The recovery of benthos following the impact of low oxygen content in the German Bight

The oxygen deficiency in the German Bight and in Danish waters 1981–1983 with ensuing benthos mortality was the reason for studying the development and re-establishment of the macrofauna in the following years. These years, from 1984–1987, exhibited more favourable oxygen conditions.The macrofauna of this region with its predominantly sandy substrate belongs to the Tellina fabula community. It is dominated by regular seasonal and ephemeral species such as Spio filicornis, Phoronis spec., Spiophanes bombyx and Lanice conchilega.In 1983, a year with exceptionally low oxygen content in bottom waters (< 1 mg O₂ dm⁻³), the macrofauna showed, in some parts of the investigated area, as reduction of approx. 30–50% in species numbers. Concurrently, the individual numbers of the remaining species were reduced. Beginning in 1984, a rapid recovery of benthic communities was observed. By 1986, biomass as well as species- and individual numbers rose to values similar to those determined by other authors in earlier surveys of the German Bight. In particular, juvenile Echinocardium cordatum and crustaceans have been observed since 1984 in larger numbers.     

Denitrification and nitrous oxide in the North Sea

In situ sediment denitrification rates were determined in the major areas of deposition of the North Sea, using the acetylene block technique. In addition, nitrous oxide profiles of the water column were determined. Nitrous oxide production generally occurred in the photic zone possibly due to nitrification; and throughout the water column in the German Bight region. Consumption at depth was possibly due to reduction in the anoxic microzones of faecal pellets, concentrated at the thermocline. Saturation of surface waters was 102.2% compared to 130.3% in the German Bight region. Calculated flux of nitrous oxide to the atmosphere was 9.5 × 10⁶ kg yr⁻¹, over half of which was produced in the German Bight.Sediment denitrification rates varied through three orders of magnitude; the highest value of 150 μmol m⁻² d⁻¹ was recorded in the Norwegian Trench. Nitrous oxide production by the sediments was low (1.1 μmol m⁻² d⁻¹ max.), and was undetectable at half of the sites. Sediment nutrient profiles exhibited porewater nitrate concentrations exceeding that of the overlying water suggesting that denitrification was fuelled by nitrification, which, in turn was related to other environmental variables. A significant positive relationship existed between in situ denitrification rate and the nitrate content of the upper sediment. Extrapolation of the rate to the total area of deposition in the North Sea suggests that denitrification is responsible for a minimum loss of 7.5–12% of the total annual nitrogen contaminant input.     

Primary production and nitrogen assimilation in the North sea during July 1987

An extensive survey of the whole of the North Sea, carried out in July 1987, is described. The study concentrated on the measurement of surface water inorganic nutrient concentrations and concomitant rates of primary production (¹⁴C) and nitrogen assimilation (¹⁵N). Primary production was investigated using size fractionation techniques. Three vertical profiles of primary production and nitrogen assimilation were also investigated.Much of the North Sea exhibed thermal stratification. Surface nutrient concentrations were low and chlorophyll concentrations typically <1 mg m⁻³. More than 75% of the primary production was attributable to cells <5 μm in diameter. Ammonium assimilation accounted for most of the nitrogen assimilation.The water column was vertically well mixed in the coastal zones. Here, inorganic nitrogen concentrations were high (e.g. up to 25 μmol NO₃⁻ with chlorophyll concentrations up to 10 mg⁻³, and organisms >5 μm diameter accounted for most of the primary production. As in offshore regions ammonium accounted for the major part of the nitrogen assimilated.A 115 km section obtained using an undulating oceanographic recorder showed that in certain regions of the North Sea physical features acted to increase the dependence of the phytoplankton on nitrate.     

