Estuarine benthic ecology: A European perspective

The study of benthic ecology within European estuaries is discussed. The history of estuarine science, and the definition of an estuary is briefly reviewed. Estuarine benthic ecology is considered with regard to its geographical, economic and political context. It is shown that much recent study has been motivated by attempts to clean up estuaries, which were polluted after decades, or even centuries of pollution. Much has been achieved in the control of pollution, and many estuaries show clear signs of recovery. Particular focus has been on the estuaries entering the North Sea, with the realization that it is not the North Sea that is polluted, but its estuaries. The scientific appraisal of the benthos of estuaries has been particularly assisted by the ecosystem approach, which has both aided our understanding of their function as well as answering some of the questions posed by the impact of mankind. The European Union is identified as a powerful factor in benthic ecology, either through legislation or through the encouragement of collaborative programs, some of which have been particularly important in identifying common factors across a wide geographical spread of estuaries. It is concluded that in general estuaries have got cleaner, but that many problems remain, ranging from eutrophication through to sea-level change. The future of estuarine benthic ecology lies in meeting the new challenges.     

Variation in populations of Littorina obtusata and L. mariae (Gastropoda) in the Severn Estuary

Populations of both species of flat periwinkle were examined at five discrete sites along the Severn Estuary, and at a marine site in Dyfed. The vertical distribution of the two species showed a clear partitioning of the shore – Littorina obtusata occupied the high/mid shore and L. mariae the low shore area. L. obtusata also extended further up the Severn Estuary than L. mariae. Maximum densities of both species were found in Milford Haven. Biometric investigations revealed intra and inter-specific differences in colour morphs, shell damage and size between populations. The possible influence of differential selection pressures at the various sites is discussed.     

Pre- and post-barrage scenarios of the relative productivity of benthic and pelagic subsystems of the Bristol Channel and Severn Estuary

The relative importance of benthic and pelagic production within the ecosystem energy cycle, varies considerably for different regions of the Bristol Channel and Severn Estuary. In regions of high turbidity, pelagic primary production is negligible whereas, in the clearer waters of the Outer Channel, the much higher pelagic production contributes to both benthic and pelagic secondary production, The construction of a tidal power scheme would most probably reduce turbidity, so increasing the overall productivity of the enclosed estuary. A range of scenarios, and their implications, are discussed.     

Aerial inputs of pollutants to the Severn Estuary

Pollutants deposited on the Severn Estuary from the atmosphere derive mainly from local industrial and urban centres. Atmospheric deposition accounts for 50% of the lead and zinc inputs, 10–20% of the cadmium, copper and nickel to the Estuary, but only a small proportion of the chromium, iron and manganese. Most of the cadmium, copper and lead comes from the lower atmosphere near Avonmouth and Cardiff Bay. Westerly winds have much higher sodium and chloride concentrations but generally carry lower pollution loads than the less frequent, offshore easterlies. Strong winds recycle pollutants into the atmosphere via sea spray. The hills on both sides of the Estuary encourage deep vertical mixing, and effluents may be carried well inland. Stable atmospheres, associated with southerly and easterly airflows, cause trapping and entrainment of stack emissions. Thus easterlies may deposit much higher levels of pollutants. This paper reviews present knowledge regarding the deposition of metals from the atmosphere into the waters of the Severn Estuary. Preliminary results (1989) indicate that, while distribution patterns remain broadly similar, the quantities deposited were considerably less than they were six years previously.     

Changes in the wildfowl populations wintering on the Severn Estuary

Monthly winter counts of seven major wildfowl species show that, since 1960, Bewick's swan, shelduck, wigeon, teal and pintail have significantly increased in both the Severn and Britain as a whole; European White-fronted geese have shown no trend on the Severn but have significantly declined in Britain; mallard show no trend on the Severn but have increased in Britain. All except shelduck show a correlation between the trends in Britain and the Severn Estuary, while all but mallard and pintail rank as internationally important on the Severn. In recent years the Severn has assumed a considerable importance for pochard and tufted ducks. The status of many species in the Severn Estuary is dependent on the management of key areas.     

The ecology of river algae

The Botanical Review -     

The use of benthic invertebrate data for evaluating impacts of urban runoff

The benthic macroinvertebrate population of a stream in an urbanized watershed was compared to the benthos in a rural stream. Using buried samplers, no significant difference between streams was found in total numbers of invertebrates, indicating no long term loss of colonization potential in the urban stream. Classifying the benthos in functional family groupings (based on Cummins, 1973) showed the rural stream to have nearly twice the functional diversity of the urban stream. The benthos of the urban stream was dominated by a few groups of invertebrates which could adapt to the erosional/depositional nature of the substrate and could utilize transient, low quality food sources. The density of invertebrates was adequate to support a coho salmon and cutthroat trout population in the urban stream. Apparently, the salmonids feed on available benthos and do not select specific benthic trophic groups. An evaluation of six similarity coefficients using cluster analysis showed that only the Canberra Metric index was able to represent the raw data according to known data associations.     

