Trophic State Variables Research Articles

The Florida experience: time for a revitalized and restructured Clean Lakes Program

Canfield, DE Jr., Bachmann RW, Hoyer MV. 2023. The Florida experience: time for a revitalized and restructured Clean Lakes Program. Lake Reserv Manage. 39:191–212. Anthropogenic nonpoint source nutrient enrichment of lakes is a worldwide problem, but inputs from the watershed to Florida lakes may not be as severe as speculated. Long-term trends for total phosphorus (TP), total nitrogen (TN), chlorophyll (Chl), and Secchi disk transparency (SDT), enrichment surrogates, in 381 lakes monitored for 20–43 yr and having extensive management to none demonstrated improvements in trophic conditions following point source removals. Afterward, TP (R2 = 0.85), TN (R2 = 0.49), and Chl (R2 = 0.50) continued to decline significantly (P < 0.05) and SDT (R2 = 0.63) increased. For 99 state-designated “impaired” lakes, TP (R2 = 0.73), TN (R2 = 0.45), and Chl (R2 = 0.44) also declined and SDT (R2 = 0.69) increased. To consider natural background conditions, lakes were assigned to their ambient TP and TN zones. Geometric TP (93%) and TN (82%) zone averages for each lake after removal of point sources remained within each zone’s 95% confidence interval. Individual lake trend analyses documented that ∼80% showed no (most) or an improving trend for the trophic state variables. After correcting for “statistically meaningful” results or to remove false significances, <5% of the lakes had trophic state variable changes associated with eutrophication. If lakes are to be rehabilitated and/or protected to maintain designated uses, exclusive focus on watershed management of nonpoint source nutrients and associated regulatory actions should not be relied on to correct “impairments” in a timely manner. Management priorities should focus on actual causes of impairment and the most effective and efficient approaches for improvement.

Pathways to water sustainability? A global study assessing the benefits of integrated water resources management

Integrated water resources management (IWRM) has been central to water governance and management worldwide since the 1990s. Recognizing the significance of an integrated approach to water management as a way to achieve the Sustainable Development Goals (SDGs), IWRM was formally incorporated as part of the SDG global indicator framework, thus committing the UN and its Member States to achieving high IWRM implementation by 2030 and measuring progress through SDG indicator 6.5.1. This paper examines the extent to which the implementation of IWRM improves the sustainable management of water and the health of water-related ecosystems—a first-of-its-kind in terms of quantitative analysis on a global scale. To achieve this objective, we conducted regression analyses between SDG 6.5.1 (both IWRM (total score) and the dimensions of SDG 6.5.1) and key water-related environmental sustainability indicators: SDG 6.2.1a (access to basic sanitation), 6.3.1 (treated wastewater), 6.4.1 (water-use efficiency), 6.4.2 (water stress), 6.6.1 (freshwater ecosystems, although here the trophic state and turbidity variables were used) and 6.3.2 (ambient water quality). Our analysis covers 124 countries for all these SDGs, with the exception of SDG 6.3.1 and SDG 6.3.2, which cover 112 and 85 countries, respectively. Results show that IWRM—to different degrees—is mainly associated with the good status of water-related sustainability indicators, with the exception of water stress, water quality, and turbidity. We observe a strong impact of control variables such as governance arrangements, economic situation and environmental and geographical conditions. Lagged effects and the scope of the framework may also explain some observed variations in the degree of association. Our study highlights the importance of further uncovering the interlinkages between IWRM implementation and the achievement of water-related environmental sustainability. Overall, the results suggest that although IWRM implementation is primarily linked to sustainable water management and the health of water systems, context-specific factors should be taken into account when evaluating its effectiveness, to enable policy- and decision-makers to make the necessary adjustments to optimize its outcomes.