Cyclic organochlorines in the hermit crabs Pagurus bernhardus and P. pubescens from the North Sea. A comparison between winter and early summer situation

The body burdens of 5 cyclic organochlorine compounds were determined in abdomens of Pagurus bernhardus and P. pubescens from the North Sea during two seasons. The seasonal bioaccumulation of ΣPCB (as the sum of concentrations of 24 individual components) is influenced by the substances associated with food originated from the spring plankton bloom and the resuspended sediments during the winter.The patterns of individual PCB congeners depended on the weight of the abdomens, and the geographical position of sampling. In offshore samples, the findings indicated a relationship between the weight of the abdomen (roughly the age) and the bioconcentration of higher chlorinated biphenyls. The results from the coastal areas demonstrated that the uptake by food determines the steady state concentration of the higher chlorinated congeners in the hermit crab. Pagurus pubescens probably possesses a mixed-function oxygenase (MFO)-system different from that of Pagurus bernhardus because the congeners 2,2′, 3,3′, 4,5,6′-heptachlorobiphenyl (174) and 2,2′, 3,3′, 4′, 5,6-heptachlorobiphenyl (177) were absent in all samples of the former species but were always present in the latter.The geographical differences between concentrations of ΣPCB reflect the general mean-routes of water and suspended particular matter transport through the North Sea: the Southern Bight and the continental coasts were more polluted with PCBs than the central North Sea. Maximum concentrations of p-p′-DDE were found in the southern parts and parts of the central North Sea. Samples from the German Bight showed the highest concentrations of Lindane. The results indicate that the atmospheric transport of lower chlorinated biphenyls, HCB, α-HCH and Lindane may be important in the distribution of these components, because they only showed slight gradients or even none at all in the area investigated.     

Environmental characteristics and distribution of macrobenthos in a mudflat of the west coast of Korea (Yellow Sea)

The spatio-temporal changes of environmental factors and the distribution patterns of macrobenthos were investigated on an intertidal mudflat near Panweol on the west coast of Korea. Temperatures of the surface sediments and salinities of the intertitial water were highest in summer and lowest in winter, while water content of the sediment was lowest in summer and highest in winter. Hourly variations of sediment temperature, water content and interstitial salinity during the daytime depended largely upon both the timing of ebb tide and daylight intensity.Temperature, water content and interstitial salinity showed a marked variation with sediment depth. Temperatures decreased sharply from the surface to 10 or 15 cm depth, and below this depth they were nearly constant, except in summer. Water content and interstitial salinity underwent a rapid decline in this upper layer and began to rise from this depth. From the high tide mark toward the main tidal channel, water content increased but salinity decreased. The sand fraction increased with the decrease of clay content toward the main tidal channel. This tidal flat could be divided into two zones: the “upper intertidal zone” and the “lower intertidal zone”, based on the spatial differences of the water content, interstitial salinity and grain-size composition in the surface sediment.There were three distinct zones of macrobenthos. The upper intertidal zone was dominated by burrowing deposit feeders, such as Ilyoplax dentimerosa, Helice tridens sheni, Cleistostoma dilatatum and Perinereis vancaurica tetradentata. The middle intertidal zone was characterized by other burrowing deposit feeders, such as Ilyoplax pingi, Macrophthalmus japonicus and Periserrula leucophryna. And the lower intertidal zone was dominated by suspension feeders represented by Laonome tridentata and Potamocorbula amurensis. Each zone was named after these characteristic species.Although the species composition of the upper intertidal community was different from that of the middle intertidal, the way of living and the feeding behaviour of the species occurring in these two zones were very similar. But the lower intertidal zone showed a characteristic community structure differing not only in species composition but also in ways of living and feeding of the species. Some sets of environmental factors are considered to explain the zonation. Water content of the sediment and interstitial salinity will be the main factors causing the observed zonation.     

Competitive interaction between Ditylum Brightwellii and Skeletonema Costatum by toxic metabolites

Comparative growth experiments were carried out in order to examine the role of toxic metabolites in the competition between two marine diatom species. Ditylum brightwellii and Skeletonema costatum exhibited mutual inhibition and auto-inhibition. Charcoal filtration did not entirely remove the toxicity. Algal extracts were more toxic than algal filtrates. Cell lysis induced by osmotic-shock treatment caused auto-inhibition in a dense culture of D. brightwellii; cells of this species recovered from a low salinity treatment after addition of charcoal to a culture. In mixed cultures the growth of both species may be affected by mutual inhibition. Toxicity of media depends on the growth phase of the competitors. In dense cultures, comparable with algal blooms in eutrophic waters, exocrines may be more effective than in diluted cultures (cf. mesotrophic waters.) Substances excreted in dense blooms of S. costatum may inhibit competing species.     

