Showing papers in "Hydrobiologia in 1990"

Engineering and biological approaches to the restoration from eutrophication of shallow lakes in which aquatic plant communities are important components

Abstract: Engineering approaches (nutrient removal, sediment pumping, hypolimnion oxygenation, alum treatments) may be most appropriate to deep lakes where the aim of restoration from eutrophication is simply to reduce the production and crop of one component, the phytoplankton. They do not always give the desired results because the nutrient loading may only be reduced to a limited extent. There are additional problems in shallow lakes where change of state between community dominance (aquatic plants versus plankton) is wanted. Each community has powerful buffering mechanisms and biomanipulation may be essential to switch one state to another even with considerable nutrient reduction.

Multiplicity of stable states in freshwater systems

Abstract: It is shown with the use of minimal models that several ecological relationships in freshwater systems potentially give rise to the existence of alternative equilibria over a certain range of nutrient values. The existence of alternative stable states has some implications for the management of such systems. An important consequence is that signs of eutrophication are only apparent after the occurrence of changes that are very difficult to reverse. Reduction of the nutrient level as a measure to restore such systems gives poor results, but biomanipulation as an additional measure can have significant effects, provided that the nutrient level has been reduced enough to allow the existence of a stable alternative clear water equilibrium.

Fish manipulation as a lake restoration tool in shallow, eutrophic, temperate lakes 2: threshold levels, long-term stability and conclusions

Abstract: In order to evaluate short-term and long-term effects of fish manipulation in shallow, eutrophic lakes, empirical studies on relationships between lake water concentration of total phosphorus (P) and the occurrence of phytoplankton, submerged macrophytes and fish in Danish lakes are combined with results from three whole-lake fish manipulation experiments. After removal of less than 80 per cent of the planktivorous fish stock a short-term trophic cascade was obtained in the nutrient regimes, where large cyanobacteria were not strongly dominant and persistent. In shallow Danish lakes cyanobacteria were the most often dominating phytoplankton class in the P-range between 200 and 1000 pg P 1−1. Long-term effects are suggested to be closely related to the ability of the lake to establish a permanent and wide distribution of submerged macrophytes and to create self-perpetuating increases in the ratio of piscivorous to planktivorous fish. The maximum depth at which submerged macrophytes occurred, decreased exponentially with increasing P concentration. Submerged macrophytes were absent in lakes > 10 ha and with P levels above 250−300 μg P 1−1, but still abundant in some lakes 10 cm numerically contributed more than 80 per cent of the total planktivorous and piscivorous fish (> 10 cm) in the pelagical of lakes with concentrations above 100 μg P 1−1. Below this threshold level the proportion of planktivores decreased markedly to ca. 50 per cent at 22 μg P 1−1. The extent of the shift in depth colonization of submerged macrophytes and fish stock composition in the three whole-lake fish manipulations follows closely the predictions from the relationships derived from the empirical study. We conclude that a long-term effect of a reduction in the density of planktivorous fish can be expected only when the external phosphorus loading is reduced to below 0.5−2.0 g m −2 y −1. This loading is equivalent to an in-lake summer concentration below 80−150 μg P 1−1. Furthermore, fish manipulation as a restoration tool seems most efficient in shallow lakes.

Are blue-green algae a suitable food for zooplankton? An overview

Abstract: One of the reasons suggested to explain the dominance of blue-greens in eutrophic lakes is that they are not used as food by zooplankton; and even when ingested, they are poorly utilized.

