Lake Trophic Status Research Articles

A two-part model linking multidimensional environmental gradients and seasonal succession of phytoplankton assemblages : Part I. Cyanoprokaryotes and Chlorophytes across Lake Trophic Status

Algal assemblages and their respective niches are analysed for a large riverine impoundment in semi-arid Australia. The extreme variability of the flow regime and the extensive period of low or no flow produced conditions including prolonged stratification akin to those of a lake during the dry season. Hierarchical agglomerative clustering of phytoplankton data and comparison with a principal component analysis of corresponding environmental data are used to demonstrate the linkage between steep environmental gradients and variations in the phytoplankton assemblages. The factor score plots separate the algal assemblages according to key environmental parameters including increasing clarity of the water column, temperature and modification of water chemistry. Assemblages normally contained cyanoprokaryotes (Oscillatoriales), euglenophytes or non-flagellated chlorophytes during flows followed by flagellated chlorophytes and cyanoprokaryotes (Nostocales) during the dry season. Species present during the period of reduced flow occupied conditions similar to those presented in the literature for temperate and/or tropical lakes. Common species included Pandorina morum, Eudorina elegans, Anabaena circinalis, Aphanizomenon issatschenkoi, Aphanizomenon aphanizomenioides and Cylindrospermopsis raciborskii. A two-part model is proposed which predicts the most likely genera of phytoplankton with respect to multidimensional environmental gradients.

Changes in fossil midge (Chironomidae) assemblages in response to cultural activities in a shallow, polymictic lake

Chironomid (Diptera: Chironomidae) head capsules were studied from a core of recent sediments from shallow, macrophyte-dominated Lake Opinicon, Ontario, Canada, to determine if assemblages have changed in response to lake-level changes and other watershed disturbances, including deforestation and agriculture. Our results indicate that the construction of the Rideau Canal in the early 1830s and subsequent flooding of Lake Opinicon has had the greatest impact on this system, but that even this disturbance did not greatly affect chironomid assemblages. Despite other significant cultural disturbances in the watershed, the lake sediments have recorded only minor changes in its recent history, providing support for the hypothesis of alternative lake equilibria. These results correspond well with diatom inferences of only minor changes in past lake trophic status.

A Re-evaluation of the Trophic State of Mountain Lake, Virginia, USA

ABSTRACT Limnological studies of Mountain Lake, Giles County, Virginia were conducted from June to October of 1997 and April to October of 1998 to evaluate the lake's trophic state, which was considered to be in an early stage of eutrophication from the late 1980s to early 1990s. Orthophosphate-P levels averaged 1.5μg/L in 1997 and 2.2μg/L in 1998, a return from 27.0μg/L in 1990 to early and mid 1980s levels. Ammonium-N also dropped, but nitrate-N increased in 1997–98 compared to 1990. The N:P ratio of 143:1 in 1997 and 244:1 in 1998 compared to 9.4:1 in 1990 clearly indicated P as the major limiting nutrient. Dissolved silica, hardness, alkalinity, and pH in the lake were similar to earlier studies. Phytoplankton taxa and cell densities were similar to those of the mid-1980s. Mountain Lake is thus oligotrophic and presently shows no signs of eutrophication. Input streams feeding the lake were also low in key nutrients, but precipitation constituted a major source of P, N, and acidity to the lake.

Influence of lake trophic status on the occurrence of microcystin‐LR in the tissue of pulmonate snails

1. To assess the influence of lake trophic status on the occurrence of microcystin‐LR (MCLR) in the tissue of resident pulmonate snails, we sampled seven lakes of varying primary productivity in Alberta, Canada. Parameters associated with productivity were measured every 2 weeks from mid‐May through mid‐September 1995. Phytoplankton and resident pulmonate snails were collected and analysed for MCLR concentration via high‐performance liquid chromatography.2. For all species of gastropod (i.e. Lymnaea stagnalis, Helisoma trivolvis and Physa gyrina), the concentration of MCLR in the tissue was correlated (P ≤ 0.03) with toxin in the phytoplankton, but not with extracellular aqueous microcystin (P > 0.28). The concentrations of toxin in the tissues of L. stagnalis and P. gyrina were also correlated with the relative abundance of Microcystis spp. (P < 0.01).3. Given that Microcystis spp. abundance and MCLR concentration within phytoplankton are correlated with indicators of productivity, we conclude that trophic status is important in influencing the occurrence and concentration of MCLR in pulmonate snails.

