Abstract
Alterations in phytoplankton biomass, community structure and timing of their growth (phenology), are directly implicated in the carbon cycle and energy transfer to higher trophic levels of the marine food web. Due to the lack of long-term in situ datasets, there is very little information on phytoplankton seasonal succession in Cyprus (eastern Mediterranean Sea). On the other hand, satellite-derived measurements of ocean colour can only provide long-term time series of chlorophyll (an index of phytoplankton biomass) up to the first optical depth (surface waters). The coupling of both means of observations is essential for understanding phytoplankton dynamics and their response to environmental change. Here, we use 23 years of remotely sensed, regionally tuned ocean-colour observations, along with a unique time series of in situ phytoplankton pigment composition data, collected in coastal waters of Cyprus during 2016. The satellite observations show an initiation of phytoplankton growth period in November, a peak in February and termination in April, with an overall mean duration of ~4 months. An in-depth exploration of in situ total Chl-a concentration and phytoplankton pigments revealed that pico- and nano-plankton cells dominated the phytoplankton community. The growth peak in February was dominated by nanophytoplankton and potentially larger diatoms (pigments of 19’ hexanoyloxyfucoxanthin and fucoxanthin, respectively), in the 0–20 m layer. The highest total Chl-a concentration was recorded at a station off Akrotiri peninsula in the south, where strong coastal upwelling has been reported. Another station in the southern part, located next to a fish farm, showed a higher contribution of picophytoplankton during the most oligotrophic period (summer). Our results highlight the importance of using available in situ data coupled to ocean-colour remote sensing, for monitoring marine ecosystems in areas with limited in situ data availability.
Highlights
Long-term time series of phytoplankton phenology based both on ocean-colour remote sensing and in situ datasets can improve our understanding of phytoplankton seasonal succession
The initiation of the phytoplankton growth period seen from the satellite in November coincides with increased concentrations of the integrated total Chl-a calculated from High-Performance Liquid Chromatography (HPLC) and with an increase in Chl-a concentrations in the surface layer
The subsurface chlorophyll maximum (SCM) in the oligotrophic Levantine is a permanent feature [55], and the increase in surface Chl-a concentration captured by the satellite in November could be attributed to the redistribution of Chl-a following the erosion of the SCM after the winter mixing, as well as to the resulting enhanced nutrient availability within the mixed layer, which triggers phytoplankton growth
Summary
The eastern Mediterranean Sea is characterised as an ultraoligotrophic region, comparable to the most oligotrophic parts of the global ocean, and is even considered as a marine desert [1,2]. This ultraoligotrophic nature is reflected in the very low primary production, chlorophyll-a (Chl-a, a proxy of phytoplankton biomass [3]) and nutrient concentrations, predominance of small-sized phytoplankton and its extremely clear waters [4,5,6,7,8,9,10,11].
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