On the Importance of Planktonic Protozoans in the Eutrophication Process of the Baltic Sea

Analysing the results of various authors recent studies in the pelagic region of the Baltic revealed that protozoan biomass is in the same range or even higher than metazooplankton biomass. The dominant groups of planktonic protozoans are heterotrophic pico- and nanoflagellates (various taxonomic groups), large heterotrophic flagellates (mainly dinoflagellates) and ciliates. Regularly the spring bloom of phytoplankton is accompanied by a maximum of protozoan biomass which declines in early summer as a result of intensive grazing pressure by metazooplankton and changing food conditions. The analysis of results from different stations indicated that biomasses of protozoans increase with an increasing degree of eutrophication. Several trophic levels within the microbial web should be added to the traditional view on the pelagic food web of the Baltic. Our knowledge regarding the quantitative aspect of the microbial matter flux of the Baltic is very limited up to now and complex ecological (and taxonomical) studies using standardized methods including all protozoan components are necessary. Protozoans (various trophic groups and levels), besides bacteria, should be viewed as the metabolically most active heterotrophic component in the pelagic region of the Baltic, their activity should increase with an increasing degree of eutrophication.     

Field ecology of freshwater macroalgae in pools and ditches,with special attention to eutrophication

A review is presented on the occurrence of macroalgae in dune pools, ditches and moorland pools in The Netherlands, which were studied in the period 1980–1987. There are clear effects of eutrophication on biomass as well as on species composition. A general trend is that under mesotrophic or moderately eutrophic conditions, the filamentous macroalgae form a diversified component of the primary producers in shallow water bodies. Especially spore forming genera asSpirogyra andOedogonium can be represented with up to about 20 species per site. In hardwater ditches alsoVaucheria is an important component, especially the sediment dwellingVaucheria dichotoma. Under distinctly eutrophic conditions dense floating algal masses contain only one or a few species asCladophora glomerata, Enteromorpha intestinalis, Hydrodictyon reticulatum. Under hypertrophic conditions the water surface is covered by dense blanketing mats of duckweeds in summer and autumn. Besides trophic factors, other factors as alkalinity and chloride concentration play a role in species composition. The results are discussed in the framework of water quality assessment of polder waters and possible applications for water purification or use as manuring substance. The need for experimental ecophysiological studies is underlined.     

Wildlife–water quality interactions in the Serengeti National Park, Tanzania

Surveys of water quality in the surface waters in the Serengeti National Park were undertaken in both the wet and dry seasons of 1996. Most rivers were ponded, with ponds having a flushing time of 1 month in the wet season and zero flushing in the dry season. pH values varied spatially from extremely alkaline conditions (pH>10) in the southern plains to acidic conditions in the northern region (pH=5·9). In the southern plains at the end of the dry season the salinity of surface waters was high (5–17‰) while there was abundant surface water and the zebras and wildebeest had started to migrate away, a finding suggesting excessive salinity may be the trigger initiating the annual migration. Most surface waters were heavily eutrophicated as a result of animal dung. As a result, the dissolved oxygen concentration near the surface fluctuated widely between 1 and 200% of saturation, smaller values occurring deeper in the water column. Stirring and mechanical aeration by hippos, crocodiles and mammals crossing the rivers prevented the formation of anoxic conditions. The oxygen stress was measurably lessened in wetland‐fringed water bodies as a result of filtering. Light penetration was high (≥10 cm) in saline waters as a result of flocculation enhanced by bacteria and vegetation detritus, elsewhere the euphotic zone was less than 1 cm thick and the waters generally inhospitable to aquatic life. The discontinuity between wooded savanna and grassland coincides with the occurrence of alkaline, high salinity waters in the dry season.     

