Relations between trophic state indicators and plant biomass in Florida lakes

We collected quantitative data on macrophyte abundance and water quality in 319 mostly shallow, polymictic, Florida lakes to look for relationships between trophic state indicators and the biomasses of plankton algae, periphyton, and macrophytes. The lakes ranged from oligotrophic to hypereutrophic with total algal chlorophylls ranging from 1 to 241 mg m^–3. There were strong positive correlations between planktonic chlorophylls and total phosphorus and total nitrogen, but there were weak inverse relationships between the densities of periphyton and the trophic state indicators total phosphorus, total nitrogen and algal chlorophyll and a positive relationship with Secchi depth. There was no predictable relationship between the abundance of emergent, floating-leaved, and submersed aquatic vegetation and the trophic state indicators. It was only at the highest levels of nutrient concentrations that submersed macrophytes were predictably absent and the lakes were algal dominated. Below these levels, macrophyte abundance could be high or low. The phosphorus–chlorophyll and phosphorus–Secchi depth relationships were not influenced by the amounts of aquatic vegetation present indicating that the role of macrophytes in clearing lakes may be primarily to reduce nutrient concentrations for a given level of loading. Rather than nutrient concentrations controlling macrophyte abundance, it seems that macrophytes acted to modify nutrient concentrations.     

Macrophyte effects on algal turbidity in subtropical versus temperate lakes: a comment on Wang et al. (2014)

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Seasonality of chlorophyll and nutrients in Lake Erken – effects of weather conditions

The effect of submerged macrophytes on interactions among epilimnetic phosphorus, phytoplankton, and heterotrophic bacterioplankton has been acknowledged, but remains poorly understood. Here, we test the hypotheses that the mean summer phytoplankton biomass (chlorophyll a): phosphorus ratios decrease with increased macrophyte cover in a series of nine lakes. Further, we test that both planktonic respiration and bacterioplankton production increase with respect to phytoplankton biomass along the same gradient of increasing macrophyte cover. Increased macrophyte cover was associated with a lower fraction of particulate phosphorus in epilimnia, with total particulate phosphorus declining from over 80% of total phosphorus in a macrophyte free lake to less than 50% in a macrophyte rich lake. Phytoplankton biomass (chlorophyll a) too was lower in macrophyte dominated lakes, despite relatively high levels of total dissolved phosphorus. Planktonic respiration and bacterioplankton production were higher in macrophyte rich lakes than would be expected from phytoplankton biomass alone, pointing to a subsidy of bacterioplankton metabolism by macrophyte beds at the whole lake scale. The results suggest that the classical view of pelagic interactions, which proposes phosphorus determines phytoplankton abundance, which in turn determines bacterial abundance through the production of organic carbon, becomes less relevant as macrophyte cover increases.     

Trophic Classification of Non‐Perennial Reservoirs Utilized for the Development of Culture‐Based Fisheries,Sri Lanka

The aim of this study was to check the suitability of some trophic models developed for temperate regions to classify the non‐perennial reservoirs of Sri Lanka in order to manage culture‐based fisheries of those reservoirs. A limnological study was carried out in 45 non‐perennial reservoirs, which have been randomly selected for stocking of fish fingerlings for the development of culture‐based fisheries. High total phosphorous (TP) content in relation to algal biomass indicates high non‐algal turbidity in all reservoirs. Carlson's trophic state indices (TSI) measured on the basis of Secchi disc depth [TSI (SDD)], TP [TSI (TP)] and chlorophyll a [TSI (Chl‐a)] show that the 45 reservoirs studied are characterized by TSI (TP) = TSI (SDD) > TSI (Chl‐a), indicating that non‐algal particulate matter or colour dominates underwater light attenuation. As TSI (Chl‐a) is positively correlated to culture‐based fisheries yield, it is useful for planning culture‐based fisheries development strategies in non‐perennial reservoirs of Sri Lanka, and has the potential to be used elsewhere in the tropics for comparable developments. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lake responses to reduced nutrient loading – an analysis of contemporary long-term data from 35 case studies

1. This synthesis examines 35 long‐term (5–35 years, mean: 16 years) lake re‐oligotrophication studies. It covers lakes ranging from shallow (mean depth <5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 μg L^?1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north‐temperate lakes were most abundant. 2. Reduction of external total phosphorus (TP) loading resulted in lower in‐lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10–15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially. 3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in‐lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables. 4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria. 5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of <100–150 μg L^?1. This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters. 6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity. 7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re‐oligotrophication.     

