Year-to-year variability in the biomass of macrobenthic animals on tidal flats of the Wadden Sea: How predictable is this food source for birds?

The Wadden Sea is important as a stop-over and wintering area for several species of waders, foraging for shorter or longer times on its tidal flats. The size of the food stocks these birds encounter varies from place to place and from year to year. We studied characteristics of the variability in time of such prey stocks, using long-term data series of annual estimates of biomass of macrobenthic animals collected on tidal flats in various parts of the Dutch and German Wadden Sea. Year-to-year fluctuations were stronger in nearly all individual benhos species than in total macrozoobenthic biomass. The various species differed significantly in their year-to-year variability. Everywhere the tellinid bivalve Macoma balthica was relatively stable, whereas species such as the cockle Cerastoderma edule and the polychaetes Lanice conchilega, Nephtys hombergii and Anaitides mucosa fluctuated heavily and rapidly in all parts of the Wadden Sea where they were found. Within species, fluctuations in biomass of individual age or size classes were stronger than those in total biomass of the species.Several species showed minimal biomass values in the same years over vast areas. This synchronization of scarcity was caused particularly by similar responses to winter character, which was each year similar over the entire Wadden Sea. Such similar responses included low over-winter survival during severe winters and recruitment failure after exceptionally mild winters. Immediately after severe winters, such cold-sensitivee species as C. edule, L. conchilega and N. hombegii were scarce all over the Waden Sea, thus liimiting the possibilities for birds to switch to other parts of the Wadden Sea to find their preferred prey in sufficient quantitu. Simultaneous recruitment failure in several important bivalve species (C. edule, Mytilus edulis, and Mya arenaria limited the possibility for specialized bivalve consumers to switch to alternative prey types in certain years.     

Scale and intensity of intertidal habitat use by knots Calidris canutus in the Western Wadden Sea in relation to food,friends and foes

In August–October 1988–1992 we studied the distribution and abundance of knots Calidris canutus around Griend in the western Wadden Sea, and the extent to which these can be explained by benthic prey availability and presence of avian predators. Numbers in the nonbreeding season showed monthly averages of 10 000 to 25 000 birds. Over 100 000 knots were recorded on three occassions. Knots feed in large flocks, individual birds usually experiencing 4 000 to 15 000 flock-mates. The Siberian-breeding/west-African wintering canutus subspecies passed through in late July and early August. Otherwise the Greenlandic/Canadian breeding islandica subspecies was present. Over the period 1964–1992 there were no clear trends in the number of knots, but canutus-knots were particularly abundant in July–August 1991, whereas in 1992 both subspecies were absent.Macoma balthica was the preferred prey of both subspecies. Hydrobia ulvae, Mytilus edulis and Cerastoderma edule were eaten when Macoma was absent close to the surface of the sediment. As Macoma buried deeper from July onwards, canutus faced better average feeding conditions than islandica later in the year. The spatial distributon of knots feeding on the intertidal flats around Griend was best explained by the harvestable biomass of the prevalent prey species in a particular year and season, i.e. Macoma (main prey when their harvestable biomass densities were greater than ca 0.8 g AFDM per m²) and Cerastoderma, and by the avoidance of situations where they run the risk of attack by bird-eating birds. Flocks of knots covered most of the intertidal flats in the Western Dutch Wadden Sea in a couple of tidal cycles. This is about 800 km², much larger than the equivalent area used by knots on their wintering grounds in Mauritania (10–15 km²), a difference that is correlated with prey spectrum, prey availability and predictability.     

Distribution patterns of two bivalve species (Nucula turgida,Tellina fabula) along a frontal system in the Southern North Sea

The distributional patterns of diverse marine soft-bottom invertebrates in the southern North Sea are considered in the light of relevant hydrodynamic and sedimentological conditions, with two species of bivalves, Nucula turgida and Tellina fabula as examples.Four possible mechanisms that may result in spatially differentiated abundances of benthic populations are discussed:

• 1.A. Settling larvae may act like passive suspended particles being deposited on the sea floor at sites where sediments with similar sinking rates would settle.
• 2.B. The upcurrent presence of rich sources of larvae, viz. dense adult populations.
• 3.C. Active habitat selection by settling larvae.
• 4.D. Spatially differentiated survival of larvae, once they have settled.

