Eutrophication in Australian Rivers, Reservoirs and Estuaries – A Southern Hemisphere Perspective on the Science and its Implications

Australian science has made rapid advances in the last decade in understanding eutrophication processes in inland waters and estuaries. The freshwater research on which these advances are based was triggered by well-publicised blooms of cyanobacteria during the 1980s and early 1990s, particularly a 1000 km long bloom on the Darling River. In estuaries the study which greatly enhanced our understanding but simultaneously served to stimulate further research into estuarine eutrophication, the Port Phillip Bay Study, was initially designed to address perceived problems of toxicants in the Bay but provided profound insights into drivers for, and ecosystem responses to, eutrophication. Subsequent estuarine research has largely been stimulated by management questions arising from Australia’s increasing coastal development for residential purposes. The research has shown that some of the beliefs extant at the time of the blooms were incorrect. For example, it is now clear that stratification and light penetration, not nutrient availability, are the triggers for blooms in the impounded rivers of southeastern Australia, although nutrient exhaustion limits the biomass of blooms. Again, nitrogen seems to play as important a role as phosphorus does in controlling the biomass of these freshwater blooms. The research has also shown that aspects of eutrophication, such as nutrient transport, are dominated by different processes in different parts of Australia. Many of the biophysical processes involved in eutrophication have now been quantified sufficiently for models to be developed of such processes as sediment-nutrient release, stratification, turbidity and algal growth in both freshwater and estuarine systems. In some cases the models are reliable enough for the knowledge gained in particular waterbodies to be applied elsewhere. Thus, there is now a firm scientific foundation for managers to rely upon when managing algal blooms. Whilst these findings have already been presented to managers and communities throughout Australia, there is still a considerable way to go before they are absorbed into their modus operandi.     

Temperature impact on the midsummer decline of Daphnia galeata: an analysis of long-term data from the biomanipulated Bautzen Reservoir (Germany)

1. The influence of water temperature on occurrence and duration of a midsummer decline (MSD) of Daphnia galeata was studied in the biomanipulated Bautzen Reservoir in Germany. The proportion of piscivores in the fish community of the reservoir has been enhanced experimentally since 1981. As a consequence, Daphnia galeata has dominated the zooplankton. Over 18 years of study (1981–1998), a long‐lasting MSD (longer than 30 days) occurred in 7 years, whereas a short MSD (shorter than 30 days) was observed in 6 years. During the remaining 5 years, an MSD was not observed.
2. Two hypotheses were examined to explain the observed patterns. First, we postulated that high water temperature during winter and early spring (January–April) leads to an MSD after an early and high spring peak of daphnids. On the other hand, low temperature during winter and early spring should not cause an MSD owing to a slower increase of the population, resulting in a later peak of daphnids. Second, we hypothesized that the mean water temperature during early summer (May and June) influences the occurrence of an MSD (by controlling young‐of‐the‐year (YOY) fish predation on daphnids).
3. The water temperature during winter and early spring explains 83%, and the early summer water temperature 55%, of interannual variation in the occurrence of an MSD.
4. The interannual variation in duration of an MSD was neither explained by temperature during winter and early spring nor by early summer temperature alone, but in 14 of the 18 years (78%) by a combination of both.
5. We conclude that water temperature during winter and early spring had a strong impact on Daphnia mortality by influencing height and timing of the spring peak which, in turn, influenced the extent of overexploitation of their food resources. By contrast, the water temperature during early summer probably influenced the mortality of daphnids caused by predation of YOY fish. The relative timing of both sources of mortality, which depends on the temperature regime during the first 6 months of the year, is the key process in controlling the occurrence and duration of an MSD. A long‐lasting MSD, therefore, is likely in Bautzen Reservoir only if temperatures are high during winter and early spring, as well as during early summer.
6. As a consequence of climate warming, recent climate records reveal warming during winter, spring and early summer in middle Europe, rather than an increase in mean annual temperatures. If our findings and conclusions are related to this regional and temporal pattern of climate warming, an increasing frequency of years with a long‐lasting MSD and, consequently, a decreasing efficiency of biomanipulation can be predicted.     

Effects of different fish species and biomass on plankton interactions in a shallow lake

Thirty-six enclosures, surface area 4 m², were placed in Little Mere, a shallow fertile lake in Cheshire, U.K. The effects of different fish species (common carp, common bream, tench and roach) of zooplanktivorous size, and their biomass (0, 200 and 700 kg ha^–1) on water chemistry, zooplankton and phytoplankton communities were investigated. Fish biomass had a strong effect on mean zooplankton size and abundance. When fish biomass rose, larger zooplankters were replaced by more numerous smaller zooplankters. Consequently phytoplankton abundance rose in the presence of higher densities of zooplanktivorous fish, as zooplankton grazing was reduced. Fish species were also significant in determining zooplankton community size structure. In enclosures with bream there were significantly greater densities of small zooplankters than in enclosures stocked with either carp, tench and, in part, roach. When carp or roach were present, the phytoplankton had a greater abundance of Cyanophyta than when bream or tench were present. Whilst top-down effects of fish predation controlled the size partitioning of the zooplankton community, this, in turn apparently controlled the bottom-up regeneration of nutrients for the phytoplankton community. At the zooplankton–phytoplankton interface, both top-down and bottom-up processes were entwined in a reciprocal feedback mechanism with the extent and direction of that relationship altered by changes in fish species. This has consequences for the way that top-down and bottom-up processes are generalised.     