Zooplankton shifts from headwater to lowland streams: Insights into the role of water quality to assist the protection and restoration of agricultural waterways

AbstractDespite the critical importance of zooplankton for many ecological processes, knowledge of its structure and distribution patterns in small streams subject to intensive land uses remains limited. Here we analysed changes in zooplankton community and composition in 10 sites located in streams from first‐order to sixteenth‐order, according to Shreve's classification, in a small catchment influenced by intensive agriculture. Using univariate and multivariate methods, we investigated the extent to which local scale environmental factors explained the spatial patterns of zooplankton community. We found that environmental conditions of headwaters differed markedly from those of lowland streams, especially with respect to nutrient levels, which were higher in the former than the latter. Zooplankton community was mainly composed by microphagous rotifers, and the overall species composition differed with the stream order. While zooplankton diversity and evenness increased with stream order and correlated negatively with nutrient level, zooplankton abundance and biomass showed the opposite trend. The mechanistic processes creating these patterns remain unknown but are likely linked to the effects of greater water mixing and dilution in lowland streams, as well as ecological processes such as predation and resource diversification. No clear pattern was found for presence or concentrations of pesticides in relation to the stream order. However, water quality seemed to be an important factor influencing zooplankton assemblages, which were significantly associated with pesticides and trophic state variables. Our findings suggest that agricultural pollution is impacting the zooplankton community, which may have implications for the food webs and overall health of the stream systems.

Long-Term Water Quality Patterns in an Estuarine Reservoir and the Functional Changes in Relations of Trophic State Variables Depending on the Construction of Serial Weirs in Upstream Reaches.

Water quality degradation is one of the major problems with artificial lakes in estuaries. Long-term spatiotemporal patterns of water quality in a South Korean estuarine reservoir were analyzed using seasonal datasets from 2002 to 2020, and some functional changes in relations of trophic state variables due to the construction of serial weirs in the upper river were also investigated. A total of 19 water quality parameters were used for the study, including indicators of organic matter, nutrients, suspended solids, water clarity, and fecal pollution. In addition, chlorophyll-a (CHL-a) was used to assess algal biomass. An empirical regression model, trophic state index deviation (TSID), and principal component analysis (PCA) were applied. Longitudinal fluctuations in nutrients, organic matter, sestonic CHL-a, and suspended solids were found along the axis of the riverine (Rz), transition (Tz), and lacustrine zones (Lz). The degradation of water quality was seasonally caused by resuspension of sediments, monsoon input due to rainfall inflow, and intensity of Asian monsoon, and was also related to intensive anthropic activities within the catchment. The empirical model and PCA showed that light availability was directly controlled by non-algal turbidity, which was a more important regulator of CHL-a than total nitrogen (TN) and total phosphorus (TP). The TSID supported our hypothesis on the non-algal turbidity. We also found that the construction of serial upper weirs influenced nutrient regime, TSS, CHL-a level, and trophic state in the estuarine reservoir, resulting in lower TP and TN but high CHL-a and high TN/TP ratios. The proportions of both dissolved color clay particles and blue-green algae in the TSID additionally increased. Overall, the long-term patterns of nutrients, suspended solids, and algal biomass changed due to seasonal runoff, turnover time, and reservoir zones along with anthropic impacts of the upper weir constructions, resulting in changes in trophic state variables and their mutual relations in the estuarine reservoir.

Long-term zooplankton composition data reveal impacts of invasions on community composition in the Waikato lakes, New Zealand

Data collected on zooplankton community composition over longer time periods (> 10 years) are rare. We examined among-lake spatial and temporal trends of zooplankton communities from a monitoring programme undertaken in the Waikato region, New Zealand. A total of 39 lakes were sampled over a period of 12 years, between 2007 and 2019, with varying degrees of temporal effort. We focussed particularly on eight lakes, considered here as ‘long-term lakes’, where samples were collected with greater regularity (including 5 with 12 years of data). Among lakes, suspended sediment concentrations and indicators of lake trophic state were inferred to be important in determining the zooplankton distributions; as this region is dominated by shallow lakes, the relative importance of suspended sediments was high. Among the long-term lakes, the greatest dissimilarities in zooplankton community composition among years were in Lake Waahi, where the Australian Boeckella symmetrica was first detected in 2012. That is, the greatest temporal changes to zooplankton composition during the study period were due to the invasion by non-indigenous species, rather than changes in trophic state or other environmental variables; non-native species commonly dominated the individual counts of species through much of 2014 and 2015, with most samples since 2016 being again dominated by native species. Following this lake, the largest and shallowest lakes in the dataset—Whangape and Waikare—exhibited the greatest variability in community composition among years.

Spatiotemporal variability in a copepod community associated with fluctuations in salinity and trophic state in an artificial brackish reservoir at Saemangeum, South Korea.