Headspace fingerprint mass spectrometry to characterize strawberry aroma at super-atmospheric oxygen conditions

This paper reports on the relation between super-atmospheric oxygen packaging and aroma production in strawberries. Volatile compounds were extracted from the headspace of strawberries stored at 7 °C for 0, 4 and 7 days under different gas conditions. Super-atmospheric oxygen concentrations were applied in combination with or without elevated CO₂ concentrations and the volatile production was measured with GC–MS. The potential of headspace fingerprint mass spectrometry (HFMS), a rapid technique for aroma profiling, was evaluated. Canonical discriminant analysis (CDA) based on the 16 most abundant volatile compounds and 26 HFMS signals (m/z) was applied to discriminate the samples according to gas storage treatment. The results showed that after 4 and 7 days of storage under super-atmospheric oxygen concentrations (without carbon dioxide) the production of ethyl acetate was suppressed. Ethyl acetate is one of the most important off-flavors in strawberries. Samples treated with only oxygen were mainly characterized by the ester methyl hexanoate with pineapple notes. Similar results were obtained with HFMS. CDA also showed that it is possible to classify samples according to days of shelf-life and that the effect of gas conditions was smaller than the effect of days of shelf-life.     

Diversity of the nematode communities in the Southern Bight of the North Sea

The nematode communities from 102 stations in the Southern Bight of the North Sea, sampled between 1972 and 1984, are examined. Overall, 456 species, belonging to 159 genera and 37 families were found.Sediment structure and chlorophyll a content were found to be determining environmental factors in the species composition of the nematode communities.Diversity is determined at different levels of the nematode community; i.e. species diversity of the whole community, species diversity of eight dominant families (Chromadoridae, Comesomatidae, Cyatholaimidae, Desmodoridae, Microlaimidae, Oncholaimidae, Thoracostomopsidae and Xyalidae), species diversity of the four feeding types, family diversity and trophic diversity within the whole community.The open sea stations are characterized by nematode communities which are comparable in terms of species diversity; the number of species S is about 30–35 per sample (sample size is about 100) with H′ between 4.00–4.50 bits/ind. The communities in the coastal stations are less diverse (S=7−22); H′=1.4−3.5 bits/ind.). The stations along the Belgian east coast have communities with low diversity (S=7; H′=1.41 bits/ind.) and with a pronounced dominance of a few species (Simpson index=0.54); the diversity within the eight families is also very low.The relationship between environmental stability (or disturbance) and stability and diversity of the nematode communities can be explained by the following factors; habitat heterogeneity, food availability, productivity, density and population growth rates.     

Cell abundance and fluorescence of picoplankton in relation to growth irradiance and nitrogen availability in the red sea

The vertical distribution and cellular fluorescence characteristics (chlorophyll and phycoerythrin, PE) of picoplankton (cyanobacteria and prochlorophytes) in the southern Red Sea were investigated in relation to physico-chemical properties of the water column. At all stations two subpopulations of Synechococcus sp. (type A and B) co-occurred, with maximal numbers up to 75 000·cm⁻³. Type B, with dim fluorescence signals, dominated the surface waters whereas type A, with bright fluorescence signals, dominated at greater depth. The divinyl a and b containing Prochlorococcus sp. peaked below the cyanobacteria at the deep chlorophyll maximum (DCM) with maximal cell numbers of 276 000·cm⁻³.Due to thermal stratification the cellular fluorescence (chlorophyll and phycoerythrin) increased with decreasing growth (PAR) irradiance, in an S-shaped manner, but magnitude and slope for the three picoplankters differed. The Synechococcus sp. type B had only a moderate increase in chlorophyll and phycoerythrin fluorescence signals with depth (3.4 and 6.6 fold, respectively), with values saturating at 3% (L_d) of the surface irradiance. The deeper-water type not only possessed much higher values for cellular fluorescence than the B type, but the increase with decreasing light level was also much higer (for chlorophyll by a factor of 11 and PE increased by a factor of 23). In addition, maximal values for these fluorescence signals occurred at an isolume of 1 to 0.5%. These differences in concentrations and responses of the pigment content to the prevailing light climate explain the variation in abundance of both types over the water column.Although the prochlorophytes dominated almost the entire euphotic zone, their adaptation to low light levels was even better than in the two types of cyanobacteria. With depth their increase in chlorophyll fluorescence was similar to that observed in the cyanobacteria (with an increase from surface to bottom of the euphotic zone by a factor of 25). The maximum fluorescence signal was not reached until an I_d of 0.01%. The great ability of the prochlorophytes to adapt their pigmentation to changing light and nitrogen conditions suggests that they can maintain growth under a wide range of environmental conditions.     