     

Cropping system effects of maize on infiltration,runoff and erosion on loess soils in South-Limbourg (The Netherlands): A comparison of two rainfall events

Two natural rainfall events are compared to evaluate the effects of three cropping systems of silage maize on soil moisture content, infiltration, runoff and erosion. Both rainfall events took place in early summer. One was a low intensity event with 27.6 mm of rain in 9 hours, and the other a high intensity event with 33.4 mm in 42 minutes. Cropping systems were:

• 1.(I) a spring tilled system (conventional),
• 2.(II) an autumn and spring tilled system with summer barley as spring cover crop, and
• 3.(III) an autumn tilled system with winter rye as winter cover crop and direct drilling of silage maize.

During the low intensity event, soil moisture content of the top 5 cm rose to field capacity on all three cropping systems. No runoff was generated. During the high intensity event, soil moisture content rose to field capacity on the two spring tilled cropping systems but was only slightly raised in the direct drill system, in spite of 17.7 mm of infiltrated rain. Runoff coefficients of the high intensity event were 41.7% (conventional system), 14.9% (autumn and spring tilled system) and 47.0% (direct drill system). The direct drill system showed a severely slaked soil surface in early summer, caused by winter rain.The response to rainfall of soil moisture content is ascribed to:

• 1.(I) a predominance of matrix infiltration on all cropping systems during the low intensity event and on the spring tilled systems during the high intensity event, and
• 2.(II) a predominance of infiltration via continuous macropores, open to the surface (of biologic origin), by-passing the soil matrix, on the direct drill system during the high intensity event.

The presence of continuous, vertical macropores on the direct drill system explains its surprisingly high infiltration capacity, considering its strongly slaked appearance. The smooth soil surface of the direct drill cropping system may have delayed infiltration during the flooded stage of the high intensity event by not providing vent points for the escape of soil air. Soil loss from the direct drill system during the high intensity event was only 15.6% of that from the conventional system. This is ascribed to low detachment rates of soil material by drop impact and/or overland flow, due to the presence of winter rye remains and, especially, the relatively high soil surface shear strength of the direct drill cropping system in early summer.     

Susceptibility of soils to physical degradation in Hungary

A simple category system was elaborated by the authors expressing the susceptibility of Hungarian soils to physical degradation, as structure destruction and compaction. The main soil characteristics which were taken into consideration in the category system were as follows: type, subtype and local variant of the soil, parent material, texture, hydro-physical properties, carbonate-, salinity/alkalinity- and organic matter status, and depth of the soil. Based on detailed territorial information on the above mentioned soil characteristics Hungarian soils were classified into 8 physical degradation sensitivity (susceptibility) categories:

• 1.1. Non-susceptible soils;
• 2.2. Slightly susceptible soils;
• 3.3. Moderately susceptible soils;
• 4.4. Soils susceptible to compaction and surface crusting, but not to structure destruction;
• 5.5. Soils susceptible to both structure destruction and compaction;
• 6.6. Soils susceptible to both structure destruction and compaction due to salinity and/or alkalinity;
• 7.7. Organic soils;
• 8.8. Shallow soils (solid rock or cemented layer near to the surface).

The map of these categories was prepared in the scale of 1:500 000.For medium (1:100 000) and large-scale (1:25 000 - 1:10 000) mapping a more detailed category-system (including more categories and subcategories with precise numerical limit values) will be elaborated.     