Classification, assessment and trophic reconstruction of Danish lakes using chironomids

1. Surface‐sediment assemblages of subfossil chironomid head capsules from fifty‐four primarily shallow and nutrient‐rich Danish lakes were analysed using multivariate numerical techniques. The species data, comprising forty‐one chironomid taxa, were compared to environmental monitoring data in order to establish a relationship between chironomid faunal composition and lake trophic state.2. The subfossil assemblages were compared to the chironomid bathymetric distributions along transects from four lakes. Correspondence analysis and similarity coefficients showed that the subfossil assemblages, sampled in the lake centre, reflect the chironomid communities in the littoral at a depth of 2–7 m.3. Two‐way indicator species analysis (TWINSPAN) was used to classify the Danish lakes into five groups defined by trophic state, lake depth and pH. Eighteen chironomid taxa showed significant differences in abundance among the five groups. Canonical correspondence analysis (CCA) showed the chlorophyll a concentration ([Chl a]) and Secchi depth to be the variables best correlated to the faunal data, and fourteen taxa were significantly correlated to [Chl a].4. The strong correlation between chironomid data and the ln‐transformed ([Chl a]) was used to create a weighted averaging (WA) model to infer lake trophic state. Several models were tested by cross validation (leave‐one‐out jack‐knifing), and a simple WA model using inverse de‐shrinking had a RMSEPjack of 0.65 (ln units) and a r2jack of 0.67.5. The results can be used in the assessment and reconstruction of lake trophic state for long‐term monitoring and palaeoecological investigations of shallow, temperate lakes in the mesotrophic to hypertrophic nutrient range.

Management of Eutrophication for Lakes Subject to Potentially Irreversible Change

We analyzed management policies for ecosystems subject to alternate states, thresholds, and irreversible changes. We focused on the problem of lake eutrophication by excessive phosphorus (P) input. Eutrophic lakes may be classified, with respect to their response to reduced P input alone, as reversible (recovery is immediate and proportional to the reduction in P input), hysteretic (recovery requires extreme reductions in P input for a period of time), or irreversible (recovery cannot be accomplished by reducing P input alone). A model with one state variable and one control variable describes the responses of lake trophic state to changes in P input and other management interventions. Activities that generate P input to the lake are assumed to create profits, while the value of ecosystem services provided by the lake declines at high P levels. We then calculated P input policies that maximize the discounted net benefits from polluting activities and ecosystem services. If “optimality” is defined as maxim...

MANAGEMENT OF EUTROPHICATION FOR LAKES SUBJECT TO POTENTIALLY IRREVERSIBLE CHANGE

We analyzed management policies for ecosystems subject to alternate states, thresholds, and irreversible changes. We focused on the problem of lake eutrophication by excessive phosphorus (P) input. Eutrophic lakes may be classified, with respect to their response to reduced P input alone, as reversible (recovery is immediate and proportional to the reduction in P input), hysteretic (recovery requires extreme reductions in P input for a period of time), or irreversible (recovery cannot be accomplished by reducing P input alone). A model with one state variable and one control variable describes the responses of lake trophic state to changes in P input and other management interventions. Activities that generate P input to the lake are assumed to create profits, while the value of ecosystem services provided by the lake declines at high P levels. We then calculated P input policies that maximize the discounted net benefits from polluting activities and ecosystem services. If “optimality” is defined as maxim...

