Lakes

Figure 18.8 Nutrient spiraling in a river channel and adjacent wetland areas. (After Ward, 1988.)

the displacement of nutrients is better represented as a spiral (Elwood et al., 1983), where fast phases of inorganic nutrient displacement alternate with periods when the nutrient is locked in biomass at successive locations downstream (Figure 18.8). Bacteria, fungi and microscopic algae, growing on the substratum of the stream bed, are mainly responsible for the uptake of inorganic nutrients from streamwater in the biotic phase of

In lakes, it is usually the phytoplankton and their consumers, the zooplankton, which play the key roles in nutrient cycling. However, most lakes are interconnected with each other by rivers, and standing stocks of nutrients are determined only partly by processes within the lakes. Their position with respect to other water bodies in the landscape can also have a marked effect on nutrient status. This is well illustrated for a series of lakes connected by a river that ultimately flows into Toolik Lake in arctic Alaska (Figure 18.10a).

nutrient flux in lakes: important roles for plankton and lake position

Figure 18.9 Downstream trends in the Vindel River in Sweden (shown as distance from the confluence with the larger Ume River) in the concentration of fecal pellets (number of fecal pellets per liter ± SE) of blackfly larvae (family Simuliidae). The generally lower concentrations in the 'runs' reflect the higher probability of pellets settling to the river bed in these sections compared to the 'rapids' sections. The numbers above the error bars are percentages of the mass of total organic matter in the flowing water (seston) made up of fecal pellets. (After Malmqvist et al., 2001.)

Figure 18.10 (a) Spatial arrangement of eight small lakes (L1-L8) interconnected by a river that flows into Toolik Lake (TL) in arctic Alaska. (b) Mean values, averaged over all sampling occasions during 1991-97 (±SE), for magnesium (Mg) and calcium (Ca) concentrations in the study lakes. (c) Pattern in primary productivity down the lake chain. (d) Mean values for carbon (C), nitrogen (N) and phosphorus in particulate form. (After Kling et al., 2000.)

Figure 18.10 (a) Spatial arrangement of eight small lakes (L1-L8) interconnected by a river that flows into Toolik Lake (TL) in arctic Alaska. (b) Mean values, averaged over all sampling occasions during 1991-97 (±SE), for magnesium (Mg) and calcium (Ca) concentrations in the study lakes. (c) Pattern in primary productivity down the lake chain. (d) Mean values for carbon (C), nitrogen (N) and phosphorus in particulate form. (After Kling et al., 2000.)

The main reason for the downstream increase in magnesium and calcium was increased weathering (Figure 18.10b). This comes about because a greater proportion of the water entering downstream lakes has been in intimate contact with the parent rock for longer; put another way, the higher concentrations reflect the larger catchment areas that feed the downstream lakes. The pattern for calcium and magnesium may also partly reflect progressive evaporative concentration with longer residence times of water in the system as well as material processing by the biota in streams and lakes as the water moves downstream. The nutrients that generally limit production in lakes, nitrogen and phosphorus, were in very low concentrations and could not be reliably measured. However, the downstream decrease in productivity that was observed (Figure 18.10c) suggests that the available nutrients were consumed by the plankton in each lake and this consumption was sufficient to lower the nutrient availability in successive lakes downstream. The downstream decrease of nitrogen, phosphorus and carbon in particulate matter (Figure 18.10d) simply reflects the lower downstream rates of primary productivity. Note that it is unusual to have a downstream decline in productivity. In less pristine conditions, productivity is more likely to increase in a downstream direction (e.g. Kratz et al., 1997), partly because of the addition of more nutrients from larger catchment areas but also because of increasing human inputs in lowland areas through fertilizer application and sewage.

