Description: The present paper reviews the literature related to the life cycle of the prymnesiophyte Phaeocystis and its controlling factors and proposes novel hypotheses based on unpublished observations in culture and in the field. We chiefly refer to P. globosa Scherffel as most of the observations concern this species. P. globosa exhibits a complex alternation between several types of free-living cells (non-motile, flagellates, microzoopores and possibly macrozoospores) and colonies for which neither forms nor pathways have been completely identified and described. The different types of Phaeocystis cells were reappraised on the basis of existing microscopic descriptions complemented by unpublished flow cytometric investigations. This analysis revealed the existence of at least three different types of free-living cells identified on the basis of a combination of size, motility and ploidy characteristics: non-motile cells, flagellates and microzoospores. Their respective function within Phaeocystis life cycle, and in particular their involvement in colony formation is not completely understood. Observational evidence shows that Phaeocystis colonies are initiated at the early stage of their bloom each by one free-living cell. The mechanisms controlling this cellular transformation are still uncertain due to the lack of information on the overwintering Phaeocystis fomms and on the cell type responsible for colony induction. The existence of haploid microzoospores released from senescent colonies gives however some support to sexuality involvement at some stages of colony formation. Once colonies are formed, at least two mechanisms were identified as responsible of the spreading of colony form: colony multiplication by colonial division or budding and induction of new colony from colonial cells released in the external medium after colony disruption. The latter mechanism was clearly identified, involving at least two successive cell differentiations in the following sequence: motility development, subsequent flagella loss and settlement to a surface, mucus secretion and colony formation, colonial cell division and colony growth. Aggregate formation, cell motility development and subsequent emigration from the colonies, release of non-motile cells after colony lysis on the other hand, were identified as characteristics for termination of Phaeocystis colony development. These pathways were shown to occur similarly in natural environments. In the early stages of the bloom however, many recently-formed colonies were found on the setae of Chaetoceros spp, suggesting this diatom could play a key-rôle in Phaeocystis bloom inception. Analysis of the possible environmental factors regulating the transition between the different phases of the life cycle, suggested that nutrient status and requirement of a substrate for attachment of free-living cells would be essential for initiation of the colonial form. Physical constraints obviously would be important in determining colony shape and fragmentation although autogenic factors cannot be excluded. Some evidence exists that nutrients regulate colony division, while temperature and nutrient stress would stimulate cell emigration from the colonies.

Description: The objective of COMWEB was to develop efficient analytical, numerical and experimental methods for assessing and predicting the effects of nutrient (N, P, Si) supply on the stability and persistence of pelagic food web structure and function in coastal waters. The experimental comparative work included a geographic gradient covering Baltic, Mediterranean, and NE Atlantic waters and a NE Atlantic gradient in state of eutrophication. COMWEB has been an experimental approach to coastal eutrophication, studying effects of enhanced nutrient supply on components and flows of the entire lower pelagic food web. Flow network representations of pelagic food webs has been a framework of data reduction and flows were established by sophisticated inverse modelling. Fundamental information on physiological properties of functional key species in the pelagic food web was used to constrain flow estimations. A main conclusion derived from the flow networks was that very little energy and materials were transferred from the microbial food web to the main food chain. The lower food web could therefore be described as two parallel food chains with relatively limited interaction between heterotrophic groups. Short-term effects of nutrient perturbations were examined in mesocosms along the geographic gradient. The response was comparable in all systems, with a stronger effect on the activity and biomass of autotrophic groups than those of heterotrophic ones. Mediterranean waters showed much lower autotrophic biomass response than Baltic and NE Atlantic waters, which responded almost equally. The response of primary production was, however, more comparable. High phytoplankton lysis rate explained this low accumulation of biomass in Mediterranean waters. The study of Atlantic coastal waters of different eutrophic states revealed that the ecological response was higher in the closed nutrient perturbed mesocosms than in open systems exposed for 〉4 summer months (summer/autumn season). The Atlantic lagoon evolved gradually from the natural oligotrophic situation towards the more eutrophicated North Sea during fertilisation. The responses observed on seasonal and long-term scale (〉10 years) may therefore be equal. The differences between short-term (weeks) and intermediate-term (seasonal) responses is most likely a result of the different time scales of perturbation and observation and the variable exchange rates with surrounding waters (water dilution rate). The analysis of pelagic flow networks provided a framework of diagnostic criteria for state and quality assessment of coastal waters. The nutrient loading rates related better to estimates of biotic fluxes than to concentrations of biotic compartments and total nutrients. On the contrary, the concentration of biotic compartments, or the biomasses, related better to total nutrient concentrations. Primary production, mesozooplankton grazing and growth, fraction of primary production consumed by grazers, bacterial production relative to primary production, cycling indices, and path lengths were all well related to nutrient loading rate. Autotrophic biomass, ratio of autotrophic to heterotrophic biomass, and fraction of pico-cyanobacteria of total autotrophic biomass were all related to total nutrients. Some of these variables, which responded equally in all systems, have the potential of becoming unified response functions in a management model for European coastal waters. COMWEB has provided further insight into the mechanisms behind coastal eutrophication. A main achievement is the conceptual framework for unified response functions, important components of management models for nutrient emission to coastal waters.