Increased mortality in alternative bivalve prey during a period when the tidal flats of the Dutch Wadden Sea were devoid of mussels

In the course of 1990, stocks of mussels (Mytilus edulis) declined to unprecedentedly low levels in the Dutch Wadden Sea. Hardly a wild mussel bed was left on the tidal flats as a consequence of three years (1988, 1989, and 1990) with failing recruitment and intensive fishing for seed mussels. During these three years, recruitment of cockles (Cerastoderma edule) also failed, whereas fishing was continued. Bird species taking these bivalves as staple food, the oystercatcher (Haematopus ostralegus) and the eider (Somateria mollissima), experienced food shortage. Significant numbers of eiders left the Dutch Wadden Sea area or died, whereas oystercatchers remained abundant trhoughout the winter in most of the Dutch Wadden Sea.Alternative prey species of oystercatchers experienced unsually high mortality rates in the appropriate size classes. This was so in all other common species of bivalves, viz. first-year and older cockles, adult Macoma balthica, and juvenile Mya arenaria. This led to minimal stocks of food for oystercatchers in the late winter of 1991. In March 1991, cockles were depleted and the combined stocks Mya and Macoma would soon have run out of food supply to the overwintering oystercatcher population. Apparently, oystercatchers are able to reduce the stocks of their various bivalve prey species to very low levels.     

Wind effects on prey availability: How northward migrating waders use brackish and hypersaline lagoons in the sivash,Ukraine

Large numbers of waders migrating northward in spring use the Sivash, a large system of shallow, brackish and hypersaline lagoons in the Black Sea and Azov Sea region (Ukraine). The bottoms of these lagoons are often uncovered by the wind. Hence, for waders the time and space available for feeding depend on wind conditions. In hypersaline lagoons the benthic and pelagic fauna was very poor, consisting mainly of chironomid larvae (0.19 g AFDM·m⁻²) and brine shrimps Artemia salina, respectively. Brine shrimp abundance was correlated with salinity, wind force, wind direction and water depth. Dunlin Calidris alpina and curlew sandpiper Calidris ferruginea were the only species feeding on brine shrimp. As brine shrimp densities are higher in deeper water, smaller waders such as broad-billed sandpipers Limicola falcinellus are too short-legged to reach exploitable densities of brine shrimp. In brackish lagoons the benthic and pelagic fauna was rich, consisting of polychaetes, bivalves, gastropods, chironomid larvae, isopods and amphipods (8.9 to 30.5 g AFDM·m⁻²), but there were no brine shrimps. Prey biomass increased with the distance from the coast, being highest on the site that was most frequently inundated. Dunlin, broad-billed sandpiper and grey plover Pluvialis squatarola were the most abundant birds in the brackish lagoon. Due to the effects of wind-tides only a small area was usually available as a feeding site. Gammarus insensibilis was the alternative prey resource in the water layer, and their density varied with wind direction in the same way as brine shrimp. Curlew sandpipers and dunlins in the hypersaline lagoons and broad-billed sandpipers in the brackish lagoons often changed feeding sites, probably following the variation in prey availability. Only because of the large size and variety of lagoons are waders in the Sivash always able to find good feeding sites.     

Size-differential control of phytoplankton and the structure of plankton communities

We provide evidence and discuss the possibility that the main factors determining food web structure in oligotrophic and eutrophic marine environments are:

• 1.-1. Small algae are better competitors for light and nutrients than larger algae.
• 2.-2. The potentially high reproduction rate of their predators makes the smaller algae more susceptible to grazing control by microzooplankton than larger algae.
• 3.-3. Larger algae escape from microzooplankton grazing, due to their size, but experience losses through sedimentation.
• 4.-4. Microzooplankton is an important food source for mesozooplankton in oligotrophic areas.

