Scarcity of medium-sized phytoplankton in the northern Red Sea explained by strong bottom-up and weak top-down control

Abstract

ABSTRACT. This study tested whether the extreme scarcity of larger nanophytoplankton and rnicrophytoplankton in the Gulf of Aqaba and in the open northern Red Sea is caused by nutrient limitation or by selective removal by grazers. Samples of near surface phytoplankton were incubated on board under a fully factorial combination of release from grazing pressure and release from nutrient stress.Release from grazing pressure by different size classes was obtained by sieving through 100, 20, and 10 pm size mesh screens. Release from nutrient stress was obtained by enrichment of Si alone and a full enrichment by N, P, Si and trace elements. Growth rates of most phytoplankton taxa showed a strong, positive response to the full nutrient enrichment and a weaker, but significant response to grazer exclusion. Several diatom taxa showed a weak positive response to Si enrichment. Thus, bottom-up control of medium-sized algae appears to he more important than top-down control.

Picophytoplankton responses to changing nutrient and light regimes during a bloom

“…In contrast to the picoeukaryotes, Synechococcus and Prochlorococcus declined slightly in abundance for all treatments and the control in the LL nutrient enrichment experiment ( Fig. 7b.2, b.3), possibly due to the presence of small (<20 m) grazers that were able to pass through the mesh at the start of the experiment (Sommer 2000). The results of this LL nutrient enrichment experiment show that when light (rather then nutrients) limits growth, both photoacclimation and considerable shifts in community composition can occur within the phytoplankton community without a net gain or loss of photosynthetic biomass.…”

“…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). …”

Grazing during early spring in the Gulf of Aqaba and the northern Red Sea

“…Samples were taken at the beginning and end of incubation for later microscopic counts. Samples for the enumeration of picoplankton and nanoplankton Sommer (2000); P: grazing on picoplankton according to Landry et al (1995); Z: mesozooplankton net hauls were preserved in 1.25% glutaraldehyde and stored at 4°C; 15 ml of the fixed sample were filtered onto 0.2 µm Nuclepore filters and stained with the fluorochrome DAPI (1.0 µg l -1 ). Heterotrophic pico-and nanoplankton and autotrophic ultraplankton (< 8 µm) were counted using a blue filter set, whereby autotrophs were differentiated from heterotrophs on the basis of their red or orange autofluorescence.…”

The interplay between upwelling and deep convective mixing in determining the seasonal phytoplankton dynamics in the Gulf of Aqaba: Evidence from SeaWiFS and MODIS

“…The majority of phytoplankton (Ͼ95%) in the Gulf of Aqaba are ultraphytoplankton (Ͻ8 m) (Lindell and Post 1995;Post et al 1996), including Synechococcus, Prochlorococcus (Lindell and Post 1995;Post et al 1996;Li et al 1998;Sommer et al 2002), and small eukaryotes, comparable to classic oligotrophic regions. However, unlike some oligotrophic regions (e.g., Claustre and Marty 1995), the phytoplankton community structure exhibits strong seasonality, with Synechococcus and Prochlorococcus dominating during the spring and summer, respectively, and eukaryotic phytoplankton (specifically dinoflagellates and diatoms) dominating during winter (Reiss and Hottinger 1984;Lindell Fig.…”