A calanoid copepod Gladioferens imparipes,holding to surfaces

Gladioferens imparipes, a calanoid copepod from estuaries in Western Australia, displays behaviour and associated morphology which is unusual among calanoids. Adult and copepodid stages make temporary attachment to underwater surfaces using fine hair sensillae on the surface of the prosome. A clear pattern can be seen in the number and arrangement of hair sensillae in early copepodid stages. Line drawings, photomicrographs and an S.E.M. are used to illustrate the structures. Laboratory studies with live copepods in a flow chamber show that adult animals may hold position against water currents. It is hypothesized that this behaviour influences the distribution pattern in an estuary where low velocity tidally induced water movement occurs, and may be of value in enabling the copepod to exist beyond the distribution of a major predator.     

Production and ecology of eelgrass (Zostera marinal L.) in the Grevelingen estuary,the Netherlands,before and after the closure

The Grevelingen estuary was cut off from the North Sea and from the influences of the river Rhine by a dam in 1971, and became a stagnant salt-water lake. Production and ecology ofZostera marina L. were studied in 1968 and in 1973–1975, both through standing stock estimations, biomass increases in permanent quadrats, and correlation of distribution patterns with ecological factors. After the closure of the estuary the intertidal eelgrass population extended downwards to 5 m below lake level, probably owing to the increased transparency of the water; the area occupied, and the density of the eelgrass beds increased strongly. Eelgrass annual overground production, based on doubled maximum standing crop values in July–August, was estimated at 50 g C/m² in 1968, 121 g C/m² in 1973 and 91 g C/m² in 1975 inZostera beds, and 4 g C/m² in 1968, 18 g C/m² in 1973 and 23 g C/m² in 1975 for the entire Grevelingen area. A minimum estimate of net production inZostera beds at a depth of 0.50–0.75 m, based on short term changes in biomass in 2 permanent quadrats in 1974 and 1975, was 40.5 g C/m²/yr for overground parts and 12.7 g C/m²/yr for underground parts. Horizontal distribution of celgrass is not primarily limited by grainsize distribution, but more by exposure to wave action and currents. On account of irradiance reduction light is a limiting factor in the vertical distribution of the eelgrass population in Lake Grevelingen. Communication no. 146 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands.     

Zooplankton biomass in the Oosterschelde (SW Netherlands) before,during and after the construction of a storm-surge barrier

The hydrodynamic consequences of large coastal engineering (barrier-construction) works in the Oosterschelde were: prolonged residence times of the water, increased sinking of particulate material, and higher water transparencies. This strongly influenced the phytoplankton (Bakker et al., 1990; 1994) and phytoplankton biomass increased in the shallow Eastern compartment of the Oosterschelde (Bakker & Vink, 1994) while phytoplankton concentration of the seston rose.Zooplankton biomass, especially of copepods (Temora) and meroplankton (barnacle larvae) increased during the post-barrier period in the eastern compartment. It is hypothesized that this is caused by the improved feeding conditions and the increased retention times in this area.The barrier years 1985 and 1986 were characterized by low current velocities. In the Eastern compartment, this may have favoured the development of the rotifer Synchaeta (Bakker, 1994) and of the important copepod predator Pleurobrachia (Ctenophora).In the Western compartment, zooplankton developments in the post-barrier years were rather similar to those in the pre-barrier period. This led to the disappearance of the previously existing biomass gradients West-East (maxima in West). At present a trend in the opposite direction (maxima in East) is observed.     

Population dynamics of phototrophic bacteria in three basins of Lake Banyoles (Spain)

The annual dynamics of the phototrophic bacterial populations developing in the anoxic layers has been monitored in three basins of the northern area of Lake Banyoles (Spain). Although two of the studied basins are meromictic and one is holomictic, chemical properties of the water are almost identical. Therefore, differences in both the spatial and temporal distribution, as well as in the composition of phototrophic bacterial communities, dominated by Chlorobium phaeobacteroides and Chromatium minus, are discussed on the basis of the structural and morphometric characteristics of each basin. Both species showed the same physiological adaptations to light intensity changes by modifying the carotenoid/bacteriochlorophyll ratio. Light reaching the oxic-anoxic boundary appears to be the most important factor controlling the growth of phototrophic bacteria in Lake Banyoles. The oxic-anoxic boundary becomes shallower as summer advances, until enough light is available for bacterial photosynthesis.     

