Ciliates, microbes and nutrients: interactions in the seasonally mixed Gulf of Aqaba

sakti silaen;

Experiments were conducted in the Gulf of Aqaba to investigate top-down and bottom-up influences on microbial food webs. Experiments were run during winter mixing, spring bloom and summer stratification, periods with increasing nutrient limitation and with picophytoplankton (.98% of total year-round biomass) dominated by different species. Size fractionation (,64, ,6 and ,0.8 mm) was cross-classified with or without the addition of nutrients (0.3 mMNH4 þ and 0.19 mMPO4, equivalent to winter concentrations), using the same design for the six experiments. All experiments showed minimal top-down and strong bottom-up control, independent of season. Among phototrophs, small eukaryotic algae responded more strongly to nutrient addition within the 24-h experiment duration than did Synechococcus or Prochlorococcus. Ciliates responded most strongly to nutrient addition, becoming markedly more abundant in treatments with added nutrients relative to those without; a trend seen to a more limited degree in ciliate biomass, as smaller species responded faster. Treatments with added nutrients were also more dissimilar to one
another than those without added nutrients: which species benefitted from added N and P appeared random. This could explain the high ciliate species diversity in the Gulf of Aqaba, if small nutrient pulses are rapidly transferred to different random collections of ciliate species in the vicinity of the pulse.