Food web manipulation in Lake Zwemlust: Positive and negative effects during the first two years

The hypertrophic Lake Zwemlust, a small water body used as a swimming pool, was characterized by algal blooms in summer, reducing the Secchi disk transparency to less than 0.3 m. Since in The Netherlands a Secchi disk transparency of 1 m is obligatory for swimming waters, corrective measures were called for to improve the light climate of the lake. In March, 1987, as an experiment, the lake was drained by pumping out the water to facilitate fish elimination. Planktivorous and benthivorous fish species, which were predominant, were removed by seine- and electro-fishing. After the lake had refilled by seepage it was restocked by a new simple fish community comprising pike (Esox lucius) and rudd (Scardinius erythrophthalmus) only. Stacks of willow twigs (Salix) and macrophytes (roots ofNuphar lutea and seedlings ofChara globularis) were introduced into the lake as spawning grounds and refuges for the pike against cannibalism and as shelter for the zooplankton. The effects of this food web manipulation on the light climate, phytoplankton, zooplankton, fish, macrophytes, macrofauna and on the nutrient concentrations were monitored during 1987 and 1988. In summer 1987, despite of high nutrient concentrations, the phytoplankton density was low, due to control by zooplankton, causing a Secchi disk transparency of 2.5 m, the maximum depth. Chlorophyll-a concentrations were low (<5 g Chl.l^–1), blooms of cyanobacteria did not occur and a shift from rotifers to cladocerans took place. In 1988, however, also some negative effects were noticed. Macrophytes and filamentous green algae reached a much higher biomass (50–60% cover of the lake bottom) than in 1987; some species, growing through the entire water column, interfered with the lake's recreational use. Associated with the macro-vegetation and possibly with the absence of larger cyprinids, the diet of which also comprises snails, a large scale development of the snail population, among themLymnaea peregra var.ovata took place. This species is known to act as an intermediate host of the bird-parasitizing trematodeTrichobilharzia ocellata, the cercariae of which cause an itching sensation at the spot of penetration of the human skin, accompanied by rash (schistosome dermatitis or swimmers' itch); in July, 1988, about 40% of the bathers complained about this itching. A positive effect of the macrophytes and filamentous green algae was the high uptake of nitrogen, resulting in a low nitrogen concentration in the lake and growth limitation of the phytoplankton population by nitrogen in the summer of 1988. In 1988 the cladocerans were abundant in April only; and unlike in 1987, in the summer of 1988 there was a shift from cladocerans to rotifers. Therefore, only in early spring (April) zooplankton grazing controlled phytoplankton growth and in summer nitrogen limitation was the major controlling factor, keeping chlorophyll-a concentrations low.     

Consumption, abundance and habitat use of Anguilla anguilla in a mesotrophic reservoir

The role of the eel Anguilla anguilla as a piscivorous species was investigated in a biomanipulation experiment in the mesotrophic Saidenbach Reservoir. The distribution and abundance of the eels were investigated by point abundance sampling, snorkelling and scuba diving. Also, the total yearly consumption of the eel population was compared with the standing stock of prey fishes and the production of roach Rutilus rutilus eggs. A restricted availability of shelter habitats at low water levels had no influence on the distribution of the eels during the nocturnal activity period. Fishes were consumed in low numbers but in high proportions of biomass by large eels. The maximum estimate of the annual consumption of the prey fish standing stock by the eels was 19%. Fish eggs were consumed during a small time period by a large proportion of the eels but the total consumption was <10% of the total annual production of roach eggs. By consuming small individuals of planktivorous fishes, the eels contribute to the biomanipulation programme in the Saidenbach Reservoir.     

The influence of biomanipulation on fish community development in a southeastern United States cooling reservoir

L Lake, a reactor cooling reservoir in South Carolina, USA was managed after filling to promote the development of healthy ecological communities similar to those in mature regional cooling reservoirs. Two types of biomanipulation were undertaken to achieve this goal, the introduction of typical southeastern US reservoir fishes (bluegill and largemouth bass) and artificial planting of native aquatic macrophytes. Fish assemblages were monitored by electrofishing from reservoir filling in 1986 until 1998. Multivariate analysis divided the fish samples into five sequential periods resulting from species replacements and additions. Small species that colonized L Lake from a feeder stream predominated in the first period but were mostly eliminated, as bluegill, largemouth bass, and other lentic species increased in the second period. A rapid increase in threadfin shad abundance characterized the third period, and small littoral zone and phytophilous fishes increased during the fourth and fifth periods coincident with the proliferation of aquatic macrophytes. Analysis of Bray-Curtis similarities and the species accumulation rate indicated that the rate of fish community change decreased with time and that fish community structure changed little during the last several years of the study. By the end of the study, community structure was similar to that in a nearby cooling reservoir that supported diverse and resilient biota. Biomanipulation contributed to the rapid establishment of lentic species and later increases in small littoral and phytophilous species suggesting that biomanipulation may be useful in accelerating fish community development in new cooling reservoirs.     