Basically, the maintenance system (also known as the retention or regenerative system), which consists of pico- and nano-algae, microzooplankton, carnivorous mesozooplankton and in which bacteria and heterotrophic nanoflagellates act as decomposers and mineralisers, is typical in consuming most fixed energy within the photic zone. The overflow response, being an extension of the maintenance system with larger micro- and macro-algae and herbi-/omnivorous copepods, shows extended export of fixed energy out of the photic zone through sedimentation. The maintenance system is present in all marine environments, whether light- and/or nutrient-controlled, well-mixed or stratified, and oligotrophic or eutrophic. Typical overflow response is observed during an upward shift of the import of the controlling factor, which is either light or nutrients.In agreement with the hypothesis of size-differential control it was found that light-controlled phytoplankton consisted mainly of small flagellates (<8 μm) during winter in the well-mixed central North Sea.The dominance of large diatoms during the early spring bloom in the Marsdiep area (The Netherlands) was not necessarily caused by their growth characteristics, but was due to their escape from size-selective microzooplankton grazing.     

An evaluation of intertidal feeding habitats from a shorebird perspective: Towards relevant comparisons between temperate and tropical mudflats

Macrozoobenthic communities of intertidal soft sediments are reviewed worldwide from the perspective of a mollusc-eating shorebird species. Based on 19 sites, total biomass figures varied between 5 and 80 g AFDM per m² (average 24 g AFDM per m²); no latitudinal trends are apparent. The contribution made by bivalves and gastropods varies between 1% and 99%, north-temperate intertidal flats having relatively more molluscs than tropical flats. Intertidal flats in the tropics contain a greater variety of taxa, with brachiopods in Indonesia and echinoderms in northwest Australia contributing significantly to biomass only there. Limits to the occurrence of avian predators of intertidal benthos are set by the harvestable fraction of the biomass on offer and the costs of living at a particular site. No systematic differences in the harvestable fraction of the total mollusc-biomass for a worldwide occurring shorebird species specializing on molluscs (knots Calidris canutus) were apparent between temperate and tropical intertidal areas, in spite of large differences in maintenance metabolism incurred by these birds. The harvestable fractions of bivalves in the two West African areas (Banc d'Arguin, Mauritania and Guinea-Bissau) tended to be high (23–84% of total biomass in six species), they were relatively low (2–52% in five species) in the temperate Wadden Sea and the tropical northwest Australian site. Harvestable biomass determines the intake rate of shorebirds, as illustrated by functional-response curves of knots feeding on two bivalves species. We argue that the collection of information on size-depth relationships along with faunal and biomass surveys at a range of sites is bound to greatly increase our understanding of both the biology of tidal-flat invertebrates and the resource base underpinning the spectacular seasonal migrations of shorebirds.     

Iso-erodent map of Zambia: Part I: The calculation of erosivity indices from a rainfall data bank

The first stage in the establishment of an iso-erodent map of Zambia is the development of a Zambian rainfall data bank, the calculation and discussion of three erosivity indices and correlating the erosivity indices with rainfall amount.The data bank is made up of 10 years of data from eight different meteorological stations representing the different rainfall zones in Zambia. The data bank includes

• 1.1. total rainfall amounts split into component rainfall amounts with constant intensity
• 2.2. kinetic energy calculations
• 3.3. the erosivity indices.

Three erosivity indices have been calculated: E130 (WISCHMEIER 1959), KE (KINNELL 1981) and ΣN_pI_p (LAL 1976). Daily amount of rainfall explained 96, 95, and 89% of the variation in those indices respectively.Although the three indices are of comparable value, advantages and disadvantages of each of them are discussed.This work will be the basis for modelling the monthly erosivity and finally for making the iso-erodent map of Zambia (see part II, this volume).     

Environmental effects of the growth rate of intertidal invertebrates and some implications for foraging waders

The paper describes effects of intertidal height and sediment type on growth rate of the bivalves Cerastoderma edule, Macoma balthica, Mya arenaria, Mytilus edulis and Scrobicularia plana, and of the worms Arenicola marina, Nephtys hombergii and Nereis diversicolor in the eastern part of the Dutch Wadden Sea. In most species, exposure time was negatively correlated with length growth, although interfering effects of sediment type could not be ruled out. When controlled for the effects of exposure time, clay content of the sediment appeared to affect the growth of all species, but in different ways. The variation was related to the foraging methods of the invertebrates. Foraging waders may use the spatial variation in growth rate of the invertebrates to optimize the exploitation of individual cohorts.     