Saemangeum Reservoir in South Korea is an estuarine system enclosed by a dyke construction, where seawater inflow and retained water outflow are managed by the opening/closing of sluice gates installed in the southern part of the dyke. An exchange of the reservoir water can cause spatiotemporal fluctuations in the salinity and trophic state, which are major drivers determining variation in the composition of biological communities in estuarine systems. Here, we investigated the seasonal and spatial variability in the copepod community and environmental conditions (water temperature, salinity, transparency, chlorophyll a concentration, total nitrogen, total phosphorus and Carlson’s trophic state index) based on seasonally conducted field monitoring in the Saemangeum Reservoir from July 2013 to January 2018. In addition to the role of temperature, salinity and chlorophyll a concentration in structuring the copepod community and diversity, the biological indices of copepods with respect to salinity range and trophic state, were evaluated. The spatiotemporal variability in the salinity and trophic state variables showed contrasting patterns, and chlorophyll a concentration was negatively affected by salinity, indicating that the reservoir water was being highly exchanged with opening of the sluice gates. The mean trophic state index values, however, were constant in the eutrophic state (50―70). Dominant copepods were Acartia (A. hudsonica, A. sinjiensis, Acartia spp.) and Oithona (O. davisae and Oithona spp.), which are common species in eutrophic neritic water. Variation in the copepod community was mainly associated with the seasonal succession of the dominant species rather than a spatial gradient (from around the estuary to the sluice gates); however, site-specific differences in frequencies of several non-dominant species could be detected around the estuary (Sinocalanus tenellus) and the sluice gates (Centropages spp., Tigriopus spp. and Labidocera rotunda). The copepod diversity increased with species-richness from around the estuary to the sluice gates, which could result from variation in the site-specific location of non-dominant species. The frequency of particular species was also able to discriminate in terms of the salinity range (oligohaline: A. pacifica, S. tenellus and A. sinjiensis; mesohaline: Pseudodiaptomus inopinus; and polyhaline: C. abdominalis and Centropages spp.) and the trophic state (mesotrophic: C. abdominalis, Calanus sinicus and Centropages spp.; and hypereutrophic: S. tenellus, P. inopinus and Sinocalanus spp.). The findings from this study not only identify the factors determining spatiotemporal variation in the copepod community in the Saemangeum Reservoir, but also expand the applicability of copepods as biological indicators of conditions associated with salinity range and trophic state in other enclosed estuarine systems.

External Nutrient Load and Determination of the Trophic Status of Lake Ziway

Lake Ziway is shallow freshwater located in Northern part of Ethiopian Rift Valley. Expansions of the flower industry, fisheries, intensive agricultural activities, fast population growth lead to deterioration of water quality and depletion of aquatic biota. The objectives of the present study are to evaluate the spatial and temporal variations in the external nutrient load and determine the trophic status of Lake Ziway in 2014 and 2015. The nutrients and Chlorophyll-a were measured according to the standard procedures outlined in APHA, 1999. From the result Ketar and Meki Rivers catchment showed the major sources of external nutrient loads to the lake ecosystem. The mean external soluble reactive phosphorus (SRP), total phosphorus (TP), total inorganic nitrogen (TIN) and total nitrogen (TN) loads to Lake Ziway were 230, 2772, 4925 and 24016 kg day-1, respectively. A general trend which was expected that the nutrient loads would be much higher in rainy season than in dry season. The mean concentrations of trophic state variables for TN, TP and Chla were 6700, 212 and 42 mg L-1, respectively. The mean values of TSI-TP, TSI-Chl-a, TSI-TN and TSI-SD were 79, 66, 81 and 83, respectively and the overall evaluation of Carlson Trophic State Index (CTSI) of Lake Ziway was 77. Therefore, the lake is under eutrophic condition. The mean values of TN: TP ratio was 48 which were very high. The trophic state index determined with chlorophyll-a showed lower value than those determined with all trophic state indices values of TN, TP, and SD which indicated that non-algal turbidity affected light attenuation for algal growth. This suggested that phosphorus was the limiting nutrient in Lake Ziway. Due to its importance as being the lake is an intensive agricultural site, management solutions must be urgently developed in order to avoid the destruction of the lake.