Trace metals in the benthic habitat of the Maarsseveen Lakes system,The Netherlands

The cadmium, zinc, lead and copper concentrations in benthic invertebrates and sediment were determined during two consecutive winters in the Maarsseveen Lakes system. A sequential extraction procedure was applied to estimate the bioavailability of the trace metals in the sediment. Based on the trace metal analyses of organisms and sediment, it is concluded that the Maarsseveen Lakes system has background levels of cadmium, zinc, lead and copper. As the majority of metals was present in geochemically more stable sediment phases, the sequential extractions provided limited additional information on trace metal bioavailability.     

Some aspects of the feeding biology of benthic herbivores

Summary The methods used in collection, preservation, and gut content and grazing analysis of benthic herbivores are reviewed. In addition, the results of experiments conducted on feeding periodicities and on the suitability for feeding studies of animals which were 1) collected with an electric shocker, 2) collected from the downstream drift of rivers and 3) killed with certain preservatives are presented. It is concluded that since most methods involve such a high degree of error, many of our concepts regarding the nutritional importance of different foods, selective feeding and the magnitude of herbivorous grazing may be largely misleading.     

Effects of floods on epilithon and benthic macroinvertebrate populations in an unstable New Zealand river

Temporal changes in epilithon biomass and benthic macroinvertebrate density were investigated in the Ashley River, a flood-prone river with an unpredictable discharge regime. Biomass, primary production and respiration of the epilithic community were highest in spring when filamentous algae were present and lowest following two large floods that occurred in close succession. Sixty invertebrate species were taken in benthic samples including 24 species of Trichoptera, 15 Diptera and 4 Ephemeroptera. Larvae of the mayfly Deleatidium (Leptophlebiidae) were numerically dominant and comprised up to 83 % of the fauna in any one month. Mean benthic invertebrate density was highest (9170–18 580 m^{–2) following long periods of low stable flow (< 30 m–3} s^–1) and lowest (230 m^–2) after a major flood (454 m^–3 s^–1). Reductions in benthic density occurred when flow exceeded about 30 m^–3 s^–1, the minimum discharge at which small cobbles are moved. Refuge seeking behaviours, flexible life histories and effective recolonization mechanisms enable the benthos of the Ashley River to persist and recover from frequent, temporally unpredictable disturbances.     

The effects of mine drainage and organic enrichment on benthos in the river nent system,Northern Pennines

A preliminary survey of the benthic fauna of sites on the Nent system affected by elevated zinc levels, acid water and organic enrichment is described. Data on faunal composition are presented for 17 sites. A clustering technique was applied to the taxa/site matrix to demonstrate the degree of association between sites varying in zinc concentration. Chandler biotic scores were found to be lowest at sites affected by high zinc levels and/or dense growths of the alga Stigeoclonium tenue. The role of elevated zinc levels and algal growth in determining the diversity and abundance of benthos is discussed.     

Factors determining the spatial structure of benthic communities of a shallow bay

The species composition and the spatial structure of benthos were studied along a transect running at a depth of 1–18 m on the soft bottom substrates of Zapadnaya Bight of Furugelm Island (Peter the Great Bay, Sea of Japan). This area has been proposed as a major site for long-term monitoring of the Far Eastern State Marine Reserve. Hydrobiological samples of different sizes (16 cm², 0.025, and 1 m²) were taken by a diving method at 5-m intervals from the shore. Benthic communities were distinguished using principal component analysis (PCA), Sørensen's similarity indices, and similarity dendrograms. The grain size composition of bottom sediment was determined. Three benthic communities at depths of 1–3, 4–8, and 14–18 m and two transitional zones were distinguished in Zapadnaya Bight. The benthic communities are characterized by a relative constancy of species composition within their boundaries, and by stable ratios of quantitative characteristics of other species from different ecological groups. The transitional zones, which are comparable in extension with the communities themselves, are characterized by changes in species composition and the presence of characteristic species; here, the substrate type changes and sediment grain sorting is decreased.     

An overview of the assessment of aquatic ecosystem health using benthic invertebrates

Community structure or species composition of benthic invertebrates has frequently been used in environmental monitoring and assessment of aquatic systems. Three general approaches have been taken: the saprobic approach, which requires detailed knowledge of taxonomy and is most effective in measuring impacts from sewage effluents; diversity indices, which do not require detailed knowledge of species requirements but ignore information provided by important species and tend to lose information; and biotic indices, which combine both approaches. In the past few years considerable advances have been made by applying multivariate statistical techniques to large data matrices and relating benthic community structure to key environmental variables. Using these techniques it is possible to establish reference communities for a set of environmental conditions, to predict the benthic community that should occur at new sites and thus measure deviation from an expected community type. This suggests that environmental criteria and objectives can be established based on biological variables as opposed to the more traditional chemical approach.Measurement of ecosystem health using functional attributes of benthic invertebrates is generally in the development stage. In the future, functional measures of ecosystem health, such as chronic measures of toxicity or stress, should be incorporated into any assessment process.     

