The decrease in aquatic vegetation in Europe and its consequences for fish populations (1987)

A diverse aquatic vegetation is essential to maintain a diverse fish fauna. The fish is an important part of a complex network of relations between nutrients, phytoplankton, epiphytes, herbivorous invertebrates, the aquatic vegetation and fish. In Northwest Europe and North America and probably in the rest of the industrialized world, the (submersed) aquatic vegetation (macrophytes) is rapidly disappearing from eutrophicated waters. The decrease is well documented. As a consequence of abun dant growth of epiphytes, which are better competitors for inorganic carbon and light in highly eutrophicated waters than submersed aquatic macrophytes are, the condition of the aquatic vegetation becomes worse. Shallow, eutrophic, relatively clear water that is rich in water plants, can change to phytoplankton dominated turbid water, within short time. This change may occur without a remarkable increase in the actual nutrient loading. Invertebrate grazers like snails, macrocrustaceans and cl adoceran zooplankters are able to protect aquatic macrophytes against the negative effects of this competition by removing epiphytes and phytoplanktonic algae. As a man predator on invertebrates, the fish indirectly influences the well-being of the aquatic vegetation. There is evidence that aquatic macrophytes are the source of biochemical compounds that negatively affect the growth of algae (allelopathy) and attract grazers. These processes are mainly found in model systems and under semi-na tural conditions. Their ecological significance still has to be tested in the field.A situation with turbid, phytoplankton dominated, water without aquatic vegetation can continue after removing nutrients from effluents because: (i) blue-green algae (phytoplankters) may excrete toxic substances, negatively affecting the growth of aquatic macrophytes; (ii) abundantly occurring young fish, but also invertebrate animals like mysids, prey on the bigger (phytoplankton grazing) cladocerans; (iii) acid rain, polluted bottom sediments and/or bird flocks contribute to the nutrient loading of a water body. Restoration techniques are: lowering the nutrient loading in combination with protection of the remaining stands of reed, replanting of aquatic plants, creation of artificial refugia for zooplankton and manipulation of young-of-the-year fish populations. Chemical and mechanical control of "nuisance" growth and heavy stocking with herbivorous fish including the common carp (Cyprinus carpi o) have to be omitted or executed very carefully to avoid phytoplankton-dominated turbid water. In small systems with "nuisance" growth, stocking (50-150 kg/ha, max. 250 kg/ha) with grass carp (Ctenopharyngodon idella) can improve the water quality.

Bibliographic Details

Main Author:	Henrik W. de Nie
Format:	Book (stand-alone) biblioteca
Language:	English
Published:	1987
Online Access:	https://openknowledge.fao.org/handle/20.500.14283/ac858e http://www.fao.org/3/a-ac858e.pdf
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