Eutrophication decreases ecological resilience by reducing species diversity and altering functional traits of submerged macrophytes

Positive feedback is key to producing alternative stable states and largely determines ecological resilience in response to external perturbations. Understanding the positive feedback mechanisms in macrophyte-dominated lakes is crucial for resilience-based management and restoration. Based on the field investigation of submerged macrophyte communities in 35 lakes in China, we found that morphological complexity (MC) and morphological plasticity (MP) are correlated with the stoichiometric homeostasis of phosphorus (H_P) and are related to ecosystem structure, functioning, and stability. We also found that the positive feedback strength of lakes dominated by macrophytes is biomass- and diversity-dependent. Eutrophication can decrease the community biomass by decreasing community MC, MP, and H_P and the species diversity through low-light availability, ultimately decreasing the positive feedback strength and resilience of clear water states. We argue that functional traits and species diversity should be considered to build more resilient ecosystems in future changing environment scenarios.     

Quantification of allochthonous nutrient input into freshwater bodies by herbivorous waterbirds

1. Waterbirds are considered to import large quantities of nutrients to freshwater bodies but quantification of these loadings remains problematic. We developed two general models to calculate such allochthonous nutrient inputs considering food intake, foraging behaviour and digestive performance of waterbirds feeding in terrestrial habitats: an intake model (IM), mainly based on an allometric relationship for energy requirements and a dropping model (DM), based on allometric relationships for defaecation. 2. Reviewed data of nitrogen (N) and phosphorus (P) content of herbivorous food varied according to diet type (foliage, seeds and roots), season and fertilization. For model parameterization average foliage diet contained 38.20 mg N g^?1 and 3.21 mg P g^?1 (dry weight), whereas mean faeces composition was 45.02 mg N g^?1 and 6.18 mg P g^?1. 3. Daily allochthonous nutrient input increased with body mass ranging from 0.29 g N and 0.03 g P in teals Anas crecca to 5.69 g N and 0.57 g P in mute swans Cygnus olor. Results from IM differed from those of DM from ducks to swans by 63–108% for N and by ?4 to 23% for P. Model uncertainty was lowest for the IM and mainly caused by variation in estimates of food retention time (RT). In DM food RT and dropping mass determined model uncertainty in similar extent. 4. Exemplarily applying the models to Dutch wetlands resulted in mean annual contribution of herbivorous waterbirds to allochthonous nutrient loading of 382.8 ± 167.1 tonnes N a^?1and 34.7 ± 2.3 tonnes P a^?1, respectively, which corresponds to annual surface‐water loadings of 1.07 kg N ha^?1 and 0.10 kg P ha^?1. 5. There was a distinct seasonal pattern with peak loadings in January, when bird abundances were highest. Lowest inputs were in August, when bird abundance and nutrient content in food was low and birds foraged less in terrestrial habitats. Three‐quarters of all nutrient input was contributed by greater white‐fronted goose Anser albifrons, greylag goose Anser anser, wigeon Anas penelope and barnacle goose Branta leucopsis alone. 6. We provide general, easy to use calculation methods for the estimation of allochthonous nutrient inputs by waterbirds, which are applicable to a range of waterbird species, a variety of potential diets and feeding behaviours, and across spatial scales. Such tools may greatly assist in the planning and execution of management actions for wetland nutrient budgets.     

Marine pollution and coral reefs

Coral reefs are exposed to many anthropogenic stresses increasing in impact and range, both on local and regional scales. The main ones discussed here are nutrient enrichment, sewage disposal, sedimentation, oil-related pollution, metals and thermal pollution. The stress comprising the main topic of this article, eutrophication, is examined from the point of view of its physiological and ecological mechanisms of action, on a number of levels. Nutrient enrichment can introduce an imbalance in the exchange of nutrients between the zooxanthellae and the host coral, it reduces light penetration to the reef due to nutrient- stimulated phytoplankton growth, and, most harmful of all, may bring about proliferation of seaweeds. The latter rapidly outgrow, smother and eventually replace, the slow-growing coral reef, adapted to cope with the low nutrient concentrations typical in tropical seas.
Eutrophication seldom takes place by itself. Sewage disposal invariably results in nutrient enrichment, but it also enriches the water with organic matter which stimulates proliferation of oxygen-consuming microbes. These may kill corals and other reef organisms, either directly by anoxia, or by related hydrogen sulfide production. Increased sediment deposition is in many cases associated with other human activities leading to eutrophication, such as deforestation and topsoil erosion.
Realistically achievable goals to ensure conservation, and in some instances, rehabilitation of coral reefs are listed.     