Nitrate depletion in the riparian zone and stream channel of a small headwater catchment

Abstract: A mass balance procedure was used to determine rates of nitrate depletion in the riparian zone and stream channel of a small New Zealand headwater stream. In all 12 surveys the majority of nitrate loss (56–100%) occurred in riparian organic soils, despite these soils occupying only 12% of the stream's border. This disproportionate role of the organic soils in depleting nitrate was due to two factors. Firstly, they were located at the base of hollows and consequently a disproportionately high percentage (37–81%) of the groundwater flowed through them in its passage to the stream. Secondly, they were anoxic and high in both denitrifying enzyme concentration and available carbon. Direct estimates of in situ denitrification rate for organic soils near the upslope edge (338 mg N m−2 h−1) were much higher than average values estimated for the organic soils as a whole (0.3–2.1 mg N m−2 h−1) and suggested that areas of these soils were limited in their denitrification activity by the supply of nitrate. The capacity of these soils to regulate nitrate flux was therefore under-utilized. The majority of stream channel nitrate depletion was apparently due to plant uptake, with estimates of the in situ denitrification rate of stream sediments being less than 15% of the stream channel nitrate depletion rate estimated by mass balance. This study has shown that catchment hydrology can interact in a variety of ways with the biological processes responsible for nitrate depletion in riparian and stream ecosystems thereby having a strong influence on nitrate flux. This reinforces the view that those seeking to understand the functioning of these ecosystems need to consider hydrological phenomena.

Toxic cyanobacteria (blue-green algae) in Finnish fresh and coastal waters

Abstract: A survey of the occurrence of toxic blooms of cyanobacteria in Finnish fresh and coastal waters was made during 1985 and 1986. Toxicity of the freeze-dried water bloom samples was tested by mouse-bioassay (i.p.). Forty-four per cent (83/188) of the bloom samples were found to be lethally toxic. Hepatotoxic blooms (54) were almost twice as common as neurotoxic ones (29). Anabaena was the most frequently found genus in toxic and non-toxic blooms and it was present in all neurotoxic samples. Statistical associations were found between hepatotoxicity and incidence of Microcystis aeruginosa, M. viridis, M. wesenbergii, Anabaena flos-aquae and Anabaena spiroides. Neurotoxicity was statistically associated with Anabaena lemmermannii, Anabaena flos-aquae and Gomphosphaeria naegeliana. Isolation of strains of cyanobacteria confirmed the occurrence of hepatotoxic and neurotoxic strains of Anabaena, as well as hepatotoxic strains of Microcystis and Oscillatoria species. Toxic blooms caused cattle poisonings at three different lakes during the study period. Toxic blooms also occurred in drinking water sources. Our study shows that toxic cyanobacteria are more common in Finnish lakes than would be expected on the basis of animal poisonings. The results of this study show the existence of toxic cyanobacteria in Finnish water supplies and the need for their continued study as agents of water based disease.

Why do cladocerans fail to control algal blooms

Abstract: Field studies show that even at high nutrient loads phytoplankton may be kept at low levels by filter-feeding zooplankton for a period of weeks (spring clear water phase in lakes) or months (low-stocked fish-ponds). In the absence of planktivorous fish, large-bodied cladocerans effectively control the abundance of algae of a broad size spectrum. Laboratory experiments show that, although difficult to handle and of poor nutritional value, filamentous algae can also be utilized by large-bodied Daphnia and prevented from population increase, exactly as the principles of the biomanipulation approach would predict.

Dynamics of bacterioplankton in oligotrophic and eutrophic aquatic environments: bottom-up or top-down control?

Abstract: Measurements of bacterial biomass, production and mortality have been carried out in a large range of aquatic environments, including eutrophic and oligotrophic ones. The general trends of variations of bacterial biomass, size, specific growth rate and mortality rate in all these environments are examined. The overall flux of bacterial production is taken as an index of the flux of organic matter available to bacteria, thus characterizing the richness of the environment. Bacterial biomass is roughly proportional to richness, while mean cell size increases with it. The turnover rate of biomass, as revealed either by growth or by mortality rates, appears to be fairly independent of richness. These observations are compatible with a simple resource-limited (bottom-up controlled) model of the dynamics of bacterioplankton. On the other hand, they are in contradiction with the predictions of a predator-controlled (top-down controlled) model.