Temporal variability in algal biomass and primary productivity in Florida lakes relative to latitudinal gradients,organic color and trophic state

Seasonal patterns in primary productivity and algal biomass in subtropical Florida lakes along increasing gradients of both dissolved organic color and phytoplankton biomass are presented. Chlorophyll a concentrations and gross primary productivity generally reached maxima during the summer and were most depressed in winter months, regardless of color or trophic classification. Primary productivity was more strongly correlated with chlorophyll a, nutrient concentrations and water clarity in clearwater (< 75 Pt units) than in colored (> 75 Pt units) systems. Amplitudes in algal biomass were considerably smaller than temperate lakes. Variability in primary production in Florida lakes was intermediate to patterns in the temperate zone and tropics, but was more closely aligned to northern latitudes. Within the Florida peninsula, variability of primary productivity decreased from north to south and corresponded to latitudinal gradients in climatic regimes.     

Ambiguous climate impacts on competition between submerged macrophytes and phytoplankton in shallow lakes

1. Shallow lakes may switch from a state dominated by submerged macrophytes to a phytoplankton‐dominated state when a critical nutrient concentration is exceeded. We explore how climate change may affect this critical nutrient concentration by linking a graphical model to data from 83 lakes along a large climate gradient in South America. 2. The data indicate that in warmer climates, submerged macrophytes may tolerate more underwater shade than in cooler lakes. By contrast, the relationship between phytoplankton biomass [approximated by chlorophyll‐a (chl‐a) or biovolume] and nutrient concentrations did not change consistently along the climate gradient. In warmer climates, the correlation between phytoplankton biomass and nutrient concentrations was overall weak, especially at low total phosphorus (TP) concentrations where the chl‐a/ TP ratio could be either low or high. 3. Although the enhanced shade tolerance of submerged plants in warmer lakes might promote the stability of their dominance, the potentially high phytoplankton biomass at low nutrient concentrations suggests an overall low predictability of climate effects. 4. We found that near‐bottom oxygen concentrations are lower in warm lakes than in cooler lakes, implying that anoxic P release from eutrophic sediment in warm lakes likely causes higher TP concentrations in the water column. Subsequently, this may lead to a higher phytoplankton biomass in warmer lakes than in cooler lakes with similar external nutrient loadings. 5. Our results indicate that climate effects on the competitive balance between submerged macrophytes and phytoplankton are not straightforward.     

Responses of four submerged macrophytes to freshwater snail density (Radix swinhoei) under clear‐water conditions: A mesocosm study

Macrophytes play a key role in stabilizing clear‐water conditions in shallow freshwater ecosystems. Their populations are maintained by a balance between plant grazing and plant growth. As a freshwater snail commonly found in shallow lakes, Radix swinhoei can affect the growth of submerged macrophytes by removing epiphyton from the surface of aquatic plants and by grazing directly on macrophyte organs. Thus, we conducted a long‐term (11‐month) experiment to explore the effects of snail density on macrophytes with distinctive structures in an outdoor clear‐water mesocosm system (with relatively low total nitrogen (TN, 0.66 ± 0.27 mg/L) and total phosphorus (TP, 36 ± 20 μg/L) and a phytoplankton chlorophyll a (Chla) range of 14.8 ± 4.9 μg/L) based on two different snail densities (low and high) and four macrophyte species treatments (Myriophyllum spicatum, Potamogeton wrightii, P. crispus, and P. oxyphyllus). In the high‐density treatment, snail biomass and abundance (36.5 ± 16.5 g/m² and 169 ± 92 ind/m², respectively) were approximately twice that observed in the low‐density treatment, resulting in lower total and aboveground biomass and ramet number in the macrophytes. In addition, plant height and plant volume inhabited (PVI) showed species‐specific responses to snail densities, that is, the height of P. oxyphyllus and PVI of M. spicatum were both higher under low‐density treatment. Thus, compared with low‐density treatment, the inhibitory effects of long‐term high snail density on macrophytes by direct feeding may be greater than the positive effects resulting from epiphyton clearance when under clear‐water conditions with low epiphyton biomass. Thus, under clear‐water conditions, the growth and community composition of submerged macrophytes could be potentially modified by the manual addition of invertebrates (i.e., snails) to lakes if the inhibitory effects from predatory fish are minor.     