The respiration of common benthic invertebrate species from the shallow littoral zone of Lake Esrom,Denmark

To enable estimation of the total assimilation of benthic populations, we measured the oxygen consumption of macroinvertebrates from the littoral zone of a eutrophic lake. The animals were collected all the year round, and their respiration was measured at field temperatures using a closed-bottle method. Multiple regressions relating the rate of oxygen consumption to temperature and body size were established for 22 taxa, including data derived from the literature for meiofaunal groups.     

Production ecology in the Northern Baltic

Production biology in the Bothnian Bay is discussed and compared to that of the southern parts of the Baltic. Severe ice-conditions, low water temperatures in spring and early summer and a pronounced water-colouring cause a delay of the spring development of phytoplankton and a low annual production in the Bothnian Bay. This delay makes possible a higher efficiency of the pelagic system as the zooplankton fauna can develop in harmony with the food resources. The downward transport of energy and matter to the bottom communities from an efficient pelagial should be small and temporally unpredictable, which is thought to be an adequate explanation of the observed very low benthic fauna biomasses in the Bothnian Bay.     

The response of two estuarine benthic communities to the quantity and quality of food

Experimental manipulations of food supply were performed on soft sediment cores from two European estuaries, the Westerscheldt and the Gironde, with a view to determining benthic macrofaunal community response. Over a period of twenty weeks in a laboratory mesocosm system, both communities showed losses in terms of numbers of individuals and small, but non-significant, losses in terms of numbers of species. Whereas no effect of the different types of foods or the dose levels at which they were supplied was detected for the Westerscheldt benthic community, that of the Gironde showed some significant response. This was largely attributed to the differential mortality of spionid polychaetes across the dose levels used, with the highest dose, equivalent to 200 g C m^–2 yr^–1, only just maintaining their initial population densities. The results are discussed in terms of the importance of lateral advection of food particles at the benthic boundary layer and the general insufficiency of many estimates of carbon input to shallow benthic systems.     

Associations and distribution of benthic nematodes in the Ethiopian Rift Valley lakes

The nematode associations in six Ethiopian rift lakes, one man-made lake, and three crater lakes were studied and related to water conductivity, lake depth and sediment particle size. The lakes investigated lie at moderate altitudes (1200–2000 m), are alkaline and vary in their conductivities. The rift lakes Abijata and Shala and the crater lakes Aranguadi, Kilotes and Chitu are soda lakes (K₂₅ > 6000 μS cm⁻¹), while the remaining five lakes and the man-made Lake Koka are within the freshwater range of conductivity (K₂₅ < 6000 μS cm⁻¹). A total of 16 species were identified including six new species. The nematode associations varied between lakes and were restricted to very few species in each lake. Most nematodes were restricted to non-saline lakes. The most dilute L. Zwai had a higher number of nematode species (12) and abundance than other lakes. The saline lakes Shala and Kilotes had nematode assemblages made up of only a single species (Mesodorylaimus macrospiculum). No nematodes were found in the hypersaline crater L. Chitu. Tobrilus africanus appeared more euryhaline than other nematodes occurring at conductivities as high as 15 000 μ S cm⁻¹ (L. Abijata). Five of the most common species (Dorylaimus sp., Monhystera stagnalis, Aphanolaimus tudoranceai, Tobrilus africanus and Actinolaimus perplexus) were associated with medium grained sand. A monthly variation in density of nematodes in L. Zwai is also shown.     

Oligochaeta of the middle Po River (Italy): principal component analysis of the benthic data

The application of principal component analysis to two types of habitat (the benthos of macrophytes and of central river bed) enabled us to single out some of the factors that affect the dynamics and the structure of the oligochaete population and its various reactions to environmental conditions. As regards macrophytes, the distribution of the variables on the basis of the first component is correlated, to a certain extent, with a seasonal factor without any significant differences among sites. The largest population is most closely correlated with the summer months. In fact, we found that the Naididae and Tubificidae species generally develop in larger numbers at higher temperatures. For the Tubificidae, we could detect a precise seasonal cycle. In the central river bed habitat, the first component was correlated with the river discharge, which determines the granulometric characteristics of the sediment; we noticed a correlation among the sites that have the same characteristics, regardless of sampling site or date. The species which correlate most closely among themselves are the Tubificidae Limnodrilus hoffmeisteri, Tubifex tubifex, L. udekemianus and L. profundicola, which are very characteristic of environments that contain abundant organic matter. The second component is correlated with temperature, and hence with the availability of oxygen, which determines the presence and the abundance of more sensitive species.