Salinity as a determinant of salt lake fauna: a question of scale

Abstract: High and often variable salinity is an obvious feature of salt lakes. Correspondingly, salinity is usually assumed to be an important ecological determinant in such lakes. An investigation of the macroinvertebrate fauna of 79 lakes (salinities from 0.3 to 343 g 1−1) in the Western District of Victoria, Australia, examined this assumption. Over the total range of salinity, species richness and composition are highly correlated with salinity. However, these relationships become nonsignificant over intermediate ranges of salinity. Furthermore, many taxa have very broad tolerances to salinity at these intermediate ranges, implying that factors other than salinity may determine their distribution. An appreciation of scale (that is, the range of salinity over which observations are considered) resolves the paradox that, despite these broad tolerances by most taxa, species richness and composition strongly reflect salinity over the entire salinity range.

Effect of crab effluent and scent of damaged conspecifics on feeding, growth, and shell morphology of the Atlantic dogwhelk Nucella lapillus (L.)

Abstract: Juvenile Nucella lapillus of two different shell phenotypes, exposed shore and protected shore, were maintained in running seawater under each of three experimental conditions for 94 d: a) laboratory control, b) exposed to the effluent of crabs (Cancer pagurus) fed frozen fish (‘fish-crab’), and c) exposed to the effluent of crabs fed live conspecific snails (‘snail-crab’). Rates of barnacle consumption and rates of body weight change varied significantly between phenotypes and among experimental conditions. Individuals from the protected-shore consumed consistently fewer barnacles and grew consistently less than those from the exposed shore. Body weight increases in the fish-crab treatments were from 25 to 50% less than those in the controls and body weights in the snail-crab treatment either did not change or actually decreased. The perceived risk of predation thus appears to have a dramatic effect on the rates of feeding and growth of N. lapillus.

Conditions for effective biomanipulation ; conclusions derived from whole-lake experiments in Europe

Abstract: Since indirect effects, feedback mechanisms and time lag play an important role in top-down manipulated food webs, conclusions regarding the reliability of biomanipulation as a management tool should be derived only from whole-lake studies, the results of which were followed for at least three to five years. There are only a few experiments and applications of that type from which the following conclusions can be drawn: (1) Food web manipulation should be combined with resource limitation measures (nutrients or light) if a ‘biomanipulation-efficiency threshold of the phosphorus loading’ is exceeded. (2) The critical biomass of undesirable fish shows a very wide range of variation (20–1000 kg ha−1) and depends on several factors, like species and age of fish, total food assemblage, refugia etc. (3) The maximum possible stability of food web manipulation is probably achieved at a biomass of undesirable fish a little below the critical biomass rather than by extermination of these fish. (4) High piscivore diversity increases stability and reliability of food web manipulations. (5) Various techniques of food web manipulation should be combined, except poisoning the entire fish community.

Is reduction of the benthivorous fish an important cause of high transparency following biomanipulation in shallow lakes

Abstract: Experimental reduction of the fish stock in two shallow lakes in The Netherlands shows that such a biomanipulation can lead to a substantial increase in transparency, which is caused not only by a decrease in algal biomass, but also by a decrease in resuspended sediment and detritus. A model was developed to describe transparency in relation to chlorophyll-α and inorganic, suspended solids (resuspended sediment). With the use of this model it is shown that more than 50% of the turbidity in these shallow lakes before biomanipulation was determined by the sediment resuspension, mainly caused by benthivorous fish.

