We studied the development and decline of the 1990 phytoplankton spring bloom in the Marsdiep area of the North Sea (The Netherlands) with emphasis on the cause of the decline of the Phaeocystis bloom, the role of microbial organisms and the utilization of organic material produced by the algae. At the top of the bloom Phaeocystis was nitrogen limited. The bloom declined through cell lysis. Sinking of colonies and grazing were found to be relatively unimportant as loss factors. Biomass in the microbial foodweb (bacteria and protozooplankton) remained low during the bloom but increased sharply as the bloom started to decline indicating that organic carbon released by the phytoplankton was rapidly utilized in the lnlcrobial foodweb Results suggest that dissolved organic carbon produced by phytoplankton through excretion and lysis was the main source of carbon for the microbial foodweb includ~ng copepods.

The influence of grazing, sedimentation and phytoplankton cell lysis on the dynamics of a coastal pelagic ecosystem in the Southern Bight of the North Sea was studied during spring/summer 1992. Diatoms in the > 8 pm size fraction dominated during early spring, due to size-differential control by microzooplankton. This diatom spring bloom became silicate depleted and declined by sedimentation. A Phaeocystis bloom developed in early summer. Phytoplankton cell lysis was the major loss factor for Phaeocystjs, accounting for 75% of the decline of the bloom. Bacterial production was positively correlated with phytoplankton cell lysis, and bacterial carbon demand could be supplied by cell lysis. This illustrates the importance of phytoplankton cell lysis in providing energy for the microbial loop. A new method (nicotine addit~on technique) was used to estimate mesozooplankton grazing on microzooplankton. h4esozooplankton appeared to prefer microzooplankton as a food source, though there occasionally was substantial grazing on phytoplankton. We conclude that grazing and sedimentation, as well as call lysis, are structuring mechanisms for algal bloom dynamics.

We assessed the influence of coral 'bleaching on the regenerative potential of corals in a senes of field expenments with artificial lesions on colony surfaces These lesions were made d u n n g a mass bleaching event in 1990 on normally coloured and bleached colonies of 3 species A4ontastrea annulans, the main reef-building coral in the Caribbean P o r~t e s astreo~des, a relatively conmon and opportunistic coral, and M e a n d r~n a meandntes, a species common on the deeper reef slope Regeneration charactenshcs studied included rate of tissue growth, recovery of lesion area and restoration of hssue colour Tissue regenerahon rates were lower in all species when companng bleached with unbleached colonies, although there were species-specific differences Long-tern~ exposure to increased temperature probably affects regeneration as bleached colonies of P astreo~des in the thermal effluent of a power plant regenerated as rapidly as normal colonies situated upcurrent and downcurrent of the power plant Colour recovery of bleached colonies was closely associated with seawater temperature When ambient seawater temperature started to decrease (<3OoC), colonies began to regain thelr normal colour Colour restoration was slowest in M a n n u l a r~s Normal colour of coral colonies in the power plant effluent returned at the same t~rne, but at temperatures 2°C hlgher than elsewhere on the reef Mortality on bleached colonies of M annulans and P astreo~des was hlgher than on normal colonies 131 d after the start of the regeneration expenments Large parts of the colony died when the tissue was already regalning its normal colour and lesion regeneration was almost completed Mortality was most severe for colonies In the effluent We recorded the sensitivity of coral species to bleaching in surveys over the reef downcurrent and upcurrent of the power plant These showed that community structure and colony condit~on on the shallow terrace downcurrent differed noticeably from the upcurrent site Downcurrent, specles diversity was lower malnly because of high densit~es of Pontes astreo~des and D~plona stngosa Bleaching was also more prominent downcurrent The decreased regeneration rates and increased mortality of bleached colonies (in Montastrea annular~s in 30 % of the colonies), dunng and subsequent to bleaching clearly Indicate the increased vulnerability of coral reefs dunng penods of thermal stress Coral reef management should consider limiting activities resulting in small lesions under penods of Increased environmental stress

Water samples were collected at a fringing coral reef in overlying water, in bottom water between corals and in crevices under coral colonies, and analyzed for nutrient concentrations, bacterial numbers and production. We found decreasing bacterial densities from overlying water through bottom water into crevices (range 9 to 2 X 105 ml-l). Bacterial specific growth was enhanced in reef crevices (range 0.005 to 0.04 h-'). Although bactenal growth was enhanced, bacterial numbers were reduced, showing a transfer of bacterial biomass into the reef. The differences in bacterial numbers and grnwth between water types depended on water movement and bottom relief Nutrients were enhanced in reef crevices as a result of mineralization. Mineralization of bacterla removed by filterfeeders could contribute 11 and 21 '70 to the increase in N and P, respectively, in coral reef crevices.

The distribution of Trididemnum solidum (Van Name) was investigated in 42 transects over the coral reef on the leeward coast of C u r a~a o .Ascidian colonies were abundantly present at depths from 1.5 to 34 m along the central part of this coast, but absent towards the tips of the island. This reflects characteristics of the lift, history of T. solidum, such as short pelagic life, and the local current regime. Another factor influencing the distribution is the ambient light level. For 9 mo, July 1978 to April 1979, w e measured the monthly in sjtu growth of a population varying between 38-50 colonies. Each fortnight these colonies were checked for presence/absence. The constancy in population size masks a highly dynamic system. During our study period 85 novel colonies appeared through division. 43 colonies disappeared through f u s~o n with other colonies and 33 colonles ceased to exist (dead) or were lost. Growth rates were extremely r a p~d , u p to 13 cm mo-' i n linear direction, compared with the most common s p a t~a l competitors (Sclel-actinia). Although T solidum overgrows corals, growth rates are reduced over I~ving coral tissue Hornogenates of the ascldlan had variant but adverse effects on different coral spccles and, apart from s~n i p l c overgrowth, allelochem~cals could be involved in spatial competition. There I S I~ttle e v~d e n c e of predation on Trididemnum solidum and drtificial lesions were rapidly rrgenpl-ated. Our ohservat~ons show I t i <~t T solidum had a life span largely exceeding our study p e r~o d crnd we suspect that clones !nay survive indefinitely.