Many lakes in arid regions, lacking a stream outflow, lose water only by evaporation. The waters of these endorheic lakes (internal flow) are thus more concentrated than their freshwater counterparts, being particularly rich in sodium (with values up to 30,000 mg l-1 or more) but also in other nutrients such as phosphorus (up to 7000 |lg l-1 or more). Saline lakes should not be considered as oddities; globally, they are just as abundant in terms of numbers and volume as freshwater lakes (Williams, 1988). They are usually very fertile and have dense populations of blue-green algae (for example, Spirulina platensis), and some, such as Lake Nakuru in Kenya, support huge aggregations of plankton-filtering flamingoes (Phoeniconaias minor). No doubt, the high level of phosphorus is due in part to the concentrating effect of evaporation. In addition, there may be a tight nutrient cycle in lakes such as Nakuru in which continuous flamingo feeding and the supply of their excreta to the sediment creates circumstances where phosphorus

Figure 18.11 Conceptual model of nitrogen (N) flux through the food web of the upper Parker River estuary, Massachusetts, USA. Dashed arrows indicate suspected pathways. DIN, dissolved inorganic nitrogen. (After Hughes et al., 2000.)

saline lakes lose water only by evaporation, and have high nutrient concentrations

Figure 18.11 Conceptual model of nitrogen (N) flux through the food web of the upper Parker River estuary, Massachusetts, USA. Dashed arrows indicate suspected pathways. DIN, dissolved inorganic nitrogen. (After Hughes et al., 2000.)

is continuously regenerated from the sediment to be taken up again by phytoplankton (Moss, 1989).

18.3.3 Estuaries nutrient flux in estuaries: roles for planktonic and benthic organisms...

In estuaries, both planktonic organisms (as in lakes) and benthic organisms (as in rivers) are significant in nutrient flux. Hughes et al. (2000) introduced tracer levels of a rare isotope of nitrogen (as nitrate-containing 15N) into the water of an estuary in Massachusetts, USA, to study how nitrogen derived from the catchment area is used and transformed in the estuarine food web. They focused their study on the upper, low salinity part of the estuary where water derived from the river catchment first meets the saline influence of tidal seawater. The planktonic centric diatom Actinocyclus normanii turned out to be the primary vector of nitrogen to some benthic organisms (large crustaceans) and particularly pelagic organisms (planktonic copepods and juvenile fishes). Certain components of the sedimentary biota received a small proportion of their nitrogen via the centric diatom (10-30%; e.g. pennate diatoms, harpacticoid copepods, oligochaete worms, bottom-feeding fishes such as mummichog, Fundulus heteroclitus, and sand shrimps). But many others obtained almost all their nitrogen from a pathway based on plant detritus. The patterns of nitrogen flow through this estuarine food web are shown in Figure 18.11. The relative importance of nutrient fluxes through the grazer and decomposer systems can be expected to vary from estuary to estuary.

The chemistry of estuarine (and ... and human coastal marine) water is strongly influ-

activities enced by features of the catchment area through which the rivers have been flowing, and human activities play a major role in determining the nature of the water supplied. In a revealing comparison, van Breeman (2002) describes the forms of nitrogen in water at the mouths of rivers in North and South America. In the North American case, where the river flows through a largely forested region but has been subject to considerable human impact (fertilizer input, logging, acid precipitation, etc.), nitrogen was almost exclusively exported to estuaries and the sea in inorganic form (only 2% organic). In contrast, a pristine South American river, subject to very little human impact, exported 70% of its nitrogen in organic form. In Australian rivers too, pristine forested catchments export little nitrogen or phosphorus, and the predominant form of nitrogen is organic. As human population density increases (greater agricultural runoff and sewage) and forests are cleared (less tight retention of nutrients), however, the export to river mouths of both nitrogen and phosphorus increases and the predominant form of nitrogen changes to inorganic (Figure 18.12).

Figure 18.12 (a) Export of total nitrogen (TN) in relation to population density in 24 catchment areas near Sydney, Australia. (b) Rivers with low TN export rates (more pristine) contain nitrogen predominantly in organic form and the percentage of TN that is inorganic increases with TN. DIN, dissolved inorganic nitrogen. (After Harris, 2001.)

Figure 18.12 (a) Export of total nitrogen (TN) in relation to population density in 24 catchment areas near Sydney, Australia. (b) Rivers with low TN export rates (more pristine) contain nitrogen predominantly in organic form and the percentage of TN that is inorganic increases with TN. DIN, dissolved inorganic nitrogen. (After Harris, 2001.)

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