Die kohäsion ungesättigter sandböden und deren beeinglussung durch organische substanz

It is well known in soil science, that organic matter contributes considerably to the stability of soil structure. There are also many methods to determine this stabilizing effect of soil organic matter. But as a matter of fact those methods are mostly developed under chemical or colloido-chemical aspects and do only satisfy the demands of special purposes. The physical and mechanical aspects are often neglected. In this work, however, the mechanical parameter “cohesion of soils” c is selected to describe the stabilizing effect of organic matter on soil structure of sandy soils.Organic matter (black peat) was mixed with a fine sand in gravimetric proportions of 0:100, 1:100, 3:100, 5:100, 8:100. The mixtures were then wetted with distilled water and left undisturbed for a few weeks. A natural sandy soil (Haplo-humod) was also taken, part of the sample was treated with H₂O₂ to destroy the organic matter, so the stabilizing effect of organic matter could be evaluated. The cohesion of all the samples was determined with an annular shearing apparatus under a constant soil water suction between 0 and 300 cm H₂O. Results were calculated using the Coulomb equation τ = c + σ_n tan ϕ⁰, where τ shear strength, c cohesion, σ_n normal stress, and ϕ⁰ friction angle respectively. The results show,

1. 1.1. all samples have no cohesion under saturated conditions (water suction = 0 cm H₂O);
2. 2.2. with all samples the cohesion at first increases with increasing soil water suction, reaches a maximum under a soil water suction of about 40 to 70 cm H₂O, and then decreases with further increasing soil water suction;
3. 3.3. organic matter enhances the cohesion of both the artificial mixtures of sand and organic matter as well as the natural sandy soil, the increase being the higher, the higher the content of organic matter.
4. 4.4. no difference between the artificial mixtures of sand and organic matter and the natural sandy soil was found.

     

Effects of land reclamation practices on physical properties of an acid,infertile Oxisol

Large areas of soils in tropical rainforests have been damaged by careless mechanical land clearing procedures. The objective of this study was to evaluate the effect on soil physical properties of various management practices to reclaim an acid, infertile Oxisol cleared by bulldozer. Crops would not grow on these soils due to low pH, poor fertility or poor physical properties. Properties in the 0- to 25-cm depth of the Typic Haplorthox soil, measured at the beginning of the study, were: 74% clay, bulk density 1.09 Mg m⁻³, pH 3.66, ECEC 8.9 cmol_c 1⁻¹, and available P 0.4 mg 1⁻¹. Land reclamation treatments included all combinations of six tillage practices and three fertility levels. Tillage practices were: hand hoeing to a depth of 15 cm; hoeing plus applying mulch; hoeing and incorporation of green manure; deep forking, i.e., hand digging to 30 cm with a digging fork; deep forking in alternate 20-cm wide strips; and roto-tilling to the 10-cm depth. Fertility levels were:

• 1.1. no fertilizer or lime added;
• 2.2. addition of lime to maintain an Al saturation near 40% and fertilizer to maintain nutrients just above their critical levels;
• 3.3. lime to neutralize all exchangeable A1 and fertilizer to satisfy the P fixation capacity of the soil and to maintain nutrients at optimum levels.

Soil physical properties monitored periodically while growing five consecutive crops were bulk density, mechanical impedance, and infiltration rate. Macroporosity was evaluated after growing the fifth crop. Deep forking and forking in strips decreased bulk density and mechanical impedance and increased macroporosity and infiltration rate. Deep tillage by forking resulted in the driest soil 10 days after rainfall and the rice crop on this treatment experienced temporary wilting. Increasing the fertility level increased root activity which in turn increased mechanical impedance (the soil was drier) and water infiltration rate. We conclude that the soil physical properties of this soil were damaged very little by the land clearing operation and that the inability of the soil to grow crops was due to other factors.     