Can macrophytes be useful in biomanipulation of lakes? The Lake Zwemlust example

Abstract: Lake Zwemlust (area 1.5 ha, Zm 1.5 m) has been the object of an extensive limnological study since its biomanipulation involving removal of planktivorous fish (bream) in March 1987 and emptying of the lake. In the subsequent summer period of 1987 the Secchi depth increased to the lake bottom (2.5 m), compared with ca 30 cm in the earlier summers. The reaction of submerged macrophytes to improving under-water light climate was rapid. In summer 1987, besides the introduced Chara globularis, 5 species of submerged macrophytes occurred and colonized 10% of the lake area. In 1988 and 1989 only quantitative changes were observed; new species did not appear, but the area colonized by macrophytes increased by 7 and 10 times, respectively. Elodea nuttallii was dominant among the macrophytes and Mougeotia sp. among the filamentous green algae. Their abundance, contributed to transient N-limination of phytoplankton causing a persistent clear water phase in 1988 and 1989, unlike in 1987 when zooplankton grazing contributed chiefly to the water clarity. Laboratory bioassays on macrophytes confirmed nitrogen limitation.

Zebra mussels (Dreissena polymorpha): a new perspective for water quality management

Abstract: In the evaluation of the role of lake restoration programmes in situ measurements of the filtration rate of the freshwater mussel Dreissena polymorpha have been performed in Lake Wolderwijd, The Netherlands. The filtration rate mainly depends on the suspended matter content of the water, and shows an inverse exponential relationship with this factor. The filtration activity is temperature indifferent between approx. 5 and 20 °C. At low temperatures the filtration rate drops abruptly, at high temperatures gradual inhibition occurs. The filtration rate shows a sigmoidal relation with the length of the mussel. The largest D. polymorpha have a diminished filtration rate compared to animals of smaller size. This might be a degenerative feature of the oldest mussels. In Lake Wolderwijd a population density of 675 per m2 is required to compensate phytoplankton growth by grazing. Manipulation of the size of the population can be executed by adding suitable substrates for the mussel.

Disturbance and organization of macroalgal assemblages in the Northwest Atlantic

Abstract: Large seaweeds are often structurally dominant in subtidal and intertidal rocky shore benthic communities of the N.W. Atlantic. The mechanisms by which these algal assemblages are maintained are surprisingly different in the two habitats. In the subtidal community, kelps are dominant space competitors in the absence of strong grazing interactions. In contrast, the large perennial seaweeds of intertidal zones (fucoids and Chondrus crispus) are competitively inferior to both sessile filter feeders and ephemeral, pioneer algal species. Intertidal seaweed beds are maintained by carnivory of whelks, which reduces filter feeder populations, and by herbivorous periwinkles which reduce ephemeral algal populations. Through most of the intertidal zone, disturbance, both biological and physical, dictates which species shall compete and equilibrium conditions obtain subsequently. The roles of subtidal consumers are quite different. Sea urchins are the major algal herbivores and these voracious animals maintain an equilibrium state in which large tracts of subtidal coralline pavement are kept free of kelp forests. Urchins do not seem to play a successional facilitative role for kelps in the way that periwinkles do for fucoids in the intertidal. Control of herbivore populations is thus a key to the maintenance of subtidal foliose algal beds. It is clear that parasitic amoebas can decimate sea urchin populations so that kelp forest dominance is assured. However, the importance of carnivory in limiting urchins in the subtidal community is unclear in the absence of appropriate manipulation experiments. It is possible that carnivorous decapods and fin fish control sea urchin populations and hence foliose algal abundance, but this must remain speculative. The seaweed-dominated state of the subtidal system is an alternative equilibrium condition to the urchin/coralline alga configuration. The structure of the kelp beds is relatively uniform in responding to frequent small-scale, infrequent large-scale, or no, disturbance.

Organization of macroalgal assemblages in the Northeast Pacific: the assumption of homogeneity and the illusion of generality

Abstract: Evidence for the geographic generality of the causes of intertidal zonation and the indirect effects of a ‘keystone’ predator, the sea otter, on subtidal kelp assemblages was examined. Most research on intertidal algal assemblages has been done at a few protected sites where zonation is distinct. Surveys of wave-exposed intertidal sites in central and northern California show that assemblage structure is highly variable. This indicates that our present understanding of assemblage organization, including the effects of mussel-algal interactions, may not be widely applicable. Surveys of kelp forest habitat along the entire coast of California suggest that deforestation by sea urchins is uncommon in the absence of sea otters. These examples indicate that the generality of commonly accepted causes of algal assemblage structure in the Northeast Pacific may be an illusion based on assumptions of environmental homogeneity.