Biomanipulation as an Application of Food-Chain Theory: Constraints, Synthesis, and Recommendations for Temperate Lakes

The aim of this review is to identify problems, find general patterns, and extract recommendations for successful biomanipulation. An important conclusion is that the pelagic food chain from fish to algae may not be the only process affected by a biomanipulation. Instead, this process should be viewed as the “trigger” for secondary processes, such as establishment of submerged macrophytes, reduced internal loading of nutrients, and reduced resuspension of particles from the sediment. However, fish reduction also leads to a high recruitment of young-of-the-year (YOY) fish, which feed extensively on zooplankton. This expansion of YOY the first years after fish reduction is probably a major reason for less successful biomanipulations. Recent, large-scale biomanipulations have made it possible to update earlier recommendations regarding when, where, and how biomanipulation should be performed. More applicable recommendations include (1) the reduction in the biomass of planktivorous fish should be 75% or more; (2) the fish reduction should be performed efficiently and rapidly (within 1–3 years); (3) efforts should be made to reduce the number of benthic feeding fish; (4) the recruitment of YOY fish should be reduced; (5) the conditions for establishment of submerged macrophytes should be improved; and (6) the external input of nutrients (phosphorus and nitrogen) should be reduced as much as possible before the biomanipulation. Recent biomanipulations have shown that, correctly performed, the method also achieves results in large, relatively deep and eutrophic lakes, at least in a 5-year perspective. Although repeated measures may be necessary, the general conclusion is that biomanipulation is not only possible, but also a relatively inexpensive and attractive method for management of eutrophic lakes, and in particular as a follow-up measure to reduced nutrient load. Received 14 April 1998; accepted 31 August 1998     

Long term study on the influence of eutrophication, restoration and biomanipulation on the structure and development of phytoplankton communities in Feldberger Haussee (Baltic Lake District, Germany)

Feldberger Haussee provides a classic example of eutrophication history of hardwater lakes in the Baltic Lake District (Germany) and of changes in their algal flora during the 20th century. The lake originally was regarded as slightly eutrophic. A process of drastic eutrophication from the 1950s until the end of the 1970s caused mass developments of blue-green and green algae. A restoration program was started in the 1980s to improve the water quality of the lake using both diversion of sewage outside the catchment area, and biomanipulation by altering the fish community. This restoration program led to positive changes in the lake ecosystem. Direct effects of biomanipulation resulted in an increase of herbivorous zooplankton, a decrease of phytoplankton biomass, and an increase of water transparency. The recovery of Feldberger Haussee also may have been indirectly enhanced by an increase in nutrient sedimentation as a consequence of intensified calcite precipitation, decrease in phosphorus remobilization due to a pH-decrease, increased NIP-ratio, and recolonization of the littoral zone by macrophytes. This paper concentrates on the long term development of the phytoplankton community as a response to changes in the food web structure as well as to alterations in the chemical environment of the algae. Both are reflected in four major stages passed by the algal assemblage between 1980 and 1994: (1) From 1980-summer 1985 dense green algal populations were found indicating similar conditions as in the 1970s during the period of maximum eutrophication. (2) A diverse phytoplankton community during summer 1985–1989 showed the first effects of a recovery. (3) From 1990–1992 the phytoplankton was characterized by ungrazeable filamentous blue-green algae first of all as a response to increased herbivory of zooplankton on edible species and to increasing N/P-ratios. (4) Finally, the algal species diversity increased in 1993 and 1994 whereas the phytoplankton biomass decreased showing the success of the combined restoration measures.     

Development of fish communities in lakes after biomanipulation

Biomanipulation measures in the Netherlands are usually a combination of a drastic fish stock reduction and an introduction of pike fingerlings. In three small shallow lakes (Noorddiep, Bleiswijkse Zoom and Zwemlust) these measures resulted in a clear water state and the development of macrophytes. After the measures the fish community developed differently because of the new physical and biological conditions. Results of lake Noorddiep and lake Bleiswijkse Zoom showed that the fish community became more divers. Bream and carp became less dominant and were partly replaced by roach and perch. The importance of the main predator pike-perch was strongly reduced and replaced by pike and perch. The share of piscivorous fish in the total fish stock increased at all sites. The recruitment of young-of-the-year was similar or even higher in the clear overgrown areas than in the turbid water before the measures, but the recruitment of young-of-the-year to older fish differed between the species. Predation by pike and perch could not control the young-of-the-year cyprinids, but their predation may have contributed to the shift from bream to roach, because of selective predation on bream in the open water, while roach was hiding in the vegetation. The macrophytes provide new refugia and feeding conditions that favour roach and perch, but offer relatively poor survival conditions for bream and carp.