Ultraphytoplankton succession is triggered by deep winter mixing in the Gulf of Aqaba (Eilat), Red Sea

Abstract

The seasonal dynamics of ultraphytoplankton for the northern Gulf of Aqaba (29"28'N, 34"55'E) were investigated in detail. Monthly analysis of pigments by HPLC and cell abundances by epifluorescence microscopy showed large fluctuations in ultraphytoplankton community structure concurrent with strong seasonal changes in water-column conditions. Following extensive winter mixing, ultraphytoplankton seasonal succession progressed rapidly as water-column stratification strengthened. Eucaryotic algae dominated in nutrient-replete winter mixing conditions, Synechococcus was the major component of the ultraphytoplankton during a spring bloom of massive proportions, and Prochlorococcus was dominant in nutrient-depleted summer-stratified waters. Over the fall-winter period, as water-column mixing progressed, succession was in the reverse order but at a much slower rate. Prochlorococcus was the major component of the community during summer stratification, yet was not detected at the height of the mixing event in late winter. The reestablishment of the population occurred only 3 months after the onset of stratification. This is the first report on seasonal succession involving all three ultraphytoplankton groups. We suggest that water-column stability is an important factor influencing seasonal variations in ultraphytoplankton community structure.For some time, water-column conditions have been known to affect phytoplankton community structure. In oceanic regions of low latitudes, conditions are fairly constant throughout the year, with stably stratified waters that are usually limiting in nutrients (with the exception of equatorial upwelling regions). As such, phytoplankton community structure in these environments is fairly stable (Campbell and Vaulot 1993; Cushing 1989). Subtropical and temperate latitude environments undergo noticeable changes in water-column conditions during the year. The seasonal thermocline existing in summer is progressively eroded by convective mixing that often reaches below the euphotic zone in winter. As a result, the euphotic zone alternates from a stratified water body with fairly constant levels of light and nutrients to a mixed water column in which the phytoplankton experience lower temperatures, continually changing light conditions, and higher concentrations of nutrients. Striking variations in phytoplankton community structure are

Occurrence of brackish water phytoplankton species at a closed coral reef in Nansha Islands, South China Sea

“…Further evidence of the high nutrient availability in the lagoon is the high concentration of chlorophyll a, in particular the concentration in the bottom water, which is much higher than that in reef flat areas and adjacent open sea. The bulk of the biomass is mainly comprised of microphytoplankton (>20 lm) and ultra-plankton (<8 lm) with the latter belonging to three major taxonomic groups: cyanobacteria, prochlorophytes and eukaryotic algae, which may in fact comprise major fraction of total photosynthetic biomass (Lindell and Post, 1995). In spite of the taxonomic composition, phytoplankton has been repeatedly demonstrated nutrient limited due to an alteration in N:P, Si:P and Si:N ratios (Yin et al, 2001;Domingues et al, 2005).…”

Picophytoplankton responses to changing nutrient and light regimes during a bloom

“…Beginning in the fall, cooling of surface waters initiates convective mixing, and a deeply mixed (300 m or more) water body is observed by winter (Wolf-Vetch et al 1992). Nutrients entrained from depth as a consequence of this mixing result in mesotrophic conditions (Lindell and Post 1995), with productivity likely limited by light (Labiosa et al 2003). The water column begins to re-stratify in the spring as surface waters warm, trapping nutrients and phytoplankton in the euphotic zone along a steep light gradient.…”

“…Phytoplankton take advantage of the favorable nutrient and light conditions within the newly stratiWed euphotic zone forming a surface bloom over a period of days , while the growth of phytoplankton trapped at depth is limited by low light. Picophytoplankton (cells < 2 m) are the dominant cell type in the Gulf of Aqaba; however, ultraplankton (cells < 8 m) and some larger diatoms and dinoXagellates (cells 5-100 m) also occur in phytoplankton assemblages, particularly during the mesotrophic winter season (Lindell and Post 1995;Sommer 2000;Mackey et al 2007). …”

In situ depletion of phytoplankton by an azooxanthellate soft coral

“…Phytoplankton depletion was expressed as the proportion of upstream chlorophyll concentration removed from the water after passing through the passage. The phytoplankton community consisted mostly of synechococcoide cyanobacteria and pica-eukaryotic cells (Lindell and Post 1995;A. Post unpubl, data).…”