Communities of microscopic plant life, or phytoplankton, dominate the Earth's aquatic ecosystems. This important new book by Colin Reynolds covers the adaptations, physiology and population dynamics of phytoplankton communities in lakes and rivers and oceans. It provides basic information on composition, morphology and physiology of the main phyletic groups represented in marine and freshwater systems and in addition reviews recent advances in community ecology, developing an appreciation of assembly processes, co-existence and competition, disturbance and diversity. Although focussed on one group of organisms, the book develops many concepts relevant to ecology in the broadest sense, and as such will appeal to graduate students and researchers in ecology, limnology and oceanography.

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The Ecology of Phytoplankton

Denitrification is a critical process regulating the removal of bioavailable nitrogen (N) from natural and human-altered systems. While it has been extensively studied in terrestrial, freshwater, and marine systems, there has been limited communication among denitrification scientists working in these individual systems. Here, we compare rates of denitrification and controlling factors across a range of ecosystem types. We suggest that terrestrial, freshwater, and marine systems in which denitrification occurs can be organized along a continuum ranging from (1) those in which nitrification and denitrification are tightly coupled in space and time to (2) those in which nitrate production and denitrification are relatively decoupled. In aquatic ecosystems, N inputs influence denitrification rates whereas hydrology and geomorphology influence the proportion of N inputs that are denitrified. Relationships between denitrification and water residence time and N load are remarkably similar across lakes, river reaches, estuaries, and continental shelves. Spatially distributed global models of denitrification suggest that continental shelf sediments account for the largest portion (44%) of total global denitrification, followed by terrestrial soils (22%) and oceanic oxygen minimum zones (OMZs; 14%). Freshwater systems (groundwater, lakes, rivers) account for about 20% and estuaries 1% of total global denitrification. Denitrification of land-based N sources is distributed somewhat differently. Within watersheds, the amount of land-based N denitrified is generally highest in terrestrial soils, with progressively smaller amounts denitrified in groundwater, rivers, lakes and reservoirs, and estuaries. A number of regional exceptions to this general trend of decreasing denitrification in a downstream direction exist, including significant denitrification in continental shelves of N from terrestrial sources. Though terrestrial soils and groundwater are responsible for much denitrification at the watershed scale, per-area denitrification rates in soils and groundwater (kg Nkm � 2 � yr � 1 ) are, on average, approximately one-tenth the per-area rates of denitrification in lakes, rivers, estuaries, continental shelves, or OMZs. A number of potential approaches to increase denitrification on the landscape, and thus decrease N export to sensitive coastal systems exist. However, these have not generally been widely tested for their effectiveness at scales required to significantly reduce N export at the whole watershed scale.

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Denitrification across landscapes and waterscapes: a synthesis.

The sediment plays an important role in the overall nutrient dynamics of shallow lakes. In lakes where the external loading has been reduced, internal phosphorus loading may prevent improvements in lake water quality. At high internal loading, particularly summer concentrations rise, and phosphorus retention can be negative during most of the summer. Internal P loading originates from a pool accumulated in the sediment at high external loading, and significant amounts of phosphorus in lake sediments may be bound to redox-sensitive iron compounds or fixed in more or less labile organic forms. These forms are potentially mobile and may eventually be released to the lake water. Many factors are involved in the release of phosphorus. Particularly the redox sensitive mobilization from the anoxic zone a few millimetres or centimetres below the sediment surface and microbial processes are considered important, but the phosphorus release mechanisms are to a certain extent lake specific. The importance of internal phosphorus loading is highly influenced by the biological structure in the pelagic, and lakes shifting from a turbid to a clearwater state as a result of, for example, biomanipulation may have improved retention considerably. However, internal loading may increase again if the turbid state returns. The recovery period following a phosphorus loading reduction depends on the loading history and the accumulation of phosphorus in the sediment, but in some lakes a negative phosphorus retention continues for decades. Phosphorus can be released from sediment depths as low as 20 cm. The internal loading can be reduced significantly by various restoration methods, such as removal of phosphorus-rich surface layers or by the addition of iron or alum to increase the sediment's sorption capacity.

