Surface patterns of zooplankton spatial variability detected by high frequency sampling in the NW Mediterranean. Role of density fronts

High frequency sampling was performed in daylight hours along a 35 km transect in the Ligurian Sea to investigate the upper layer zooplankton distribution during the spring phytoplankton bloom. The results show detailed spatial structure and biomass of key zooplankton functional groups, copepods, salps and krill larvae, within the different water masses characterizing this region. Although observed values of total copepod biomass distribution were rather constant along the transect, species-specific patterns were observed in the copepod spatial distribution. The larger species Calanus helgolandicus, as well as Centropages typicus, Oithona spp., and Oncaea spp., were associated with the frontal zone. However, Acartia spp. had a scattered distribution, and Clausocalanus/Paracalanus did not have a clear pattern. In addition, krill larvae were concentrated in the frontal area and salps had a scattered pattern. The cross-shore zooplankton distribution appeared strongly influenced by both the Northern Ligurian current governing inshore waters, which acts as a major flushing forcing, and the Ligurian front, which governs offshore waters and may act as retention area for zooplankton.     

The importance of estuarine gravitational circulation in the early life of the Bohai Penaeid Prawn

Estuaries along the southern shore of the Bohai Sea are the major habitat of the Bohai Penaeid Prawn (Penaeus chinensis). Since the 80's, however, many of the rivers have been dammed. Field observations, as well as laterally integrated 2-D numerical experiments, were conducted to understand both the role of estuarine gravitational circulation and the impact of the damming of the rivers on the early life of the Bohai Penaeid Prawn.For a river with runoff, especially for small discharge, the gravitational circulation tends to transport the planktonic larvae in their metamorphosis phase near the river mouth where condition is favorable for survival. The gravitational circulation inside the estuary tends to transport the mysis phase larvae towards the upstream end of the estuary when the most part of the larvae suspended in the bottom layer, and it transports the post larvae to the low salinity near upstream side of the estuary when the larvae become benthic, if the larvae enter the estuary.Damming causes long periods of zero runoff in the river, resulting in the alteration of the estuarine circulation and in the change of the estuarine environment. In addition, excess evaporation may prematurely transport the planktonic larvae into the estuary. On the other hand, sudden release of a large volume of freshwater from behind the dam may exert undesirable stress on the larvae.     

Meroplankton abundance in the Northeast Water Polynya: Insights from oceanographic parameters and benthic abundance patterns

We investigated meroplankton (planktonic larvae of benthic organisms) abundance and distribution in the Northeast Water (NEW) Polynya, located on the northeast coast of Greenland, from July 15 to August 15, 1992. Meroplankton was present at all sites visited (0.03–84.83 individuals per m³); at one station meroplankton comprised 8.28% of total zooplankton. Total meroplankton abundance was correlated with total zooplankton abundance and total benthic infaunal abundance but was not correlated with either microscopic carbon concentration or primary productivity. Examination of distribution data for barnacle nauplii and adults indicated that both adults and larvae were concentrated at the same locations. Patterns of distribution were also examined for stelleroid plutei, polychaete larvae and trochophores. There were distinct geographic patterns in total and class-specific meroplankton distributions, with maximal abundances occurring over the Belgica Bank and in the eastern regions of the Westwind Trough and minimal abundances in the Belgica Trough. The apparent control of meroplankton distribution by the hydrography of the region, coupled with the correlation between meroplankton, zooplankton and adult infaunal abundance, reinforces the hypothesis that hydrography plays a major role in controlling the distribution of biota in the NEW polynya (Ambrose and Renaud, 1995; Ashjian et al., 1995, 1997-this volume; Smith et al., 1995; Piepenburg et al., 1997-this volume).     

Mortality through ontogeny of soft-bottom marine invertebrates with planktonic larvae

The present survey covers one spawning season of marine benthic invertebrates in a large geographical area, the inner Danish waters, and includes a wide range of habitats with steep salinity and nutrient load gradients. The loss ratios of soft-bottom marine invertebrates from one development stage to the next is calculated based on average abundances of pelagic larvae, benthic post-larvae and adults of Bivalvia, Gastropoda, Polychaeta and Echinodermata, with planktonic development. This gives a rough estimate of the larval and post-larval mortality. Loss ratios between post-larvae stage and adult stage (post-larval mortality) varies from 3:1 to 7:1 (71.2–84.9%) and loss ratios between larvae and post-larvae (larval mortality) and between larvae and adult, ranging from 7:1 to 42:1 (85.2–97.6%) and from 45:1 to 210:1 (97.8–99.5%), respectively. The results show a remarkable unity in loss ratios (mortality) between the mollusc taxa (Bivalvia and Gastropoda) at the phylum/class level. This similarity in loss ratios among the mollusc taxa exhibiting the same developmental pathways suggests that the mortality is governed by the same biotic and abiotic factors. Larval mortality is estimated to range from 0.10 d^− 1 to 0.32 d^− 1 for Bivalvia and ranging from 0.09 d^− 1 to 0.23 d^− 1 for Polychaeta. The species loss ratios combined with specific knowledge of the reproduction cycles give estimated loss ratios (mortality) between the post-larvae and the adult stage of 25:1 and 14:1 for the bivalves Abra spp. and Mysella bidentata. For the polychaete Pygospio elegans the loss ratio (larval mortality) between the larvae and the post-larval stage is 154:1 and between the post-larvae and the adult stage 41:1. For Pholoe inornata the loss ratio between post-larvae and adults is 7:1. The present results confirm that the larval stage, metamorphosis and settlement are the critical phase in terms of mortality in the life cycle for Bivalvia. Assuming steady state based on actual measurements of pelagic larval densities an estimated input to the water column of pelagic bivalve larvae is ranging from 10,930 to 17,157 larvae m^− 2 d^− 1 and for Polychaeta between 2544 and 3994 larvae m^− 2 d^− 1. These estimates seem to correspond to the reproductive capacity of the observed adult densities using life-table values from the literature.The potential settlement of post-larvae is 43 post-larvae m^− 2 d^− 1 for Bivalvia and 56 post-larvae m^− 2 d^− 1 for Polychaeta. The adult turnover time for Bivalvia is estimated to be 1.5 years and 2.1 years for Polychaeta. This exemplifies that species with short generation times may dominate in very dynamic transitional zones with a high frequency of catastrophic events like the frequent incidents of hypoxia in the inner Danish waters.     

Ichthyoplankton in the neritic and coastal zone of Antarctica and Subantarctic islands: A review

Since the article published by Loeb et al. [Loeb, V.J., Kellermann, A., Koubbi, P., North, A.W., White, M., 1993. Antarctic larval fish assemblages: a review. Bull. Mar. Sci. 53(2), 416–449.] about Antarctic ichthyoplankton, many surveys were carried out in different sectors of the Southern Ocean focusing on different aspects of the ecology of fish larvae. Some of these researches were conducted in the Subantarctic Kerguelen Islands and others on the continental shelf off Terre Adélie and Georges V land. Oceanographic and geographic features influence fish larvae ecology such as island mass effects, gyres, canyons. Antarctic fishes show also temporal segregation of spawning which induces temporal succession of early stage larvae. This avoids competition and probably the predation on early stages for species having few recruits. In that case, we have to understand how these larvae can deal with the match–mismatch with their preys and how they find sufficient food to survive. But our knowledge on Antarctic fish larvae is still insufficient as we do not know larvae for quite a lot of species and because of the difficulty to sample during winter.