Effect of Catchment Land Use on Trophic State Variables of Small Water Bodies (Northern Poland)

We examined the land use impact on the water quality of small water bodies located in northern Poland. Twenty nine ponds, located in the urban area or a typical agricultural land were analyzed based on physical-chemical parameters: water temperature, conductivity, oxygen concentration, content of particulate suspended matter and chlorophyll “a”, and nutrients concentration. Additionally, to describe and compare the trophic condition of investigated small water bodies, we used the Carlson type trophic state. Performed measurements showed that nutrient concentrations significantly differentiated the agricultural ponds from urban water bodies. Mean total phosphorus and nitrogen concentration in midfield ponds were about 5 and 3 times higher, respectively, than in urban ponds. Moreover the phytoplankton chlorophyll “a” mean values were on average fivefold higher in agricultural ponds. Furthermore, urban ponds were characterized by lower values of TSI, thus lower trophy level: generally on the border of meso- and eutrophy. The agricultural ponds trophy changed from meso- to hypertrophy.

Possible Sediment Mixing and the Disparity between Field Measurements and Paleolimnological Inferences in Shallow Iowa Lakes in the Midwestern United States

Field measurements of water quality in Iowa lakes contradict paleolimnological studies that used 210Pb dating techniques in 33 lakes to infer accelerating eutrophication and sediment accumulation in recent decades. We tested this hypothesis by analyzing a series of water quality measurements taken in 24 of these lakes during the period 1972–2010. There was little change in the trophic state variables. Total phosphorus and algal chlorophylls did not increase, and Secchi depths did not decrease with no evidence that the lakes had become more eutrophic. Changes in daily sediment loads in the Raccoon River also did not match the paleolimnological inferred rates of soil erosion for the period 1905–2005, and an independent estimate of soil erosion rates showed a decline of 40% in the 1977 to 2012 period rather than an increase. We hypothesized that sediment mixing by benthivorous fish could be responsible for violating the basic assumption of 210Pb sediment dating that the sediments are not disturbed once they are laid down. We developed a mathematical model that demonstrated that sediment mixing could lead to false inferences about sediment dates and sediment burial rates. This study raises the possibility that sediment mixing in Iowa lakes and similar shallow, eutrophic lakes with benthivorous fish may cause significant sediment mixing that can compromise dating using 210Pb dating of sediment cores.

Direct and Indirect Effects of Fish on Invertebrates and Tiger Salamanders in Prairie Pothole Wetlands

Hydrologic changes have facilitated increased fish abundance in wetlands of the North American prairie pothole region (PPR). Declines in PPR wetland condition have been attributed to fish-generated changes in trophic state, and reduced abundance and diversity of plants, invertebrates, and non-fish vertebrates. We sought to gain insight into mechanisms for fish impacts on wetland condition by evaluating their direct and indirect effects on invertebrates and salamanders. We quantified relationships among trophic state variables (chlorophyll a, nitrogen, phosphorus, total suspended solids, turbidity); fish and larval tiger salamander biomass; plant cover and taxon richness; and invertebrate density, biomass, and taxon richness in 34 continuously-flooded wetlands in Iowa. Path analysis results supported hypotheses that fish indirectly caused declines in invertebrate taxon richness by increasing turbidity, which in turn reduced plant cover. Path analysis results also supported the hypothesis that fish directly reduced salamander biomass by predation. Fish biomass increased with increasing water depth. Therefore, deeper wetlands were generally in poorer condition than relatively shallow ecosystems. Our findings support conclusions of previous investigators that regardless of other management strategies, actions limiting fish biomass in PPR wetlands (e.g., eliminating surface-water connections to other aquatic systems) are essential to restore health and functions of these ecosystems.

Cladocera as indicators of trophic state in Irish lakes

We examined the impact of lake trophic state on the taxonomic and functional structure of cladoceran communities and the role of nutrient loading in structuring both cladoceran and diatom communities. Surface sediment assemblages from 33 Irish lakes were analysed along a gradient of total phosphorus concentration (TP; 4.0–142.3 μg l−1), using a variety of statistical approaches including ordination, calibration and variance partitioning. Ordination showed that the taxonomic structure of the cladoceran community displayed the strongest response to changes in lake trophic state, among 17 measured environmental variables. Trophic state variables chlorophyll-a and TP explained about 20% of the variance in both cladoceran and diatom assemblages from a set of 31 lakes. Procrustes analysis also showed significant concordance in the structure of cladoceran and diatom communities (P < 0.001). Thus, lake trophic state affects the taxonomic structure of both primary and secondary producers in our study lakes. We also found a significant decrease in relative abundance of taxa associated with both macrophytes and sediments, or sediments only, along the TP gradient (r = −0.49, P = 0.006, n = 30), as well as an increase in the proportion of the planktonic group (r = 0.43, P = 0.017, n = 30). This suggests that cladoceran community structure may also be shaped by lake trophic state indirectly, by affecting habitat properties. We found no relationship between lake trophic state and the relative abundance of each of three cladoceran groups that display different body size. We compared community structure between bottom and top sediment samples in cores from six Irish lakes. Results revealed similar trajectories of nutrient enrichment over time, as well as a strong shift in cladoceran functional structure in most systems. This study confirms that Cladocera remains in lake sediments are reliable indicators of lake trophic state. This study also highlights the fact that taxonomic and functional structure should both be considered to account for the multiple factors that shape cladoceran communities.