Siltation of stone-surface periphyton in rivers by clay-sized particles from low concentrations in suspention

Abstract: The observation that deposits of fine sediment are found on stream beds only in areas of slower water velocity promotes a common misunderstanding of the depositional behaviour of fine suspensoids in flowing water and a disregard for the potential for siltation effects on the biota on the surface of stones in fast flowing water. A model for deposition from turbulent water, whereby particles are lost from suspension where water currents are slowed by boundary friction, provides an explanation for silt infiltration into epilithic periphyton. Theoretically calculated deposition rates of clay sized mineral particles at low suspended concentrations (2 to 5 g m−3) were found to account for observed rates of silt accumulation in epilithic periphyton in a braided river in the South Island of New Zealand. At concentrations between 1 and 10 g m−3 of suspended mineral silt during normal flow, silt accumulation in epilithic periphyton accounted for about 50% of its dry weight. This caused a reduction in the mean organic content of the periphyton to 22% of the dry weight compared to 52% in a reference stream where the concentration of suspended mineral particles was less than 1.0 g m−3 during non-freshet flow. This reduction in proportional organic content is discussed in terms of diminished food value of the periphyton and potential interactions between periphyton and invertebrate consumers.

Growth rate of four freshwater algae in relation to light and temperature

Abstract: Four algae of freshwater phytoplankton were studied in monospecific culture: Chlorella vulgaris, Fragilaria crotonensis, Staurastrum pingue and Synechocystis minima. Experiments were performed to determine the growth rate over a wide range of light intensities (5–800 µE m−2 s−1, 15/9 light/dark photoperiod) and temperatures (10–35 °C). The results provide a set of parameters (particularly the maximal growth rate associated to optimal conditions of light and temperature) for a three-equation model used to described the growth rate response of a non-nutrient-limited culture.

Biomanipulation: the next phase — making it stable

Abstract: Review of the literature on biomanipulation shows that fish manipulations have occurred through the following methods: piscivore addition; piscivore catch restriction; habitat enhancement; piscivore removal; planktivore exclusion; planktivore removal by selective catch, lake emptying, fish poisons, fish diseases, winterkill, summerkill; habitat expansion or contraction; planktivore addition; and natural events. The methods can be classified as deliberate, inadvertent, or natural, all of which have successful and unsuccessful examples. However, the problem of perpetuation of successful results remains unsolved. It is proposed that this may be resolved through use of a refuge or refuges from among the following: low light intensity refuge; low temperature refuge; low dissolved oxygen refuge; physical concealment refuge; visual clutter refuge; behavior modification refuge; and predator inefficiency refuge. Perhaps through use of such mechanisms large herbivorous zooplankters can continue to exist in lakes. Examples are given.

Fish manipulation as a lake restoration tool in shallow, eutrophic temperate lakes 1: cross-analysis of three Danish case-studies

Abstract: The use of fish manipulation as a tool for lake restoration in eutrophic lakes has been investigated since 1986 in three shallow, eutrophic Danish lakes The lakes differ with respect to nutrient loading and nutrient levels (130–1000 μg P l−1, 1–6 mg Nl−1) A 50% removal of planktivorous fish in the less eutrophic cyanobacteria-diatom dominated Lake Vaeng caused marked changes in lower trophic levels, phosphorus concentration and transparency Only minor changes occurred after a 78% removal of planktivorous fish in eutrophic cyanobacteria dominated Frederiksborg Castle Lake In the hypertrophic, green algae dominated Lake Sobygard a low recruitment of all fish species and a 16% removal of fish biomass created substantial changes in trophic structure, but no decrease in phosphorus concentration The different response pattern is interpreted as (1) a difference in density and persistence of bloomforming cyanobacteria caused by between-lake variations in nutrient levels and probably also mixing- and flushing rates, (2) a difference in specific loss rates through sedimentation of the algal community prevaling after the fish manipulation, (3) a decreased impact of planktivorous fish with increasing mean depth and (4) a lake specific difference in ability to create a self-increasing reduction in the phosphorus level in the lake water This in turn seems related to the phosphorus loading