The macrozoobenthos of the subtidal western dutch wadden sea. I. Biomass and species richness

During a one-year period in 1981–1982, a survey was conducted on the macrozoobenthos of the subtidal areas of the western half of the Dutch Wadden Sea. In total 80 species were found, half of them polychaetes. In terms of biomass, Mytilus edulis dominated the macrozoobenthos, with Hydrobia ulvae, Heteromastus filiformis, Carcinus maenas and Macoma balthica as other important species. Numerically important were also the polychaetes Pygospio elegans and Scoloplos armiger. Average macrozoobenthic biomass amounted to 43.7 g·m⁻² ash-free dryweight. This value is in the same range as values from intertidal areas in the Wadden Sea. The relatively high value in comparison with data from similar subtidal areas is attributed to the important mussel culture in the area.     

Seasonal and annual variation in phaeocystis pouchetii (haptophyceae) in the westernmost inlet of the Wadden Sea during the 1973 to 1985 period

Data on seasonal and annual variation in densities of Phaeocystis pouchetii s.I. in the Marsdiep tidal inlet of the western Wadden Sea are reported for 1973 to 1985 inclusive. Each year a spring peak was observed followed by one or more smaller peaks in summer; no peaks were observed after September or before March. In 1985 Phaeocystis colonies were for the first time observed in winter (January/February). Over the 12-year observation period, an increase was found in cell numbers during the spring peak and summer, as well as in the duration of the spring peak and the length of the period that Phaeocystis pouchetii occurred in the plankton. No relation appears to exist between temperature and the timing of rapid increase or occurrence of peaksof Phaeocystis. Such inconsistency might be explained, among others, by the occurrence of different strains of Phaeocystis pouchetii s.I. More taxonomic work is also needed to indicate whether antarctic and tropical records of Phaeocystis pouchetii pertain to the same species.     

Facies analysis of Nile delta continental shelf sediments off Egypt

This study evaluates the texture and coarse fraction composition of 108 bottom samples from the Nile delta continental shelf. In total 19 petrological variables were considered for each of the samples, and Q-mode factor analysis of the textural and mineralogical variables yielded 4 factors (facies groups):

• 1.Facies I. Fine and very fine sands, light minerals, heavy minerals and ‘glauconite’, extending from the shoreline to the inner shelf and decreasing away from the shoreline.
• 2.Facies II. Silty sand rich in biogenic components occupies the entire outer shelf and its contiguous lower terraces.
• 3.Facies III. Mud (silt plus clay), mica and ‘glauconite’, covering the middle shelf and the upper terraces.
• 4.Facies IV. Fine to coarse-grained sediments and ‘glauconite’, locally distributed along the coast off lake ldku, Burullus headland, and Damietta promontory and El Gamil.

The configuration patterns of facies I (delta-front), facies III (prodelta) and facies IV (distributary mouth bar or progradational coastal sand) help identify delta lobes related to former distributary branches of the Nile river (Canopic, Saitic, Sebennitic, Atribic and Mendisian). The 4 facies-forming shelf sediments have resulted from sediment dynamics, sea-level fluctuation and differences in sediment input and provenance.     

Management of soil organic matter in the farming systems of the low land humid tropics of West Africa: a review

Soil organic matter is the key to successful and sustained productivity of soils of the tropics. This is because soil organic matter positively affects structure, aggregation, porosity, microbial activity, pore size distribution and water retention capacity of the soil.Furthermore, soil organic matter is the major nutrient storage site for the low-activity-clay soils of the tropics and so affect nutrient retention capacity, availability and mobility of macro- and micro-nutrients. It increases the water use efficiency, and therefore attenuate runoff and erosion and consequently the productivity of the soil.The low land humid tropics is characterised by high temperature, high relative humidity, high rainfall intensity and high microbial activity which all encourage rapid mineralisation, depletion and erosion of organic matter leading to soil deterioration.Many cultural practices and operations encourage rapid depletion of soil organic matter while others are associated with soil organic matter build up. Practices like crop rotation, multiple cropping, mulching, alley cropping, following and farm yard manuring, encourage soil organic matter accretion. However, the effectiveness of these practices in increasing soil organic matter depends on:

