Lydia Ignatiades

Seasonal horizontal and vertical variability in primary production and standing stocks of phytoplankton and zooplankton in the Cretan Sea and the Straits of the Cretan Arc (March 1994-January 1995)

Abstract Phytoplankton communities, production rates and chlorophyll levels, together with zooplankton communities and biomass, were studied in relation to the hydrological properties in the euphotic zone (upper 100 m) in the Cretan Sea and the Straits of the Cretan Arc. The data were collected during four seasonal cruises undertaken from March 1994 to January 1995. The area studied is characterised by low nutrient concentrations, low 14 C fixation rates, and impoverished phytoplankton and zooplankton standing stocks. Seasonal fluctuations in phytoplankton densities, chlorophyll standing stock and phytoplankton production are significant; maxima occur in spring and winter and minima in summer and autumn. Zooplankton also shows a clear seasonal pattern, with highest abundances occurring in autumn–winter, and smallest populations in spring–summer. During summer and early autumn, the phytoplankton distribution is determined by the vertical structure of the water column. Concentrations of all nutrients are very low in the surface waters, but increase at the deep chlorophyll maximum (DCM) layer, which ranges in depth from about 75–100 m. Chlorophyll- a concentrations in the DCM vary from 0.22–0.49 mg m −3 , whilst the surface values range from 0.03–0.06 mg m −3 . Maxima of phytoplankton, in terms of cell populations, are also encountered at average depths of 50–75 m, and do not always coincide with chlorophyll maxima. Primary production peaks usually occur within the upper layers of the euphotic zone. There is a seasonal succession of phytoplankton and zooplankton species. Diatoms and ‘others’ (comprising mainly cryptophytes and rhodophytes) dominate in winter and spring and are replaced by dinoflagellates in summer and coccolithophores in autumn. Copepods always dominate the mesozooplankton assemblages, contributing approximately 70% of total mesozooplankton abundance, and chaetognaths are the second most abundant group.

A Review on Toxic and Harmful Algae in Greek Coastal Waters (E. Mediterranean Sea)

The Greek coastal waters are subjected to harmful algal bloom (HAB) phenomena due to the occurrence of species characterized as toxic (TX), potentially toxic (PT), and non-toxic, high biomass (HB) producers causing harm at multiple levels. The total number of (TX), (PT) and (HB) algae reported in this work are 61, but only 16 species have been associated with the occurrence of important HABs causing damage in the marine biota and the water quality. These phenomena are sporadic in time, space and recurrence of the causative species, and are related to the anthropogenically-induced eutrophication conditions prevailing in the investigated areas.

Clearance rate in the venerid bivalve Callista chione (L) in response to endemic algal species and bacteria: effects of cell biovolume and body size

Natural diet items were used to study the feeding behavior of Callista chione, a venerid bivalve inhabiting the sandy bottom of the N. Euvoikos Gulf, Aegean Sea. For more reliable measurements and conclusions we combined in vitro and field measurements. Microscopical algal cell counting was used for measurements of phytoplankton concentrations and high performance liquid chromatography for phytopigment analysis. Clearance rates were determined in vitro from reduction in the algal cell concentration in relation to time and ranged from 0.09 to 0.65 L h−1 depending on the size of C. chione and algal species. The digestive efficiency and regression equations of data related to the clearance rate with shell length of bivalves were derived. This study broadens our understanding of feeding behavior of C. chione in vitro as well as its impact in the food web dynamics in the field.