A 2-year experimental study on nutrient and predator influences on food web constituents in a shallow lake of north-west Spain

1. A 2‐year study was carried out on the roles of nutrients and fish in determining the plankton communities of a shallow lake in north‐west Spain. Outcomes were different each year depending on the initial conditions, especially of macrophyte biomass. In 1998 estimated initial ‘per cent water volume inhabited’ (PVI) by submerged macrophytes was about 35%. Phytoplankton biomass estimated as chlorophyll a was strongly controlled by fish, whereas effects of nutrient enrichment were not significant. In 1999 estimated PVI was 80%, no fish effect was observed on phytoplankton biomass, but nutrients had significant effects. Water temperatures were higher in 1998 than in 1999. 2. In the 1998 experiment, cladoceran populations were controlled by fish and cyanobacteria were the dominant phytoplankton group. There were no differences between effects of low (4 g fresh mass m^?2) and high (20 g fresh mass m^?2) fish density on total zooplankton biomass, but zooplankton biomass was higher in the absence of fish. With the high plant density in 1999, fish failed to control any group of the zooplankton community. 3. Total biovolume of phytoplankton strongly decreased with increased nutrient concentrations in 1998, although chlorophyll a concentrations did not significantly change. At higher nutrient concentrations, flagellate algae became more abundant with likely growth rates that could have overcompensated cladoceran feeding rates. This change in phytoplankton community composition may have been because of increases in the DIN : SRP ratio. Both chlorophyll a concentration and total phytoplankton biovolume increased significantly with nutrients in the 1999 experiment. 4. A strong decline of submerged macrophytes was observed in both years as nutrients increased, resulting in shading by periphyton. This shading effect could account for the plant decline despite lower water turbidity at the very high nutrient levels in 1998.     

Regularities in Primary Production,Secchi Depth and Fish Yield and a New System to Define Trophic and Humic State Indices for Lake Ecosystems

General relationships between phytoplankton production, chlorophyll, total, dissolved and particulate phosphorus, Secchi depth, humic level, trophic level, fish production and latitude are described by regression equations using an extensive “Soviet” data base covering a wide domain of lake characteristics and a European data base. New systems for defining lake trophic and humic status are presented. The results may be used for more precise estimates of fundamental lake properties and for many practical issues of lake management, e.g., predictions of fish catch. We have used strict chlorophyll‐a concentrations for every trophic class and we have omitted Secchi depth from the trophic classes, since Secchi depth and other variables strongly related to water clarity (like suspended particulate matter and particulate organic carbon) depend on autochthonous production, allochthonous influences and resuspension. We have used the Secchi depth as a simple operational measure of the effective depth of the photic zone. It has also been shown that among these lakes there exist a very strong relationship between primary production and latitude. In fact, 74% of the variability among the lakes in mean summer primary production can be statistically related to variations in latitude. These data also show a strong relationship between primary production and fish yield, which can be used to address many fundamental issues in lake management, like “normal and abnormal fish production”.     

Phytoplankton and Epipelon Responses to Clear and Turbid Phases in a Seepage Lake (Buenos Aires,Argentina)

Annual changes in the algal density and concentrations of chlorophyll a, total phosphorus, and organic matter were analyzed in water and sediments at four sites characterized by the presence or absence of submerged and emergent macrophytes, during turbid‐ and clear‐water conditions to determine the contribution of the algal components of the plankton and the epipelon and to identify the most typical species in each community. Three states were recognized: one turbid and two clear, with different submerged macrophyte cover. The peaks of phytoplankton and epipelon occurred in the turbid phase, whereas the highest proportion of true epipelic algae in sediments was reached in the second clear phase. The Oscillatoriaceae dominated during the turbid phase in the water and throughout the entire year within the sediments. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stochastic modelling of nutrient loading and lake ecosystem response in relation to submerged macrophytes and benthivorous fish