Phytoplankton diet of Arctodiaptomus salinus (Copepoda, Calanoida) in Lake Shira (Khakasia)

A flow-through method was employed to study the algal part of the diet spectrum of the calanoid Arctodiaptomus salinus in the surface layer of the lake and in the phytoplankton biomass maximum zone (10 m deep). Daily consumption rates of this calanoid differed markedly in these layers, being 1 and 11 g ind ^–1 d^–1, respectively. The cyanobacterial taxa, Lyngbya contorta and Microcystis sp., accounted for more than a half of the ration. Nevertheless, the negative Ivlev electivity coefficient indicated that Arctodiaptomus prefers none of these two taxa. The low measured uptakes suggest that the energy expenditure is compensated by other resources, presumably microzooplankton and detritus.     

Field observations in the Oosterschelde (The Netherlands) onCoscinodiscus concinnus andCoscinodiscus granii (Bacillariophyceae) infected by the marine fungusLagenisma coscinodisci (Oomycetes)

During 1985–1990Coscinodiscus concinnus andCoscinodiscus granii from the Oosterschelde were infected by the marine fungusLagenisma coscinodisci, although not every year with the same intensity. Infected cells were only observed during the period July–October at water temperatures between 13.2 and 20.2°C. In 1986 and 1987 the course of the infection withL. coscinodisci inC. concinnus andC. granii, was recorded at three stations. The highest infection percentages varied between 22.2 and 58.3% inC. concinnus and between 7.1 and 41.9% inC. granii. It is concluded that the water temperature may play an important role in the appearance ofL. coscinodisci inC. concinnus andC. granii and that silicate limitation is of minor importance. Finally it is discussed that the infection was not of great importance for the phytoplankton community in the Oosterschelde.     

Food composition of Clarias gariepinus (= C. lazera) (Cypriniformes,Clariidae) in Lake Kinneret (Israel)

The feeding behaviour of the freshwater piscivore, Clarias gariepinus (C. lazera) (C & V 1840) was studied over two periods: 1973–1975 and 1981–1982, in Lake Kinneret (Israel). The total number of fish analysed was 264 and their sizes (SL) and weights varied between 238 and 830 mm (146 to 5728 g). More than fifty species of plants and animals from the plankton, benthos and nekton of Lake Kinneret were identified in the intestines of C. gariepinus. Preyed fish were the most abundant food component (81%) and constituted the highest biomass, with Mirogrex terraesanctae representing the majority (although other species were also found). The potential impact of piscivory in the Kinneret ecosystem is considered.     

Macroinvertebrates associated with Chara in a tropical coastal lagoon (Imboassica lagoon,Rio de Janeiro,Brazil)

Imboassica lagoon is an urban coastal lagoon located in the municipality of Macaé (RJ), which has been exposed to a process of artificial eutrophication through the inflow of untreated sewage, as well as artificial openings of the sandbar that separates it from the ocean, provoking drastic modifications in this ecosystem. The sampling for the analysis of the community of macroinvertebrates associated with Charopyceae (Chara angolensis e C. fibrosa) were performed between October 1995 and October 1997, with a total of 9 samplings at two sampling stations: one located in the innermost area of the lagoon and another at the mouth of the main sewage channel, close to the sandbar. Throughout this period, four artificial sandbar-opening events were recorded, resulting in changes in the biomass of the macroalgae and in the densities of the macroinvertebrate populations. Through the analysis of the density of organisms per square meter, and density of organisms per 100 g dry weight of Chara, it was established that the biomass of the macroalgae has a direct influence on the density of the associated macrofauna. The greater development of the periphytic community in the area close to the inflow of sewage allows for the development of a community of macroinvertebrates with high densities, composed mainly of scraping organisms, such as the mollusc Heleobia australis (Hidrobiidae).     