Nutrients, seston, and transparency of Missouri reservoirs and oxbow lakes: An analysis of regional limnology

A long-term (1978–2007) summer monitoring study of 167 Missouri reservoirs and floodplain lakes shows wide ranges in the trophic state variables total phosphorus (TP; 6–395 μg/L for reservoir means), total nitrogen (TN; 200–3290 μg/L), chlorophyll (Chl; 1–223 μg/L) and Secchi depth (0.15–4.3 m). There are strong regional contrasts between eutrophic-hypereutrophic water bodies in the predominantly agricultural Osage Plains, Glaciated Plains and Big Rivers sections and the mostly oligotrophic-mesotrophic reservoirs in the largely forested Ozark Highlands. The ecotonal Ozark Border was intermediate. Missouri reservoirs had slightly less TN relative to TP than predicted by global models, but TN:TP (median = 18.4) is typical of North American lakes and was about five- fold lower in the most enriched reservoirs compared to the least enriched ones. Concentrations of seston, measured as total suspended solids (TSS), volatile suspended solids (VSS), nonvolatile suspended solids (NVSS), filterable (small) suspended solids (fTSS) and the sum of all fractions (ΣTSS), were strongly correlated with TP, TN, and Secchi depth. The proportion of mineral seston (NVSS, fTSS) increased with TSS and was the dominant fraction in most reservoirs, especially in the agricultural regions. Phytoplankton was dominated by small forms (<11 μm), and Cyanophyta were the most common algae comprising a median 46% of phytoplankton biovolume. Chlorophyll-nutrient regressions showed a dominant effect of TP (r2 = 0.83 for reservoir means) with a lesser, negative effect of mineral seston (NAS or fTSS, partial r2 ~ 0.05). Secchi depth was more strongly controlled by mineral seston than by phytoplankton such that Secchi was a better predictor of nutrients (especially TP) than algal biomass. Frequency of algal blooms (Chl >10 μg/L) and Secchi <1 m increased sigmoidally with TP and TN with midrange nutrient concentrations (TP 20–50 μg/L, TN 400–700 μg/L) showing the greatest response to change. Trophic state criteria appropriate for Missouri reservoirs are similar to other north temperate lakes except for Secchi depth for which cutpoints are much lower because of nonalgal turbidity.

Ecological implications of artificial mixing and bottom‐sediment removal for a shallow urban lake, Lake Sheldon, Colorado

This paper describes the results of the investigation of a shallow urban lake before, and a second year after, restoration to determine the effects of aeration and bottom-sediment removal on the trophic state and environmental variables controlling the lake's phytoplankton, macrovertebrates and zooplankton compositions. The anthropogenic eutrophic process of the lake is characterized by increased nutrient concentrations from urban run-off inputs. This addition triggers a chain of events that start with a massive increase in the growth of primary producers, as these are generally growth-limited by nutrients in freshwater ecosystems. Although the lake was artificially mixed, nuisance cyanobacteria were dominant as primary producers the second year after restoration, during the summers of 2004 and 2005, until this population collapsed and were replaced by diatoms after periods of high winds, rainfall and a decreased temperature. The absence of macrophytes in the main basin because of the removal of bottom sediment had a major effect on macroinvertebrates and zooplankton. Daphnia sp. was the dominant zooplankton species before restoration, being replaced by Bosmina sp., possibly because of the lack of refuge by macrophytes. As an alternative, Bosmina sp. used the surface blooms of cyanobacteria in summer as refuge from grazing by planktivorous fish. The greatest disturbance on the macroinvertebrate community richness and evenness was the decline of scrapers and shredders, because of the lack of macrophytes after removal of the lake bottom sediment.