SUMMARY 1. The strong stabilising effect of increased submerged macrophytes (charophytes) and benthivorous fish reduction on the clear water state was shown for shallow Lake Veluwe and Lake Wolderwijd. 2. The first two links in the chain of relationships from external phosphorus (P) loading to in‐lake total‐P concentrations to chlorophyll a concentrations to water transparency, showed a significant correlation with the areal fraction of coverage with charophytes. Higher coverages lead to (i) lower ratios of the in‐lake total‐P concentration compared with the volume weighted average concentration in the inlet water, indicating a higher retention of P in the presence of charophytes (ii) lower chlorophyll a to total‐P ratios, indicating a positive effect of charophytes on top‐down control of algae, and (iii) higher water transparency because of lower algal turbidity. Transparency further improved as a result of benthivorous fish reduction and a significant positive correlation between non‐algal turbidity and benthivorous fish biomass. 3. A model was developed taking into account the inherent variability in precipitation and uncertainties in the empirical relationships determining phosphorus export from stream catchments and other sources and eutrophication variables in the receiving lakes. The model was used to compute (i) probability distributions for in‐lake total‐P, chlorophyll a and Secchi Disc transparency in relation to the coverage with charophytes and benthivorous fish biomass, and (ii) exceedence probabilities with respect to critical values for in‐lake total‐P and water transparency for several management scenarios. 4. The effects of an expected rise in external nutrient loading on the in‐lake total‐P and chlorophyll a concentrations and on water transparency can be compensated for by two proposed control measures: (i) extended treatment at a waste water treatment plant directly discharging into Lake Veluwe, and (ii) diverting the outlet of a stream draining a catchment with high fertilisation. The minimal internal charophyte coverage needed to sufficiently stabilise the clear water state and to meet with the objective of a summer mean water transparency of at least 1 m was estimated at well over 30% of the lake area, while the benthivorous fish stock should be maintained at the present level of c. 20 kg ha^?1.     

Microbial tropicalization driven by a strengthening western ocean boundary current

Western boundary currents (WBCs) redistribute heat and oligotrophic seawater from the tropics to temperate latitudes, with several displaying substantial climate change‐driven intensification over the last century. Strengthening WBCs have been implicated in the poleward range expansion of marine macroflora and fauna, however, the impacts on the structure and function of temperate microbial communities are largely unknown. Here we show that the major subtropical WBC of the South Pacific Ocean, the East Australian Current (EAC), transports microbial assemblages that maintain tropical and oligotrophic (k‐strategist) signatures, to seasonally displace more copiotrophic (r‐strategist) temperate microbial populations within temperate latitudes of the Tasman Sea. We identified specific characteristics of EAC microbial assemblages compared with non‐EAC assemblages, including strain transitions within the SAR11 clade, enrichment of Prochlorococcus, predicted smaller genome sizes and shifts in the importance of several functional genes, including those associated with cyanobacterial photosynthesis, secondary metabolism and fatty acid and lipid transport. At a temperate time‐series site in the Tasman Sea, we observed significant reductions in standing stocks of total carbon and chlorophyll a, and a shift towards smaller phytoplankton and carnivorous copepods, associated with the seasonal impact of the EAC microbial assemblage. In light of the substantial shifts in microbial assemblage structure and function associated with the EAC, we conclude that climate‐driven expansions of WBCs will expand the range of tropical oligotrophic microbes, and potentially profoundly impact the trophic status of temperate waters.     

Dynamics of submerged macrophyte populations in response to biomanipulation

1. A 6‐year study (1992–97) of changes in submerged vegetation after biomanipulation was carried out in the eutrophicated Lake Finjasjön, Southern Sweden. Ten sites around the lake were revisited each year. At each site five samples of above‐ground biomass were taken at 10 cm water depth intervals. An investigation of the seed bank at the 10 sites, and a grazing experiment where birds and large fish were excluded was also conducted. 2. Between 1992 and 1996, in shallow areas (water depth < 3 m), vegetation cover increased from < 3 to 75% and above‐ground biomass from < 1 to 100 g DW m^–2. Mean outer water depth increased from 0.3 to 2.5 m. Elodea canadensis and Myriophyllum spicatum accounted for > 95% of the increase in biomass and plant cover. The following year (1997), however, cover and above‐ground biomass decreased, mainly attributable to the total disappearance of E. canadensis. Secchi depth increased after biomanipulation until 1996, but decreased again in 1997. 3. Total and mean number of submerged species increased after biomanipulation, probably as a result of the improved light climate. However, after the initial increase in species number there was a decrease during the following years, possibly attributed to competition from the rapidly expanding E. canadensis and M. spicatum. The lack of increase in species number after the disappearance of E. canadensis in 1997 implies that other factors also affected species richness. 4. A viable seed bank was not necessary for a rapid recolonization of submerged macrophytes, nor did grazing by waterfowl or fish delay the re‐colonization of submerged macrophytes. 5. Submerged macrophytes are capable of rapid recolonization if conditions improve, even in large lakes such as Finjasjön (11 km²). Species that spread by fragments will increase rapidly and probably outcompete other species. 6. The results indicate that after the initial Secchi depth increase, probably caused by high zooplankton densities, submerged vegetation further improved the light climate. The decrease in macrophyte biomass in 1997 may have caused the observed increase in phosphorus and chlorophyll a, and the decrease in Secchi depth. We suggest that nutrient competition from periphyton, attached to the macrophytes, may be an important factor in limiting phytoplankton production, although other factors (e.g. zooplankton grazing) are also of importance, especially as triggers for the shift to a clear‐water state.     

Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient

1. Using data from 71, mainly shallow (an average mean depth of 3 m), Danish lakes with contrasting total phosphorus concentrations (summer mean 0.02–1.0 mg P L?l), we describe how species richness, biodiversity and trophic structure change along a total phosphorus (TP) gradient divided into five TP classes (class 1–5: <0.05, 0.05–0.1, 0.1–0.2, 0.2–0.4,> 0.4 mg P L?1).
2. With increasing TP, a significant decline was observed in the species richness of zooplankton and submerged macrophytes, while for fish, phytoplankton and floating‐leaved macrophytes, species richness was unimodally related to TP, all peaking at 0.1–0.4 mg P L?1. The Shannon–Wiener and the Hurlbert probability of inter‐specific encounter (PIE) diversity indices showed significant unimodal relationships to TP for zooplankton, phytoplankton and fish. Mean depth also contributed positively to the relationship for rotifers, phytoplankton and fish.
3. At low nutrient concentrations, piscivorous fish (particularly perch, Perca fluviatilis) were abundant and the biomass ratio of piscivores to plankti‐benthivorous cyprinids was high and the density of cyprinids low. Concurrently, the zooplankton was dominated by large‐bodied forms and the biomass ratio of zooplankton to phytoplankton and the calculated grazing pressure on phytoplankton were high. Phytoplankton biomass was low and submerged macrophyte abundance high.
4. With increasing TP, a major shift occurred in trophic structure. Catches of cyprinids in multiple mesh size gill nets increased 10‐fold from class 1 to class 5 and the weight ratio of piscivores to planktivores decreased from 0.6 in class 1 to 0.10–0.15 in classes 3–5. In addition, the mean body weight of dominant cyprinids (roach, Rutilus rutilus, and bream, Abramis brama) decreased two–threefold. Simultaneously, small cladocerans gradually became more important, and among copepods, a shift occurred from calanoid to cyclopoids. Mean body weight of cladocerans decreased from 5.1 μg in class 1 to 1.5 μg in class 5, and the biomass ratio of zooplankton to phytoplankton from 0.46 in class 1 to 0.08–0.15 in classes 3–5. Conversely, phytoplankton biomass and chlorophyll a increased 15‐fold from class 1 to 5 and submerged macrophytes disappeared from most lakes.
5. The suggestion that fish have a significant structuring role in eutrophic lakes is supported by data from three lakes in which major changes in the abundance of planktivorous fish occurred following fish kill or fish manipulation. In these lakes, studied for 8 years, a reduction in planktivores resulted in a major increase in cladoceran mean size and in the biomass ratio of zooplankton to phytoplankton, while chlorophyll a declined substantially. In comparison, no significant changes were observed in 33 ‘control’ lakes studied during the same period.     

Major changes in CO<Subscript>2</Subscript> efflux when shallow lakes shift from a turbid to a clear water state

Lakes can be sources or sinks of carbon, depending on local conditions. Recent studies have shown that the CO₂ efflux increases when lakes recover from eutrophication, mainly as a result of a reduction in phytoplankton biomass, leading to less uptake of CO₂ by producers. We hypothesised that lake restoration by removal of coarse fish (biomanipulation) or invasion of mussels would have a similar effect. We studied 14–22 year time series of five temperate Danish lakes and found profound effects on the calculated CO₂ efflux of major shifts in ecosystem structure. In two lakes, where limited colonisation of submerged macrophytes occurred after biomanipulation or invasion of zebra mussels (Dreissena polymorpha), the efflux increased significantly with decreasing phytoplankton chlorophyll a. In three lakes with major interannual variation in macrophyte abundance, the efflux declined with increasing macrophyte abundance in two of the lakes, while no relation to macrophytes or chlorophyll a was found in the third lake, likely due to high groundwater input to this lake. We conclude that clearing water through invasive mussels or lake restoration by biomanipulation may increase the CO₂ efflux from lakes. However, if submerged macrophytes establish and form dense beds, the CO₂ efflux may decline again.     

