C. Bakker

Size and phytoplankton selection by Oosterschelde zooplankton

Grazing of adults of the copepods Acartia spp., Temora longicornis, Centropages hamatus and nauplii of Balanus spp. on natural particulate matter from the Oosterschelde was studied using a Coulter counter. Two types of particulate matter distributions were found to occur in the Oosterschelde: 1. distributions with distinct peaks in the > 20 μm size range (A) and 2. more flattened distributions which were bell-shaped or contained multiple small peaks spread over the entire 4–100 μm size range (B). In A-type distributions, peak tracking was performed in all species studied, especially in Acartia spp. and T. longicornis. In B-type distributions, the animals tended to spread their grazing activity towards the smaller particles. Quantitative microscopical analysis of the phytoplankton distributions shows that A- and B-type distributions were caused by the presence (A) or absence (B) of blooms of phytoplankton species with an SED of > 20 μm, reaching concentrations of >1.40·106 μm3·cm−3. The demonstrated clearance rate distributions are shown to result in a concentration of phytoplankton in the ingested material as compared to the medium for all copepod species studied. Balanus spp. nauplii did not demonstrate this systematic selection of phytoplankton. When B-distributions occurred, only Acartia spp. switched its feeding activity towards smaller particles to such a degree that a substantial contribution of the <20 μm size range to the ingested material resulted. Whether this behaviour represents a higher selection capacity for small phytoplankton species or a detritivorous behaviour remains an open question.

Comparison of plankton assemblages of identical salinity ranges in estuarine tidal, and stagnant environments II. Zooplankton

Abstract A comparison was made between the zooplankton assemblages of the meso- and polyhaline ranges of the Westerschelde estuary and Lake Veere (SW Netherlands). Species composition and density of the zooplankton of brackish water ecosystems depend not only on salinity but on several other factors too. Zooplankton development in stagnant Lake Veere and tidal Westerschelde waters of the same salinity is quite different. Important factors appeared to be food supply, water movement and water exchange. Lake Veere zooplankton subsists on a diversified and dense phytoplankton as source of food during long periods of the year. Continuous turbulent water movement is absent. The combination of these factors is favourable for rotifers and polychaet larvae. Several groups, i.e. protozoans, rotifers, larvae of polychaets and molluscs, copepods, constitute important parts of total average biomass. Westerschelde zooplankton in the mesohaline area of the estuary subsists on detritus as the only important source of food throughout the year. Phytoplankton is densest in summer but small species, essential as food, are quantitatively rare. Strong tidal turbulence tends to disperse the organisms in the whole water mass. Tidal exchange causes transport of organisms in seaward direction. These conditions are unfavourable especially for rotifers. Copepods show successful adaptation to the estuarine environment. Average standing stocks in the mesohaline area are much larger than in Lake Veere.

Development time and growth rate of the marine calanoid copepod Temora longicornis as related to food conditions in the Oosterschelde estuary (Southern North Sea)

Abstract The development time of the larval stages of the marine calanoid copepod Temora longicornis was estimated by applying the multi-cohort model of Bossicart (1980) on observed densities of 3 successive cohorts in the open Oosterschelde estuary (SW Netherlands) during 1983 and 1984. The shortest development time of nauplii + copepodids of 16 to 25 days occurred in spring; the maximum development time of both series of stages was 27 to 43 days and was found in summer. This longer time of development could be ascribed to a smaller food supply during this period. The development time of the larval stages was significantly correlated with smaller phytoplankton cells (spherical equivalent diameter ⩽ 50 μm). Lowest daily exponential k-values (0.04) were calculated for summer periods (17 to 20°C) and highest values (k=0.23) were found during spring (∼10°C). From 1986 onwards, the tidal regime in the estuary has been modified by an artificial storm-surge barrier, implying a decrease in current velocity and leading to shifts in phytoplankton composition and densities in the eastern part. It is hypothesized that normal conditions for growth of nauplii and smaller copepodids of Temora in this section will continue in the future, but that those for growth and development of adults may become less favourable.

Zooplankton grazing pressure in the Oosterschelde (The Netherlands)

Grazing activity by the dominant zooplankton species in the Oosterschelde (SW Netherlands) was studied as part of an ecological survey accompanying the construction of a storm-surge barrier at the mouth of this tidal inlet. At station WEST, situated at the mouth, potential grazing pressure by the zooplankton amounted to 6% d−1 of the phytoplankton standing stock and 18% of the primary production. At station EAST, in the inland part, 16% of the phytoplankton standing stock and 11% of the primary production was potentially consumed daily. Because the dominant zooplankton species in the inland part (Acartia spp. and cirriped nauplii) tended to feed more on small particles than the dominant species in the seaward part (Temora longicornis and Centropages hamatus), the potential grazing pressure on small phytoplankton (< 20 μm Spherical Equivalent Diameter) was 3 times higher at station EAST than at station WEST. Ecological consequences of this difference are discussed in relation to developments to be expected in the post-barrier situation.