The role of the blue mussel Mytilus edulis in the cycling of nutrients in the Oosterschelde estuary (The Netherlands)

The fluxes of particulate and dissolved material between bivalve beds and the water column in the Oosterschelde estuary have been measured in situ with a Benthic Ecosystem Tunnel. On mussel beds uptake of POC, PON and POP was observed. POC and PON fluxes showed a significant positive correlation, and the average C:N ratio of the fluxes was 9.4. There was a high release of phosphate, nitrate, ammonium and silicate from the mussel bed into the water column. The effluxes of dissolved inorganic nitrogen and phosphate showed a significant correlation, with an average N:P ratio of 16.5. A comparison of the in situ measurements with individual nutrient excretion rates showed that excretion by the mussels contributed 31–85% to the total phosphate flux from the mussel bed. Ammonium excretion by the mussels accounted for 17–94% of the ammonium flux from the mussel bed. The mussels did not excrete silicate or nitrate. Mineralization of biodeposition on the mussel bed was probably the main source of the regenerated nutrients.From the in situ observations net budgets of N, P and Si for the mussel bed were calculated. A comparison between the uptake of particulate organic N and the release of dissolved inorganic N (ammonium + nitrate) showed that little N is retained by the mussel bed, and suggested that denitrification is a minor process in the mussel bed sediment. On average, only 2/3 of the particulate organic P, taken up by the mussel bed, was recycled as phosphate. A net Si uptake was observed during phytoplankton blooms, and a net release dominated during autumn. It is concluded that mussel beds increase the mineralization rate of phytoplankton and affect nutrient ratios in the water column. A comparison of N regeneration by mussels in the central part of the Oosterschelde estuary with model estimates of total N remineralization showed that mussels play a major role in the recycling of nitrogen.     

The genus Synchaeta (rotifers) in a north-western Mediterranean coastal lagoon (Etang de Thau, France): taxonomical and ecological remarks

During a survey of a north-western Mediterranean coastal lagoon, (the Etang de Thau, and at a nearby marine station, 1994–1998), six species of the genus Synchaetawere identified. The systematics of this illoricate genus is difficult and identification is virtually impossible in preserved material, although some indication may be obtained from an examination of the trophi after treatment with sodium hypochlorite. In this study, taxonomic characters (mastax, body morphology and egg size) were obtained from living material prior to preservation. We add some ecological observations: distribution of species (neither temperature nor salinity were found to be determining factors), co-existence (up to five species can co-occur) and consideration of trophic links (competition with tintinnids, copepod nauplii and cirripedes).     

Copepod biomass in an estuarine and a stagnant brackish environment of the S. W. Netherlands

In the mesohaline zone of the Westerschelde estuaryEurytemora affinis is the dominant copepod, demonstrating large biomass values nearly throughout the year. In the meso-polyhaline Lake VeereAcartia tonsa is abundant, mainly during summer. In spring a small population ofEurytemora americana is found. The tidal estuary harboured far denser copepod populations throughout the year than the stagnant brackish lake water. The average yearly copepodid+adult biomass in the Westerschelde estuary was approx. 850 mg/m³ (wet weight), in Lake Veere approx. 130 mg/m³. Temporarily low oxygen values did not influence negatively the copepod populations in the Westerschelde estuary. The seasonal distribution of the dominant copepods in both areas is explained in the light of recent litterature data. PerennialEurytemora affinis abundance in the Westerschelde estuary must be mainly caused by large concentrations of nannodetritus particles, bacteria included, throughout the year.Acartia tonsa in Lake Veere has to thrive mainly on nannophytoplankton. Communication no. 141 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherland.     

Population dynamics of Bosmina longirostris (Crustacea: Cladocera) in Lake Kariba,Zimbabwe

The population dynamics of Bosmina longirostris in a large tropical reservoir, Lake Kariba, Zimbabwe, are described. Population density showed two peaks associated with periods of high nutrient fluxes. Fecundity was always low. Two eggs per female was the maximum ever recorded. Instantaneous rates of birth, death and population growth calculated from field data using the egg ratio method are discussed. The rate of biomass change fluctuated in a regular pattern, probably linked to food availability or predation by a clupeid, Limnothrissa miodon.     

Phytoplankton growth rates in the freshwater tidal reaches of the Schelde estuary (Belgium) estimated using a simple light-limited primary production model

During the course of 1996, phytoplankton was monitored in the turbid, freshwater tidal reaches of the Schelde estuary. Using a simple light-limited primary production model, phytoplankton growth rates were estimated to evaluate whether phytoplankton could attain net positive growth rates and whether growth rates were high enough for a bloom to develop. Two phytoplankton blooms were observed in the freshwater tidal reaches. The first bloom occurred in March and was mainly situated in the most upstream reaches of the freshwater tidal zone, suggesting that it was imported from the tributary river Schelde. The second bloom occurred in July and August. This summer bloom was situated more downstream in the freshwater tidal reaches and appeared to have developed within the estuary. A comparison between phytoplankton growth rates estimated using a simple primary production model and flushing rate of the water indicated that no net increase in phytoplankton biomass was possible in March while phytoplankton could theoretically increase its biomass by 20% per day during summer. Chlorophyllaconcentrations at all times decreased strongly at salinities between 5–10 psu. This decline was ascribed to a combination of salinity stress and light limitation. Phytoplankton biomass and estimated annual net production were much higher in the freshwater tidal zone compared to the brackish reaches of the estuary (salinity > 10 psu) despite mixing depth to euphotic depth ratios being similar. Possible reasons for this high production include high nutrient concentrations, low zooplankton grazing pressure and import of phytoplankton blooms from the tributary rivers.     