Clear,crashing, turbid and back – long‐term changes in macrophyte assemblages in a shallow lake

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Lake trophic state and the limnological effects of omnivorous fish

Ecologists have hypothesized that planktivorous fish have greater effects on the plankton and water quality of oligotrophic lakes than eutrophic lakes. We tested this hypothesis in a tank-mesocosm experiment of factorial design in which five biomass levels of filter-feeding omnivorous gizzard shad (Dorosoma cepedianum) were cross-classified with two levels of lake trophic state achieved by filling tank-mesocosms with water and plankton transported by truck from two lakes with different trophic states. The presence of gizzard shad significantly increased total phosphorus, primary productivity, chlorophyll, and particulate phosphorus (PP) 2–20 and 20–200 μm and significantly decreased Secchi depth, cladocerans, copepods and PP > 200 μm. The effects of gizzard shad on chlorophyll, Secchi depth, cladocerans, copepods and PP 2–20 and > 200 μm were dependent on lake trophic state and most intense in the eutrophic lake system. This experiment suggests that filter-feeding omnivorous fish interact synergistically with trophic state so that the limnological effects of omnivorous fish become more intense with increased eutrophication.     

Does high nitrogen loading prevent clear‐water conditions in shallow lakes at moderately high phosphorus concentrations?

1. The effect of total nitrogen (TN) and phosphorus (TP) loading on trophic structure and water clarity was studied during summer in 24 field enclosures fixed in, and kept open to, the sediment in a shallow lake. The experiment involved a control treatment and five treatments to which nutrients were added: (i) high phosphorus, (ii) moderate nitrogen, (iii) high nitrogen, (iv) high phosphorus and moderate nitrogen and (v) high phosphorus and high nitrogen. To reduce zooplankton grazers, 1⁺ fish (Perca fluviatilis L.) were stocked in all enclosures at a density of 3.7 individuals m^?2. 2. With the addition of phosphorus, chlorophyll a and the total biovolume of phytoplankton rose significantly at moderate and high nitrogen. Cyanobacteria or chlorophytes dominated in all enclosures to which we added phosphorus as well as in the high nitrogen treatment, while cryptophytes dominated in the moderate nitrogen enclosures and the controls. 3. At the end of the experiment, the biomass of the submerged macrophytes Elodea canadensis and Potamogeton sp. was significantly lower in the dual treatments (TN, TP) than in single nutrient treatments and controls and the water clarity declined. The shift to a turbid state with low plant coverage occurred at TN >2 mg N L^?1 and TP >0.13–0.2 mg P L^?1. These results concur with a survey of Danish shallow lakes, showing that high macrophyte coverage occurred only when summer mean TN was below 2 mg N L^?1, irrespective of the concentration of TP, which ranged between 0.03 and 1.2 mg P L^?1. 4. Zooplankton biomass and the zooplankton : phytoplankton biomass ratio, and probably also the grazing pressure on phytoplankton, remained overall low in all treatments, reflecting the high fish abundance chosen for the experiment. We saw no response to nutrition addition in total zooplankton biomass, indicating that the loss of plants and a shift to the turbid state did not result from changes in zooplankton grazing. Shading by phytoplankton and periphyton was probably the key factor. 5. Nitrogen may play a far more important role than previously appreciated in the loss of submerged macrophytes at increased nutrient loading and for the delay in the re‐establishment of the nutrient loading reduction. We cannot yet specify, however, a threshold value for N that would cause a shift to a turbid state as it may vary with fish density and climatic conditions. However, the focus should be widened to use control of both N and P in the restoration of eutrophic shallow lakes.     

Chemical limnology of two artificial lakes used for both stormwater management and recreation

The aims of this study were to document the mainly chemical behaviour of two linked artificial lakes used for both stormwater management and recreation in the new town of Craigavon. Further, the understanding of their behaviour should help in their management and the design of other similar lakes.The lake mean total phosphorus (73 µg P l^–1), nitrate (0.50 mg N l^–1) and chlorophyll a (25 µg l^–1) concentrations, Secchi depth (1.2 m) and the estimated total phosphorus loading (1.98 g m^–2 a^–1) all classify the main lake as eutrophic. An important source of the phosphorus load on the lakes is the urban area of Craigavon (52% of the total load). The interrelationships between total phosphorus, chlorophyll a and Secchi depth in the main lake are similar to those in natural ones. In addition, the lake follows the total phosphorus load — trophic state relationships (lake total phosphorus and chlorophyll a concentrations and Secchi depth) found to apply elsewhere. These two points indicate that the artificial lakes in Craigavon behave similarly to natural ones.