Daily fecundity of Temora longicornis (Copepoda calanoida) in the Oosterschelde estuary (SW Netherlands)

Abstract Daily fecundity of Temora longicornis in the Oosterschelde was estimated in 1986 and 1987 after 24 h incubation of female copepods collected in the estuary. Daily egg production fluctuated between 3 and 25 eggs per female. Egg production rates increased with temperature in the range of 3 to 15°C when algal food was present in excess. Limiting food conditions were found at the end of the phytoplankton spring bloom and occasionally in summer at concentrations of ⩽2 μg chlorophyll·dm −3 . When chlorophyll maxima were found, predominantly consisting of Phaeocystis , daily fecundity of Temora was suppressed. It could increase, however, in Phaeocystis blooms mixed with diatoms ( Rhizosolenia delicatula ). Standing stocks of eggs in the springs of 1986 and 1987 were calculated on the basis of the established temperature/fecundity relation and compared with observed numbers, both in the years 1986 and 1987 (estuary provided with a storm-surge barrier in the mouth) and in the years 1983 and 1984 (open estuary). Smaller differences between observed and calculated egg numbers in 1986 and 1987 could be ascribed to possible errors in calculated egg development time and to the different age structure of females in the course of time. Much larger differences were found between the years 1986–1987 and 1983–1984. These are discussed in relation to the following phenomena: sinking of eggs, cannibalism, predation, food quantity and availability, density-dependent effects, hydrodynamical changes of the estuary. Arguments are given for density-dependent factors being responsible for the strongly reduced egg numbers in 1984.

Comparison of brackish water plankton assemblages of identical salinity ranges in an estuarine tidal (Westerschelde) and stagnant (Lake Veere) environment (S.W.- Netherlands). I. Phytoplankton)

SummarySpecies composition, abundance and seasonal distribution of the phytoplankton of brackish water ecosystems depend not only on salinity but on several other environmental factors too.In stagnant brackish waters (Lake Veere) the easily established stability of the water masses proved to be an ecological masterfactor in relation to phytoplankton development, equaling salinity in importance.In tidalbrackish waters (Westerschelde estuary) turbulence acts as an ecological masterfactor in relation to phytoplankton development, dominating salinity in importance within the chosen salinity ranges.

Some reflections about the structure of the pelagic zone of the brackish Lake Grevelingen (SW-Netherlands)

SummaryThe seasonal succession of the plankton in the marine brackish Lake Grevelingen, a closed sea arm in the S.W.-Netherlands, comprises the initial stagessensu Margalef and is characterized by predominantly small phytoplankton (flagellates, diatoms) and zooplankton (rotifers, tintinnids, copepods), maintaining relatively high levels of production from early spring (February) to late summer (September). The structure of the plankton in the course of seasonal succession is in agreement with the concepts of Margalef.Simplification of the pelagic food web in Lake Grevelingen has occurred as a consequence of the elimination of the tides. Some examples are given in relation to the composition of the phyto- and zooplankton and of its significance. The occurrence of rotifer-dominated zooplankton blooms in early spring is emphasized.Closed sea arms such as Lake Grevelingen, showing the same morphometry as the previous tidal estuary, contain extended shallow areas which influence strongly the pelagic zone. The abundance in the zooplankton of larval stages of several littoral-benthic species demonstrate these influences clearly. The shallows of the lake, occupied by eelgrass beds (Zostera marina) in summer, influence the pelagic zone in several ways: large quantities of detritus are given off after the growing season, sheltered habitats are supplied for small pelagic animals, and eelgrass leaves represent a substrate for epifauna species.

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.

General tendencies of phyto- and zooplankton development in two closed estuaries (Lake Veere and Lake Grevelingen) in relation to an open estuary (Eastern Scheldt). S.W. Netherlands

In previous papers (BAKKER and DE PAUW, 1974; 1975), comparisons were made between the plankton assemblages of tidal (Western Scheldt estuary) and stagnant (Lake Veere) brackish waters in the Delta area of t h e S.W. N e t h e r l a n d s , i n r e l a t i o n t o s p e c i e s c o m p o s i t i o n , s e a s o n a l d i s t r i b u t i o n and s i z e o f t h e s t a n d i n g s t o c k s . Lake Vee re was t h e f i r s t Delta lake that was created; its closing occurred in 1961. Since 1971 a second, much larger, brackish lake exists: Lake Grevelingen. NZENHUIS (1978a) gave a survey of the results of ten years of investigation in the Grevelingen, executed by the Delta Institute for Hydrobiological Research at Yerseke. Data on nutrients and primary productivity of the previous Grevelingen estuary and the present lake have been given by VEGTER (1976; 1977). BAEKR (1978) published a paper about the structure of the pelagic zone of the lake and NIENHUIS (1978b) discussed the changes of the ecosystem after the closure of the estuary. In the present study general tendencies of phytoplankton and primary productivity as well as of zooplankton development in both brackish

Interpretation of particle spectra of electronic counters by microscopical methods

Abstract1.Using monocultures or single species dominated natural phytoplankton, cell counts and volume estimations obtained by visual and electronic methods show reasonable agreement. Calibration seems possible (Figs. 1–2).2.Further examples given (Fig. 3–4) show that microscopical identification of the volume peaks in electronic counter spectra of natural seston is not quite simple. Phytoplankton peaks may not be detected in the Coulter spectrum. Shifts of Coulter peaks to the left side of the visual spectrum may be found when cylindrical, elongated or needle-like phytoplankton dominate the sample.3.Both visual and electronic methods include potentially large errors. Possibly particle volume is either overestimated by the microscope and/or underestimated by the Coulter counter (Figs. 1a, 2, 3b, 4b, c; Table 2). In grazing studies both methods should be employed. Mutual corrections may be possible, based on the type of the seston present (size and nature of phytoplankton cells and detritus particles). In each case both techniques can yield complementary information about the seston investigated. When performing multitube analysis, screening tests of the samples as described by VANDERPLOEG (1981) are recommended.4.Detritus, especially the different types of aggregated particles, offers severe problems. In the analysis of detritus-rich samples both methods give unreliable results.5.In most cases estimates of volume, obtained by microscopical and electronic methods, are used as auxiliary parameters. It is the relation between microscopical or Coulter volume and other parameters (e.g. Coulter volume versus POC and phytoplankton volume versus chlorophyll content) that can give useful ecological information.