Most shallow lakes and ponds tend to be located in the more fertile lowland regions and are therefore more sensitive to human activities in the catchments.
Nutrient loading is proportionally higher in shallow than in deep systems. In deep stratified lakes, nutrients are typically lost through sedimentation from the surface layer (epilimnion) to the bottom layers (hypo-limnion) during the summer stratification. These nutrients only return to the epilimnion when the water column becomes mixed again during the autumn turnover (in the temperate region). Shallow systems, in contrast, present a strong interaction between the sediments and the water column, which ensures a fast recirculation of the settled material and less net losses of nutrients to the sediments during the critical growing season. The activities in the catchments therefore acquire greater importance for lake functioning with decreasing lake size and depth.
In pristine waters, total phosphorus (TP) concentrations are around a few micrograms to a few tens of micrograms per liter, while total nitrogen (TN) concentrations will usually be ca. 10-20 times as high. Since P compounds are less soluble than nitrogen, P is usually scarce in aquatic systems and frequently limits algal growth. However, the sediments in shallow systems experience relatively high temperatures in summer. This leads to an increase in mineralization rates and higher release of nutrients from the sediment when the supply of organic matter is very high. These processes may enhance the eutrophication effects in shallow lakes. Much of the P that has been trapped in the sediments during periods of high loading or, in the temperate zone, during cold winters can be easily released to the water column, a process called 'internal loading.' This phenomenon is frequent in shallow eutrophic lakes even after the external loading has been reduced (due to improved wastewater treatment, for example), and may take place during most of the summer in temperate lakes. In lakes with a high phosphorus accumulation in the sediments, the recovery process may last several years and even decades, regardless of the residence time of the lakes. During the recovery, a progressive decline in the P release rate and duration takes place, particularly in winter, followed by spring and autumn. Many north temperate lakes often reach a new equilibrium with respect to TP < 10-15 years after the external loading has been reduced, whereas a new equilibrium regarding TN is typically reached after <5-10 years. However, there are examples of more than 40 years response time. Besides the consequent nutrient control of phytoplankton biomass, increased top-down control due to fast response of the fish community seems often to accompany the reduced external loading. A decline in fish biomass and an increase in the percentage of piscivores and the ratio between zooplankton to phyto-plankton biomass occur in many lakes. Lakes with reduced fish populations, such as those subjected to biomanipulation measures, exhibit a shorter period of
P release during summer, while the TP load may be reduced to 50%.
Contrarily, N will eventually be lost or removed from the system through denitrification. However, N availability and concentrations in many ground and surface waters have increased remarkably since the 1950s, mostly as a result of agricultural activities. Typically, primary production in lakes is thought to be P-limited, although the nutrient controls seem different in lakes dominated by plants from those turbid, often dominated by phytoplankton. Low N concentrations seem to entail higher aquatic plant species richness and even increase the potential for submerged plants to occur. A diverse plant community assemblage is likely more stable against external perturbations. In contrast, TN concentrations higher than 1-2 mg L_1, under moderate to high TP concentrations, seem to trigger the loss of submerged plants or at least a strong impoverishment of plant diversity. These findings impose a reconsideration of the TP and TN thresholds to be achieved after nutrient control during a restoration strategy.
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