Whole-lake food-web manipulation as a means to study community interactions in a small ecosystem

Abstract: Whole-lake food-web manipulation was carried out in the hypertrophic Lake Zwemlust (The Netherlands), with the aim of studying the effects on the lake’s trophic status and to gain an insight into complex interactions among lake communities. Before manipulation this small (1.5 ha) and shallow (1.5 m) lake was characterized by Microcystis blooms in summer and high chlorophyll-a concentrations were common (ca. 250 µg 1−1). In March 1987 the planktivorous and benthivorous fish species in the lake were completely removed (ca. 1000 kg ha −1), a new simple fish community (pike and rudd) was introduced and artificial refuges were created. The effects of this manipulation on the light climate, nutrient concentrations, phytoplankton, zooplankton, fish, macrophytes, and macrofauna were monitored during 1987, 1988 and 1989. Community interactions were investigated in phytoplankton bioassays and zooplankton grazing experiments. After the manipulation, despite the still high P and N loads to the lake (ca. 2.2 g P m − 2 y − 1and ca. 5.3 g N m − 2 y −1), the phytoplankton density was low (Chl-a 5 µg 1− 1), due to control by large-sized zooplankton in spring and N-limitation in summer and autumn. A marked increase in the abundance of macrophytes and filamentous green algae in 1988 and 1989, as well as N loss due to denitrification, contributed to the N limitation of the phytoplankton. Before manipulation no submerged macro-vegetation was present but in 1988, the second year after manipulation, about 50% of the lake bottom was covered by macrophytes increasing to 80% in 1989. This led to substantial accumulation of both N and P, namely 76% and 73% respectively of the total nutrients in the lake in particulate matter. Undesirable features of the increase in macrophytes were: 1) direct nuisance to swimmers; and, 2) the large scale development of snails, especially L. peregra,which may harbour the parasite causing ‘swimmers’ itch’. But harvesting of only about 3% of the total macrophyte biomass from the swimmers’ area, twice a year, reduced the nuisance for swimmers without adversely affecting the water clarity.

Scraping a living: a review of littorinid grazing

Abstract: Littorinid snails are predominantly herbivorous and the versatility of their radulae enables them to feed on a variety of macroscopic and microscopic plants in a diversity of habitats. Some are selective feeders preferring some species of algae to others, and rejecting some even after a prolonged period of starvation. Different species of snail exhibit different preferences. The factors affecting the attractiveness and edibility of food plants are discussed and food value considered.

Ulva lactuca as bioindicator of metal contamination in intertidal waters in Hong Kong

Abstract: The use of Ulva lactuca L. as an indicator of metal contamination was assessed by analysing the levels of Mn, Fe, Ni, Cu, Zn, Cd and Pb in the alga collected from 24 intertidal sites around the Island of Hong Kong. Twelve of the sites are in the rural southern parts of the Island where the coastal waters are relatively clean. The remaining 12 sites are located in the north and within Victoria Harbour which receives, apart from industrial effluents, untreated domestic sewage from a population of some 3.5 million. The mean levels of Mn, Fe, Ni, Cu, Zn and Pb in Ulva from the urban sites were respectively 4.0, 4.6, 1.8, 2.3, 2.4 and 4.6 folds those from the rural sites. However, similar levels of Cd were found in the alga amongst all the sites. Locations of high levels of metal contamination, particularly to the eastern end of the Harbour, have been identified. Preliminary results indicate that Ulva is a good indicator of Mn, Fe, Cu, Zn and Pb contaminations.