Variation in Water Quality and Phytoplankton Nutritional Status in an Urbanized Lake System (Lake Merced, California, USA)

Lake chemistry is influenced by land use in the surrounding watershed, particularly in complex urban landscapes, which are commonly subjected to an increase in material loadings. Because land use is rarely uniform, individual ecosystems embedded within the landscape may reflect varying water quality conditions. The Lake Merced system is composed of three lake basins with watersheds (surface area = 13 km2) residing entirely within the city of San Francisco, California. On nine occasions from January-June 1995 we sampled the two main lakes (North and South Lake Merced) within the system to evaluate their relative productivity and nutrient status. On each date, several physical-chemical conditions were measured, and nutrient enrichment bioassays were performed to evaluate the nutritional status of the phytoplankton (nitrogen (N) and phosphorus (P) enrichment in a 2 × 2 factorial experimental design). All the parameters tested except temperature were higher in the north lake compared with the south (pair-wise Wilcoxon Ranked sums), and common trophic state variables were more than 2-fold higher in the north lake. Phytoplankton growth in North Lake Merced was N-limited while growth in South Lake Merced was co-limited by N and P. In sum, our data indicate that differences among these lakes may be explained by simple volume differences between lakes (with the smaller North Lake Merced being more eutrophic), and heavy stocking of fishes to the north lake. Particulate nutrient ratios (N:P) were good indicators of phytoplankton nutritional status throughout the system, while N:P ratios based on total nutrient concentrations gave misleading results.

Lake level and trophic state variables among a population of shallow Florida lakes and within individual lakes

Monthly total phosphorus, total nitrogen, and chlorophyll concentrations, Secchi depth, and lake water level data for 84 Florida lakes were used to examine relations between trophic state variables and water level fluctuation. Lake size averaged 566 ha (range 4.0 to 5609 ha), with the period of record for individual lakes averaging 57 months (range 7 to 175 months). Lake level fluctuation for individual lakes averaged 1.3 m (range 0.1 to 3.5 m). The lakes also ranged from oligotrophic to hypereutrophic, with average chlorophyll values for individual lakes ranging from 1 to 97 µg·L–1. No overall relation between trophic state variables and lake level fluctuation could be found among the population of lakes. However, individual lakes showed direct, inverse, or no significant relations between lake trophic state variables and water level fluctuation, regardless of the magnitude of water level fluctuation. These data suggest that predicting how water level fluctuations will impact trophic state variables among a population of lakes will be difficult, if not impossible, and that any accurate predictions will have to be made after first examining several mechanisms within individual lake systems.

Simple Graphical Methods for the Interpretation of Relationships Between Trophic State Variables

Graphical methods are presented that can identify relationships between the trophic state variables, total phosphorus, total nitrogen, chlorophyll, and Secchi depth. The graphical approach extends the use of empirical models beyond predictions based solely on local data. Instead, the authors suggest comparing the data against an established set of predictive equations. These equations serve as a “standard model” from which deviations can be observed and inter-preted. The goal of the use of deviations from a standard model is to enhance our ability to understand relationships between important nutrient, transparency, and biological variables and to adjust our predictive models and manage-ment decisions accordingly. Examples are given of interpretations of deviations caused by nitrogen limitation, non-algal turbidity, zooplankton grazing, and dissolved water color.

A Proposed Framework for the Identification of Habitat Utilisation Patterns of Macrophytes in River Po Catchment Basin Lakes (Italy)

The main aim of this study, which is based on cover-abundance values of 20 species of floating-leaved and submersed macrophytes collected in 18 lakes located within the River Po catchment basin (Italy), is to investigate the relationships between lake-scale environmental features (including morphology, hydrology, trophic state and water quality variables) and the changes in species composition and species richness in macrophyte communities. The findings reveal that the main changes in community composition of the pondweed communities, identified by principal component analysis, could be effectively explained by a newly introduced morpho-hydrological parameter (the theoretical laminar water renewal rate – R L) and by the trophic state of the lakes, expressed as Carlson’s trophic state index (TSI). The results of the multivariate analyses performed also show that the grouping of species is strictly related to growth-form traits. In particular, floating-leaved and submerged macrophytes appeared mutually exclusive in dominating different habitat types defined on the basis of the R L–TSI framework. An increasing trend in species richness was also observed in relation to R L. The possibility of using the bivariate R L–TSI framework for the identification of habitat utilisation patterns by single species was also investigated, and cover-abundance prediction maps based on the R L–TSI framework were produced for several macrophyte species. Observations in another system characterised by high spatial heterogeneity for hydrological and trophic conditions, support the predictions in terms of growth-form prevalence, species richness and single species cover-abundance (where available), and also suggest a more refined application of the proposed approach.