Inshore and offshore diversity of epibenthos dredged in the North Sea

Large epibenthos (>1 cm.) was dredged in the German Wadden Sea and in adjacent offshore areas at 10 to 30 m depth in Helgoland Bight. There were apparent differences in species composition and relative abundances between the 2 regions. The hypothesis of increasing species richness in offshore direction and of increased individual numbers inshore was tested. Neither proposition was found to be generally true.Regional species number was higher offshore (85) than inshore (73). However, a direct comparison of 11 + 11 localities and of 66 + 66 dredge hauls revealed no significant differences. Ratios of species richness for localities versus regions and for hauls versus localities differed significantly, indicating a higher regional heterogeneity offshore and a higher habitat heterogeneity inshore. Thus, causes for an overall similar diversity were found on different spatial dimensions.Few species were encountered on bottoms subject to the riverine waters of Elbe and Weser. Many decapod crustaceans attained stronger populations inshore, while ophiuroids completely dominated the epibenthos offshore and caused higher individual numbers there. Historical sources suggest that the epibenthos was richer in the past than it is now.     

Carbon isotope discrimination: potential for screening salinity tolerance in rice at the seedling stage using hydroponics

Rice is a moderately salt‐sensitive crop species and soil salinity is the single most widespread soil toxicity problem lacing rice production. The quantification of salinity resistance poses serious problems in the field because of climatic factors and field heterogeneity. In the present study. rice germplasm obtained from the International Rice Research Institute (IRRI). Philippines, was screened in a naturally lit (11 h daylight) glasshouse‐based hydroponics unit at two salinity levels (4 and 6 dS/m), Phenotypic performance based on survival of tolerant, moderately tolerant and susceptible isogenic lines along with tolerant and susceptible parents was evaluated after 10 and 13 days of salt stress. Plants were harvested after second scoring and carbon isotope discrimination in the leaves (A) was measured. δ¹ ranged from 19.5 to 22.9%, A highly significant negative correlation (r =?0.95. P < 0.001) between δ and visual scoring was observed. Data indicated the potential of using δ as a physiological indicator for salinity tolerance in rice seedlings grown in hydroponics.     

Production and consumption of tropical seagrass fields in Eastern Indonesia measured with bell jars and microelectrodes

During the Indonesian-Dutch Snellius-II Expedition the production and consumption of tropical seagrass species were measured with bell jars at four sampling stations in eastern Indonesia. Applying a conversion factor of 0.29, the amount of carbon fixed and mineralized was calculated from the recorded oxygen evolution. The gross production of the different seagrass communities was found to be between 1230 and 4700 mg C·m⁻²·d⁻¹. The consumption lay between 860 and 3860 mg C·m⁻²·d⁻¹. From these data a relatively low net production of 60 to 1060 mg C·m⁻²·⁻¹ could be calculated. At one sampling station incubations were carried out at different depths in a sloping Halodule uninervis (Forssk.) Aschers meadow, which indicated that seagrasses above a depth of about 2 m may become subject to photoinhibition. A linear correlation between biomass and measured production was found for Thalassia hemprichii (Ehrenb.) Aschers. Above a biomass of 100 g DW·m⁻² the production per unit of biomass decreased due to self shading.Using microelectrodes the oxygen production of epiphytes was found to be 230 mg C·m⁻² leaf surface·d⁻¹. at 1900 μE·m⁻²·s⁻¹ assuming an epiphyte coverage of 40%. This indicated that up to 36% of the primary production in a seagrass community may be attributed to epiphytes.The seagrass fields in Indonesia were found to be healthy ecosystems with a high primary production, but organisms within the communities use the abundance of organic matter very efficiently, creating net production rates of 100 to 300 mg C·m⁻²·d⁻¹ which are similar to barren areas.     

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.     