Ground water flow paths in relation to nitrogen chemistry in the near-stream zone

Abstract: Interactions between ground water flow paths and water chemistry were studied in the riparian zone of a small headwater catchment near Toronto, Ontario. Significant variations in oxygen — 18 and chloride indicated the presence of distinct sources of water in the ground water flow system entering the near-stream zone. Shallow ground water at the upland perimeter of the riparian zone had nitrate-N, chloride and dissolved oxygen concentrations which ranged between 100–180 µg L−1, 1.2–1.8 mg L−1 and 4.6–9.1 mg L−1 respectively. Concentrations of nitrate — N in deep ground water flowing upward beneath the riparian wetland were < 10 µg L−1, whereas chloride and dissolved oxygen ranged between 0.6–0.9 mg L−1 and 0.4–2.2 mg L−1 respectively. Ammonium — N concentrations (20–60 µg L−1) were similar in shallow and deep ground water. Ground water was transported through the wetland to the stream by three hydrologic pathways. 1) Shallow ground water emerged as springs near the base of the hillslope producing surface rivulets which crossed the riparian zone to the stream. 2) Deep ground water flowed upward through organic soils and entered the rivulets within the wetland. 3) Deep ground water reached the stream as bed and bank seepage. Springs were higher in nitrate and chloride than rivulets entering the stream, whereas bank seeps had lower concentrations of nitrate and chloride and considerably higher ammonium concentrations than the rivulets. These contrasts in nitrate and chloride concentrations were related to initial differences in the ion chemistry of shallow and deep ground water rather than to element transformations within the riparian wetland. Differences in ammonium concentration between seeps and rivulets were caused by immobilization of ammonium in the substrates of aerobic rivulets, whereas little ammonium depletion probably occurred in deep ground water flowing upward through reduced subsurface organic soils around the stream perimeter.

Phytoplankton biomass reduction after planktivorous fish reduction in a shallow, eutrophic lake: a combined effect of reduced internal P-loading and increased zooplankton grazing

Abstract: No recovery was recorded in the shallow and eutrophic Lake Vaeng, Denmark, after a sewage diversion in 1981, due to an internal phosphorus loading and a dominance of planktivorous fish. In order to improve lake water quality by means of biomanipulation, a total of 2.5 tons of bream (Abramis brama) and roach (Rutilus rutilus) was removed during 1986 and the spring of 1987. The planktivorous/benthivorous fish biomass was thereby reduced by approximately 50%, from 30 to 15 g WW m−2. After the reduction, the biological structure of the lake changed markedly. The zooplankton community changed from a dominance of rotifers before the fish reduction to larger cladocerans afterwards. Zooplankton mean summer biomass increased from 0.4 mg DW l−1 in 1986 to 2.7 mg DW l−1 in 1987 and to 1.3 mg DW l−1 in 1988. Phytoplankton biomass decreased from a mean summer level of 25 mm3 l−1 in 1986 to 12 in 1987 and to 7 mm3 l−1 in 1988. Qualitatively, the phytoplankton changed from a dominance of cyanobacteria and small diatoms to a dominance of larger diatoms, larger greens and cryptophytes. Secchi depth increased from a mean summer level of 0.6 m in 1986 to 1.0 m in 1987 and 1.3 m in 1988. After the fish removal, the internal phosphorus loading was reduced markedly. This reduction is considered primarily to be caused by the improved redox conditions in the sediment due to reduced sedimentation, and by the increased micro-benthic primary production and phosphorus uptake. Submerged macrophytes, mainlyPotamogeton crispus andElodea canadensis, increased in abundance due to improved light climate at the lake bottom.