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Role of sediment and internal loading of phosphorus in shallow lakes

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The Structuring Role of Submerged Macrophytes in Lakes

Nitrate (NO3
−) leaching from agriculturalproduction systems is blamed for the rising concentrations ofNO3
− in ground- and surface-waters around the world.This paper reviews the evidence of NO3
− leachinglosses from various land use systems, including cut grassland, grazed pastures,arable cropping, mixed cropping with pasture leys, organic farming,horticultural systems, and forest ecosystems. Soil, climatic and managementfactors which affect NO3
− leaching are discussed.Nitrate leaching occurs when there is an accumulation ofNO3
− in the soil profile that coincides with or isfollowed by a period of high drainage. Therefore, excessive nitrogen (N)fertilizer or waste effluent application rates or N applications at the wrongtime (e.g. late autumn) of the year, ploughing pasture leys early in the autumn,or long periods of fallow ground, can all potentially lead to highNO3
− leaching losses. N returns in animal urine havea major impact on NO3
− leaching in grazed pastures.Of the land use systems considered in this paper, the potential for causingNO3
− leaching typically follow the order: forest< cut grassland < grazed pastures, arable cropping < ploughing ofpasture < market gardens. A range ofmanagement options to mitigate NO3
− leaching isdescribed, including reducing N application rates, synchronizing N supply toplant demand, use of cover crops, better timing of ploughing pasture leys,improved stock management, precision farming, and regulatory measures. This isfollowed by a discussion of future research needs to improve our ability topredict and mitigate NO3
− leaching.

Nitrate leaching in temperate agroecosystems: sources, factors and mitigating strategies

The influence of pH on phosphate release from lake sediments

Nutrient emissions from agricultural activities have become the dominant source of nutrient loads to freshwater in the Netherlands. This paper focuses on nutrient emissions from agriculture, emphasizing nutrient loads to surface waters, and strategies and perspectives to reduce these emissions. Although adverse environmental effects of intensive agriculture have been known for several years, it was not until 1987 that stabilization of animal manure production and application began. Since 1991 manure application rates have been reduced. Recently, application standards have been replaced by agriculturally inevitable nutrient losses and environmentally acceptable nutrient losses and these losses are reduced to an equilibrium fertilization in 2010, defined as the supply of manure and fertilizers that meets crop uptake and compensates for inevitable losses. In the 1980s, the most important tool to manage nutrient losses was a manure bookkeeping; recently a mineral bookkeeping has been introduced. Agricultural nutrient emissions to and their impact upon surface waters have been estimated from field experiments and model calculations, which indicate that the proposed legislation will not significantly improve water quality. Increasing areas will be saturated with P, especially where intensive livestock farming is located on sandy soils. Tailor-made regional programs are necessary to achieve ecological restoration of surface waters, with priority for catchment areas with vulnerable receiving waters. These programs may consist of a further reduction of nutrient application rates, hydrological measures, selection of crops that extract P from the soils or measures to increase the P adsorption capacity of the soil, and buffer strips.

Agricultural Nutrient Losses to Surface Water in the Netherlands: Impact, Strategies, and Perspectives

Nitrogen fluxes and processes in sandy and muddy sediments from a shallow eutrophic lake

An analysis of data from 49 shallow lakes showed, that the parameters of empirical models between phosphorus loading and concentration in the lake (e.g. Vollenweider type of relations) differ significantly for lakes without or with a reduced external loading. For lakes without a reduction of the external loading the summer phosphorus concentration is determined by the external phosphorus loading and the hydraulic loading. For these lakes the ‘classical’ models suffice; deviations between calculations and measurements are partly due to errors made in the determination of the loading.

P.C.M. Boers

Papers

The influence of pH on phosphate release from lake sediments

Agricultural Nutrient Losses to Surface Water in the Netherlands: Impact, Strategies, and Perspectives

Nitrogen fluxes and processes in sandy and muddy sediments from a shallow eutrophic lake

Influence of internal loading on phosphorus concentration in shallow lakes before and after reduction of the external loading

Phosphorus Release from the Peaty Sediments of the Loosdrecht Lakes (the Netherlands)