A PRELIMINARY LIMNOLOGICAL STUDY OF BUFFELSPOORT DAM AND ITS CATCHMENT

SUMMARY

A limnological survey of the Buffelspoort Dam and its catchment between 1973 and 1975 showed that the water of the area contained extremely low concentrations of dissolved minerals and that there was no point source of nutrient supply to the dam. The anion and cation dominance in the catchment water may be summarized as HCO3 >Cl >SO₄NO₃ and Na⁺ >Mg⁺⁺ >Ca⁺⁺ >K⁺, respectively, and this water may be classified as soft bicarbonate water. The impoundment was warm monomictic and had an extensive anaerobic hypolimnion during summer stratification. Changes in water transparency were related to the seasonal fluctuation in water content and the hydrological cycle. Nutrient concentrations in the dam displayed seasonal trends which were related to stratification, hydrological events and phytoplankton activity. On the basis of its nutrient content and chlorophyll a concentrations the impoundment can be classified as mesotrophic. The further development of holiday resorts in the catchment poses a cutrophication hazard, especially if expansion results in the installation of sewage treatment plants.     

Experimental nitrogen and phosphorus enrichment stimulates multiple trophic levels of algal and detrital-based food webs: a global meta-analysis from streams and rivers

Anthropogenic increases in nitrogen (N) and phosphorus (P) concentrations can strongly influence the structure and function of ecosystems. Even though lotic ecosystems receive cumulative inputs of nutrients applied to and deposited on land, no comprehensive assessment has quantified nutrient-enrichment effects within streams and rivers. We conducted a meta-analysis of published studies that experimentally increased concentrations of N and/or P in streams and rivers to examine how enrichment alters ecosystem structure (state: primary producer and consumer biomass and abundance) and function (rate: primary production, leaf breakdown rates, metabolism) at multiple trophic levels (primary producer, microbial heterotroph, primary and secondary consumers, and integrated ecosystem). Our synthesis included 184 studies, 885 experiments, and 3497 biotic responses to nutrient enrichment. We documented widespread increases in organismal biomass and abundance (mean response = +48%) and rates of ecosystem processes (+54%) to enrichment across multiple trophic levels, with no large differences in responses among trophic levels or between autotrophic or heterotrophic food-web pathways. Responses to nutrient enrichment varied with the nutrient added (N, P, or both) depending on rate versus state variable and experiment type, and were greater in flume and whole-stream experiments than in experiments using nutrient-diffusing substrata. Generally, nutrient-enrichment effects also increased with water temperature and light, and decreased under elevated ambient concentrations of inorganic N and/or P. Overall, increased concentrations of N and/or P altered multiple food-web pathways and trophic levels in lotic ecosystems. Our results indicate that preservation or restoration of biodiversity and ecosystem functions of streams and rivers requires management of nutrient inputs and consideration of multiple trophic pathways.     

Spatial and temporal variability of nutrient retention in river basins: A global inventory

Nutrient export by rivers may cause coastal eutrophication. Some river basins, however, export more nutrients than others. We model the Basin-Wide Nutrient Export (BWNE) Index, defined as nutrient export by rivers as percentage of external nutrient inputs in the basins. We present results for rivers worldwide for the period 1970–2050. The results indicate that nutrient retentions differ largely among basins. They indicate that BWNE increases with nutrient inputs to the land, indicating that the percentage of, for instance, fertilizers exported to sea increases with fertilization rate. We argue that a better understanding of the BWNE Index might help to identify where measures and technologies to reduce nutrient inputs to coastal waters are most effective.