Temporal changes in the abundance, leaf growth and photosynthesis of three co-occurring Philippine seagrasses

The analysis of the temporal changes in shoot density, areal leaf biomass, leaf growth and parameters of the photosynthesis–irradiance relationship of three tropical seagrass species (Enhalus acoroides, Thalassia hemprichii and Cymodocea rotundata), co-existing in a shallow subtidal meadow in Cape Bolinao, Philippines, shows that species-specific traits are significant sources of temporal variability, and indicates that these seagrass species respond differently to a common environmental forcing. Species-specific differences are much less important as source of variability of the temporal change in chlorophyll concentration of seagrass leaves. The results indicate that the temporal changes in photosynthetic performance of these seagrasses were driven by environmental forcing and their specific responses to it mostly, but the temporal change in their abundance and leaf growth was also controlled by other factors. The significant contribution of species-specific factors in the temporal changes of biomass, growth and photosynthetic performance of co-occurring seagrass species in Cape Bolinao should contribute to the maintenance of the multispecific, highly productive meadows characteristic of pristine coastal ecosystems in Southeast (SE) Asia.     

Abundance, frequency of dividing cells and growth rates of Synechococcus sp. (cyanobacteria) in the stratified Northwest Mediterranean Sea

The abundance, frequency of dividing cells and growth ratesof the planktonic cyano bacteria Synechococcus sp. during thesummer of 1995 and 1996 were estimated in the Northwest MediterraneanSea to test whether depth-dependent growth rates of this speciesexplain its dominance in the deep chlorophyll maximum (DCM)layer formed during summer thermal stratification in the NWMediterranean, compared to the surface layer. Abundance at theDCM layer (50–70 m) was up to two orders of magnitudegreater than that at the surface, with values ranging from 1.7to 13x10⁶ cells I-¹ and from 4 to 175 x 10⁶ cells I-¹ at thesurface and in DCM waters, respectively. Gross growth rates,however, were much higher at the surface than in the DCM layer(surface: 0.76–1.07 day DCM: 0.30–0.47 day-¹ Thehigher gross growth rates at the surface layer were supportedby a higher frequency of dividing cells (surface: 0.09–0.24;DCM: 0.01–0.12). The negative correlation between theabundance or standing stock and growth rates of these planktonicpicocyanobacteria points to losses, and not growth rate, asthe main control on the abundance of Synechococcus. Althoughwe provide some evidence that grazing alone may be able to accountfor these losses, further, direct determinations are clearlyneeded to elucidate the regulation of the abundance of Synechococcusin the NW Mediterranean.