Long-term studies on Noctiluca scintillans in the German Bight population dynamics and red tide phenomena 1968–1988

The seasonal development of Noctiluca scintillans has been studied by continuous monitoring at Helgoland-Roads over a period of 20 years. These long-term studies have revealed a temporal distribution pattern of seasonal periodicity with maximal abundances in June/July, and with a low but pronounced peak in December/January. Additionally, over a 21-year term a periodicity of high abundances in three years intervals is observed. In Helgoland waters the mean near surface abundances are higher than the near sea bed values. Apart from physico-chemical influences, buoyancy in Noctiluca is also controlled by its nutritional status.Since 1981, investigations have been extended to the German Bight. Potential centres of Noctiluca growth are found within certain areas near the east-frisian Wadden Sea, persisting from April/May until late July when the centres of high cell concentrations are separated from the coastline. Simultaneously, there is a strong tendency for vertical separation, generating the well-known red-tide phenomena with large mass aggregations at the sea surface, often misleadingly called Noctiluca-blooms, although the cell masses are irreversibly damaged and die off very rapidly.The spatial distribution of Noctiluca as determined every year in July, serves as a realistic base for biomass calculations. From 1981 to 1988 the total per season produced biomass (ashfree dry-weight) of Noctiluca ranges between 30000 (1981) and 69000 tons (1982).     

Radon profiles in the Indonesian archipelago

A series of near-bottom excess radon profiles was measured in the Indonesian archipelago during the Snellius-II Expedition. The results show considerable variations in structure, ²²²Rn concentrations and integrated excess ²²²Rn inventory in the water column. Near-bottom vertical eddy diffusion coefficients vary from 46.4 to 63.6 cm²·s⁻¹ in the basins; they are in general much higher on slopes and sills. The standing crop (the integrated amount of excess ²²²Rn) ranges from 0.9 to 49.6 dpm·cm⁻². ²²⁶Radium supported ²²²Rn concentrations range from 9 to 3888·10⁻² dpm·kg⁻¹ Compared to mid-ocean basins the vertical mixing processes of near-bottom water masses in the Indonesian archipelago is fast, especially in the water layer from 50 to 200 m above the bottom, with diffusion coefficients in the order of 10 times higher than in the Pacific Ocean. With the exception of two stations standing crop values were comparable with those found in the Pacific Ocean but higher than those from the Atlantic Ocean. ²¹⁰Pb analyses show high sedimentation rates, up to 75 cm·10⁻³·a⁻¹. This does not agree with related standing crops. One explanation can be that erosion plays an important role on the sills and slopes.     

Particulate and dissolved organic carbon and chlorophyll A in the Zaire river,estuary and plume

Data were collected on POC, DOC and phytoplankton in the Zaire river, estuary and plume. Mean river value for POC was 1.1 mg·l⁻¹, 4.7% of the suspended matter. Average DOC content of the river water was 8.5 mg·l⁻¹. These values are in accordance with the calculations of TOC input from rivers to the world's ocean. Within the estuary POC and chlorophyll decreased regularly up to a salinity of 20. Between salinities of 20 and 32 small phytoplankton bloom occurred resulting also in higher POC values.DOC mixed conservatively up to a salinity of 25; at salinities above 25, values indicate DOC production. This DOC production occurred partly in the bottom water of the canyon where low oxygen values indicated mineralization and conversion of the accumulated POC into DOC. Another area of DOC production observed inside and outside the surface waters of the plume, was partly related to autolysis and degradation of the phytoplankton bloom.This study shows that the influence of rivers on the organic carbon in the ocean will not be confined to the amount introduced directly, but that we have to add the amounts of POC and DOC resulting from enhanced phytoplankton primary production by nutrient input from rivers and by river induced upwelling.     