Factors controlling hydrochemical and trophic state variables in 86 shallow lakes in Europe

In order to disentangle the causes of variations in water chemistry among European shallow lakes, we performed standardised sampling programs in 86 lakes along a latitudinal gradient from southern Spain to northern Sweden. Lakes with an area of 0.1 to 27 000 ha and mean depth of 0.4–5.6 m located in low to high altitudes were investigated within the EC project ECOFRAME 1–4 times during June–October 2000–2001. Several variables like conductivity, alkalinity, abundance of submerged plants, concentrations of suspended solids, total nitrogen and phosphorus were latitude-dependent decreasing from south to north. Secchi depth, concentrations of total nitrogen, total phosphorus, suspended solids, and chlorophyll a correlated strongly with the presumed quality classes of the lakes. We came to the conclusion that the variability of shallow lakes in Europe is still mostly dependent on natural differences. Variables connected to lake morphometry, seasonality, basin geology and climate explained altogether nearly half of the total variability of lakes. The trophic state factor, describing mostly the human influence on lakes, was the strongest single factor responsible for nearly a quarter of the total variability of the studied European lakes.

Eutrophication and recovery in the High Arctic: Meretta Lake (Cornwallis Island, Nunavut, Canada) revisited

Meretta Lake (Resolute Bay, Cornwallis Island, Nunavut, Canada) is a polar lake that has been receiving sewage since 1949 via a series of watercourses and utilidors from the so-called `North Base' of the Canadian Department of Transport. The lake's physical, chemical and biological characteristics were studied between 1968 and 1972 as part of the Char Lake Project, which was a component of the International Biological Programme (IBP). This was the first detailed study of high arctic lake eutrophication. However, since the time of the IBP, use of the North Base has declined markedly. Between 1992 and 1999, we re-sampled Meretta Lake for a suite of limnological variables, and compared our findings to those gathered during IBP. Our data indicate that, although Meretta Lake was still more eutrophic in the 1990s than near-by, undisturbed high arctic lakes, it presently has much lower nutrient concentrations and other trophic state variables than it did during IBP. These concentrations continued to decline in the 1990s, coincident with further decreases in usage of the base. Our most recent data indicate that Meretta Lake nutrient levels are now near `natural' background levels. Furthermore, phytoplankton are characterised by higher abundances of cryptophytes than those recorded in the early 1970s, again indicating less eutrophic conditions. Diatom-based, paleolimnological techniques recorded marked species assemblage shifts coincident with the eutrophication from the North Base. However, similar to the phytoplankton data, species assemblage changes were different from those recorded following eutrophication in more temperate regions, with periphytic diatoms overwhelmingly dominating the assemblages.

A Monitoring and Classification System for New Zealand Lakes and Reservoirs

ABSTRACT Five variables gave strong indications of change in the trophic state of New Zealand lakes, namely; concentrations of chlorophyll a, total phosphorus and total nitrogen, as well as Secchi depth and dissolved oxygen depletion rate. The New Zealand Lake Monitoring Program, investigated 17 lakes for 3 to 4 years to develop a method for detection of small changes in these five variables and to develop a cost-effective monitoring system for lakes. Two different methods were developed which enabled trends observed separately for variables to be combined into an indicator of change (PAC, Percent Annual Change) and an index of trophic state (TLI, Trophic Level Index). The PAC is a relative indicator that gives the average percent annual change of the trophic state variables, and is an indicator of the magnitude of the change observed. It is calculated from the 5 variables listed above. The TLI is a numerical indicator of lake trophic level, which typically varies from 2 (oligotrophic) to 7 (supertrophic). The TLI can be examined for time trends that generally reflect the changes detected by the PAC relative indicator. These two indicators, when used together, give a good estimate of the probability and magnitude of trophic level change in a lake as well as the trophic level of the lake. Results for all the monitored lakes are given. The PAC and TLI indicators also gave useful results when used to study reservoir monitoring data.