• 1.(a) amount and frequency of residue application;
• 2.(b) the nature and C:N ratio of the mulching material, or manure;
• 3.(c) rainfall amounts, intensity and distribution, soil moisture and clay contents.

Land clearing with heavy machinery is associated with removal of biomass from the field, while conventional ploughing and harrowing lead to soil organic matter depletion. These practices should be discouraged or modified to reduce their negative effects on soil organic matter.Effects of green manuring and burning, on soil organic matter are questionable.Research is needed on processes and pathways of crop residue decomposition to provide clues to possible interventions aimed at management of soil organic matter. Work is also needed to improve the efficiency of agro forestry systems in accreting soil organic matter in soils of the low land humid tropics.     

Distribution and abundance of the zooplankton of the Ems estuary (North Sea)

From 1974 to 1977 a study was made of the abundance and the distribution of the zooplankton species of the Ems estuary (The Netherlands and the Federal Republic of Germany), an area of about 500 km² with extensive tidal flats.The most important component of the zooplankton consisted of holoplanktonic calanoid copepods with, during the summer, a significant contribution of meroplankton, mainly consisting of polychaete and cirripede larvae. Zooplankton abundance showed a marked seasonality, with a pronounced spring peak and a smaller late summer/autumn peak. In the low salinity area the spring bloom was dominated by Eurytemora affinis, which persisted in the salinities below S=5 upriver during the whole year. In the polyhaline area Acartia bifilosa was the main component of the spring bloom. The late summer maximum from August to October was dominated by Acartia tonsa in the meso- and polyhaline area and by A. discaudata and Centropages hamatus in the euhaline part of the estuary bordering on the Wadden Sea proper. The number of abundant (> 1000 ind·m⁻³) species increased from the inner, fresher part of the estuary towards the Wadden Sea but, except for April, average zooplankton density in salinities <18 was similar (2768 ind·m⁻³) to the density in salinities >18 (2817 ind·m⁻³).The distribution patterns show that there are only a few autochthonous species and that the majority of species is allochthonous and penetrates more or less deeply into the estuary from the Wadden Sea and North Sea, varying with species and season.     

Potential use of the scheduler plant stress monitor in soybean

Much of work relating plant temperature to plant water stress has been done in arid regions. In 1989–1990, investigations were carried out verifying the validity of the Scheduler Plant Stress Monitor made by the Standard Oil Engineering Materials Company of the United States, in irrigation timing, under Hungary's changeable weather conditions.Three different water treatments were used:

• 1.⊎ natural rainfall only (control),
• 2.⊎ lysimeter study, where the water supply is in accordance with the water demand of plants,
• 3.⊎ irrigation according to the instructions of the Scheduler.

The monitor compares plant and air temperature differences to calculate stress index and irrigation timing. The measured indices were corrected for wind speed.Taking the varied water levels into account, the highest grain yield occurred in canopies watered by using the Scheduler instructions. In 1989, the yield increase was more moderate (16.5%) than that of the yield surplus in dry 1990 (38.1%), comparing to the results of non-irrigated control.The water-use efficiency in plant stands grown by the Scheduler control fell between the results of non-irrigated control and lysimeter study, independent of weather conditions.Use of the Scheduler may be recommended mainly in arid growing seasons, where yield increase is high enough to compensate for the additional cost of irrigation water. In Hungary, where the weather is very changeable it cannot be stated with complete confidence that the Scheduler is always an effective tool in irrigation planning. To clarify this problem further work is needed.