). Most of the studies in coastal areas have been devoted to changes in marine biota attributed to increasing summer temperatures.…”

“…Irradiance is obviously key to photosynthesis, but it is generally high enough in Western Mediterranean waters to promote net phytoplankton growth even in homogeneous and vertically mixed water columns (Morán and Estrada, 2005;Zingone et al, 2010). In winters with high WII, the number of hours with bright sunshine between January and March was highest in winter 2008 (479 h, Zooplankton grazing is another factor that can exert a strong top-down control on phytoplankton biomass, and reduce drastically the standing stock.…”

“…The few available nutrient and phytoplankton time series concern highly urbanised areas, where long-term variability reflects the combined effects of climate and anthropogenic forcing, e.g. land-derived nutrients delivered to coastal waters by river discharges (Bernardi Aubry et al, 2004;Mozetič et al, 2010;Arin et al, 2013), fertiliser runoff and industrial effluents (Ninčević Gladan et al, 2010), urban runoff from very densely populated regions (Ribera d'Alcalà et al, 2004;Zingone et al, 2010), and aquaculture (Sara et al, 2011). The studies based on these time series could not distinguish the responses of phytoplankton to physical forcing from those induced by multiple human disturbances.…”