The effects of bioturbation on sediment transport on an intertidal mudflat

Laboratory flume studies were conducted to determine the relative importance of various biological and physical factors controlling sediment erosion threshold and transport rate on an intertidal mudflat in the Bay of Fundy. Studies were conducted in a July period of maximum solar exposure. The upper and mid-intertidal stations of the flat were dominated by silt-clay sediments, while the lower intertidal was dominated by very fine sand. The tube-dwelling amphipod Corophium volutator was the most abundant infaunal species with densities exceeding ∼1300 ind·m⁻² based on counts of burrow openings. Sediment-penetrometry and water-content measurements indicated no change in unconsolidated shear strength and porosity, respectively, along the intertidal transect. Despite the apparently cohesive nature of the sediment, erosion occurred as small ripples. Critical shear velocities (u_crit) for erosion determined with intact cores in a laboratory flume were relatively consistent between stations and sampling dates (mean = 2.1 cm·s⁻¹ ± 0.2 SD), with no relationship to Corophium density, sediment chlorophyll a, or physical variables. Field-treatment of sediment with formalin did not cause an obvious change in u_crit as determined by flume experiments. Corophium seemed to have little effect on erosion thresholds because incipient motion could be observed between tube burrows, beyond the local influence of the amphipod. In contrast to erosion thresholds, sediment-erosion rates measured with bedload traps were negatively correlated with density of small Corophium, probably due to binding of sediment into burrows and the ambient sediment microfabric, all of which reduce the availability of sediment for transport. Adult amphipods, which occurred at low density probably due to territorial/competitive interactions, had no obvious effect on erosion rate since only a small proportion of the sediment surface was impacted by their bioturbation. Although a portion of the amphipod population exerts a stabilizing influence on sediment-erosion rates, concurrent studies at the site indicate that Corophium seasonally reduces the sediment-erosion threshold by grazing on microflora which would otherwise inhibit the initiation of grain motion. Due to this decoupling of erosion rate and threshold, it is necessary to measure both processes in assessing the effects of benthic biota on sediment transport.     

Organochlorines in benthic polychaetes (Nephtys Spp.) and sediments from the southern North Sea. identification of individual PCB components

ΣPCB concentrations (as the sum of concentrations of 35 individual components) on a lipid basis varied more than an order of magnitude between samples of polychaete worms (Nephtys spp.) from different stations in the southern North Sea. PCB patterns, i.e. the relative contribution of each component to ΣPCB, were highly similar for the larger part of the area.The geographical differences between concentrations of PCB's reflected the general routes of residual water and suspended particulate matter transport throughout the North Sea. The Rhine is likely to be the main source for PCB's in Dutch coastal waters, including the entire coastline formed by the Dutch Wadden Sea islands. Theoretical patterns of PCB's in solution were calculated from the PCB patterns in Nepthys and values of bioconcentration factors reported.Concentrations of HCB, α- and γ-HCH, dieldrin, p,p′-DDD and p,p′-DDT were at least an order of magnitude lower and close to, or even below, detection limit in most samples. Although concentrations of α-and γ-HCH in solution were the highest of the compounds investigated, their concentrations in Nephtys were among the lowest, which might have been due to their relatively high solubility in water, or metabolism to more polar derivatives.Based on total sediment analyses, organochlorine concentrations in Nephtys spp. were not correlated with the concentrations in the (type of) sediment which they inhabited. In sediments the fraction of grains <63 μm appeared to be very important.A sewer outlet near Grimsby (U.K.) was found to be a source for HCB and Σ-DDT.     

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

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.     

Interactive Effects of High Boron and NaCl Stresses on Subcellular Localization of Chloride and Boron in Wheat Leaves

Salinity and boron (B) toxicity often occur simultaneously and may have interactive effects on plant responses. This study aims at further investigating long‐term effects of salinity, B toxicity and their combination on wheat plants. Plants grown in hydroponics were treated with 2.5 μm H₃BO₃ (control), 75 mm NaCl, 200 μm H₃BO₃ or 75 mm NaCl + 200 μm H₃BO₃ and were analysed on a subcellular level 6 weeks after germination. Shoot fresh and dry weight (DW), water content, transpiration rate and osmolality were reduced, while Na⁺ and Cl^? concentrations increased under salinity stress. However, Cl^? was reduced in all compartments by an additional application of B toxicity. At adequate B supply, NaCl increased apoplastic and symplastic soluble B concentrations, whereas the total B content remained unchanged. At high B level, however, soluble and total B was reduced by additional salt stress. Despite this antagonistic effect of a combined salt and B toxicity stress on Cl^? and B concentrations, an additive effect was observed regarding shoot fresh weight reduction. Our results confirm an alleviating effect of the combined stresses on toxic ion concentrations, which did not prevent additive growth reductions.