Food and feeding habits of Mirogrex fingerlings in Lake Kinneret (Israel)

The food composition of the fingerlings (13–41 mm TL) of Mirogrex terraesanctae terraesanctae was studied on specimen collected at four stations in Lake Kinneret. The fingerling feed mostly on pelagic zooplankters and zoobenthic forms whereas algae were found in the intestines in low biomass. The number of zooplankton organisms in the guts of the fingerling that were sampled during June–August was higher by 234% than in fingerlings that were sampled during March–April. The role of sardines fry in the Kinneret ecosystem is discussed.     

A redescription ofDiaphanosoma modigliani Richard, 1894 (Crustacea : Daphniiformes : Sididae)and an evaluation of its occurence outside Lake Toba (Sumatra)

A pelagic species of Diaphanosoma, D. modigliani is redescribed on material from the type locality (Lake Toba, Sumatra, Indonesia). Other records of this species in tropical Asia (India, Philippine Islands, Malaysia, Sri Lanka, Thailand) are either unsubstantiated or erroneous.     

A new interstitial cyclopoid copepod from a sandy beach on the western shore of Lake Baikal,Siberia

A new species of cyclopoid copepod, Diacyclops biceri sp. nov., is described from Lake Baikal. It was found in the interstitial water of a sandy beach at Buchta Peschanaya on the western shore of the central basin of Baikal. The new species is unique in possessing 2-segmented endopods in swimming legs 3 and 4. Swimming legs 2 to 4 have 3-segmented exopods. The slender body form, the lack of the antennal exopodal seta, and the presence of a secondary pseudosomite anterior to the genital double somite of the adult female are interpreted as adaptations to the interstitial habitat. The harpacticoid Epactophanes richardi Mrazek was found in the same interstitial habitat as D. biceri.     

Surirella sparsipunctata Hustedt and S. sparsipunctata var. laevis Hustedt (Bacillariophyceae), a light and electron microscopical study

Hydrobiologia - Three populations of S. sparsipunctata from the eastern coast of Lake Tanganyika were studied by light and electron microscopy. We found no distinct differences between the type...     

Nest-building and communal care of young byTilapia rendalli dumeril (pisces,cichlidae) in Lake Malawi

Synopsis Tilapia rendalli of Lake Malawi dig large tunnels in their nests and practise communal care of young. Both these traits are considered to be antipredator adaptations.     

Distinct life histories of peracarid crustaceans in a Ria Formosa salt marsh (S. Portugal)

Pitfall traps were exposed between February 1993 andSeptember 1994 every 4th to 6th week near the highwater mark in a Ria Formosa salt marsh (Portugal).With only a few exceptions, peracarid crustaceansaccounted for more than 95% of the individualscaught. Three amphipod species (Orchestiagammarellus(Pallas), Orchestia mediterraneaCosta, Talorchestia deshayesii(Audouin)) and3 isopod species (Tylos ponticus(Arcangeli),Halophiloscia couchi(Kinahan) and Porcellio lamellatus(Uljanin) Budde-Lund) wereidentified. In general, these species were most activefrom spring to autumn.Data collected included information on growth rate,life expectancy and timing of reproduction. Theamphipods displayed more opportunistic life-historypatterns with high growth rates, reproductive activityduring most of the year, early sexual maturity andrelatively short life expectancies. The isopods weremore persistent with slower growth rates, morerestricted reproductive periods and longer lifeexpectancies.There is no indication in the literature that most ofthese species are particularly common in salt marshes.In addition, peracarid crustaceans are not mentionedas dominant taxa in most studies of salt marsh fauna.We suggest that the peracarids play an important rolein the degradation of organic matter in salt marshesand hypothesize that the high numbers of peracaridsfound in Ria Formosa is due to the high contributionof leaf shedding plants.