Changes in lake levels, salinity and the biological community of Great Salt Lake (Utah, USA), 1847–1987

Abstract: Great Salt Lake is the fourth largest terminal lake in the world, with an area of about 6000 square kilometers at its historic high elevation. Since its historic low elevation of 1277.52 meters in 1963, the lake has risen to a new historic high elevation of 1283.77 meters in 1986–87, a net increase of about 6.25 meters. About 60 percent of this increase, 3.72 meters, has occurred since 1982 in response to greater than average precipitation and less than average evaporation.

Massive fish kills within the nyanza gulf of lake victoria, kenya

Abstract: The morphometry and hydrology of the Nyanza Gulf of Lake Victoria can be greatly affected by violent storms. This can result in a condition in which nutrient-rich bottom mud is mixed with the sediment-laden runoff water from neighbouring marshes and rivers. This situation can lead to massive fish kills. It is such an event in 1984 which will be described that caused the sudden death of over 400 000 fish weighing over 2400 tonnes. The unusually low precipitation in the catchment area during that year resulted in a drop in the lake level from 12.4 to 11.8 m. This low water level, combined with a severe storm created the conditions which caused this large fish kill. The species affected were Lates niloticus and Oreochromis niloticus. Mortality was attributed to several factors which included high levels of suspended material in the water column (detritus and algae) which clogged the gills of the fish, low dissolved oxygen, low pH, and high concentrations of algae.

The Forth estuary: a nursery and overwintering area for North Sea fishes

Abstract: The fish community of the Forth estuary, Scotland, has several components — estuarine resident species, diadromus migratory species, marine and freshwater adventitious species, marine juvenile migrants using the area as a nursery, and adults of marine species with seasonal migrations. The population changes during the period 1981–1988 in six species representative of the last two categories are described here. The species are the juvenile gadoids, whiting, Merlangius merlangus (L.) and cod, Gadus morhua L., juvenile flatfish, plaice, Pleuronectes platessa L. and common dab, Limanda limanda (L.), and the adult clupeids, sprat, Sprattus sprattus (L.) and herring, Clupea harengus L.

Incidence of mussel culture on biogeochemical fluxes at the sediment-water interface

Abstract: Upward nutrient fluxes at the sediment-water interface were studied in a mussel farming zone (Carteau, Gulf of Fos, France) in order to estimate the impact of organic matter input from biodeposition. Nitrate, nitrite, ammonia, silicate, phosphate and oxygen were measured. Fluxes were estimated by means of polyacrylate benthic chambers placed at sites located under (UM) and outside (OM) the rope hanging structures. Transformation of biodeposited organic matter increases phosphate, silicate and ammonia fluxes. No variation in nitrite fluxes could be detected and only minor differences were observed in nitrate and the oxygen production/consumption equilibrium at the two stations. Phosphate and silicate fluxes, which were always higher at the UM than at the OM site, decreased from spring to winter. Ammonia fluxes were very high under mussel cultures in May and September and lower in November. The fact that ammonia flux was always higher at the UM than at the OM sites might be explained by degradation of mussel biodeposit, as well as by benthic macrafauna excretion. Discrepancies between fluxes of the nutrients studied at the UM and OM sites increased as organic particulate matter in the water column decreased. Variations of oxygen flux followed a different pattern, since they were correlated with presence and abundance of photosynthetic microphytes on the bottom and in the water. Bottom respiration exceeded production of oxygen only in May 1988 at the UM station. As it now stands, biodeposit input into the sediment under mussel ropes does not affect the ecosystem, although the flow of nutrients towards the water column is higher than in other areas.

In situ biochemical and bacterial variation of sediments enriched with mussel biodeposits

Abstract: In order to estimate the qualitative variation of sedimented mussel deposits, biochemical and microbial measurements were undertaken after a sediment enrichment with fresh faeces and pseudo-faeces collected in a mussel farming area (Carteau, Gulf of Fos).