Addition of sulfur to organic matter during early diagenesis of lake sediments

Enrichment of organic matter with sulfur during early diagenesis has been amply documented in marine sediments. The importance of such reactions in lacustrine sediments is not as well appreciated. In this study the organic sulfur contents of sediments from seven lakes were compared, and the speciation of the sulfur in the humic acid fraction of the sedimentary organic matter was examined with X-ray photoelectron spectroscopy (XPS). Diagenetic enrichment of organic matter with sulfur occurred in five of seven lakes as evidenced by C:S ratios and stable isotope ratios. The availability of sulfide, organic matter, and reactive iron were not important determinants of the extent of organic S enrichment. The only environmental parameter that appeared to be related to S enrichment was lake trophic state. Together with data from the literature, our results suggest that sediments in most eutrophic lakes are enriched in organic S, while organic matter is enriched in only few oligotrophic lakes. Organic sulfides or thiols are the dominant forms of reduced organic S in the humic acids, and sulfur enrichment occurs primarily by formation of organic sulfides or thiols. Addition of S to polyunsaturated molecules could account for 5–10% of the S enrichment. Di- and polysulfides did not comprise a significant fraction of organic S in any of the sediment humic acids. Thiophenes could be identified tentatively only in the oldest (60 yr) sediment sample analyzed. Sulfoxides were observed in several samples. Lake trophic state and exposure to oxygen appear to be major factors influencing the extent and pathways of S addition to organic matter.

An Index of Biotic Integrity: A Test with Limnological and Fish Data from Sixty Florida Lakes

An index of biotic integrity (IBI) that used eight fish assemblage metrics was examined for effectiveness in estimating anthropogenic impacts to 60 Florida lakes ranging in size from 2 ha to more than 12,400 ha. The lakes ranged in trophic status from oligotrophic to hypereutrophic and had aquatic macrophyte abundances (percent lake volume infested) ranging from less than 1% to 100%. Fish species were classified by trophic feeding guild and tolerance to increases in turbidity or warming and decreases in dissolved oxygen concentration. Fish assemblage metrics tested were as follows: Number of fish species, number of native fish species, number of Lepomis species, number of piscivorous species, number of generalist species, number of invertivore species, number of species intolerant of increased turbidity or warming and decreased dissolved oxygen concentration, and number of species tolerant of increased turbidity or warming and decreased dissolved oxygen concentration. The total IBI scores and the data used to calculate individual metrics were unable to accurately predict the degree of anthropogenic impact to 60 Florida lakes, as estimated by personal observations of local limnologists, lake chloride concentrations, and road densities in the watersheds. Lake surface area and lake trophic status have a dominant influence on the fish assemblage metrics tested in this study. Thus, the IBI approach may be of limited usefulness for predicting anthropogenic impact in lake data sets that have wide ranges of surface areas and trophic status classifications.

Accumulation and recycling of PCBs and PAHs in artificially eutrophied Lake 227

The influence of lake trophic status on the cycling and burial of polychlorinated biphenyls (PCB's) and polycyclic aromatic hydrocarbons (PAH's) was examined in Lake 227 (L227), an artificially eutrophied lake in the Experimental Lakes Area, northwestern Ontario, Canada. Sedimentary accumulation of PCB's, PAH's, and organic carbon (OC) was determined pre- and post-eutrophication in L227 and compared with accumulation in other regional lakes. Mass and OC accumulation increased an average of 1.4 and 2.0 times, respectively, since nutrient addition began in June 1969. ΣPCB and ΣPAH sediment accumulation rates and profiles in L227 were similar to those in Lake Superior and other proximate and midlatitude lakes unimpacted by point sources. Eutrophication did not increase contaminant accumulation. In general, individual PAH accumulation rates declined following eutrophication, which coincided with similar signals elsewhere. Perylene declined markedly (4 times) since eutrophication as a result of decreased in situ formation. An increase in OC accumulation did not enhance contaminant accumulation in L227 because phytoplankton community structure shifted to species characterized by lower bioaccumulation factors, and water column recycling rates were high. Accumulation rates of mass, OC, PCB's, and PAH's, when compared with ice-free settling fluxes in 1993 and 1994, exhibited similar recycling ratios in eutrophic L227 and oligotrophic L110 due to the efficient preservation of organic matter in sediments.

X-Ray Microanalytical Study of the Protozoan Ceratium Hirundinella From Rostherne Mere (Cheshire, Uk): Dynamics of Intracellular Elemental Concentrations, Correlations and Implications for Overall Ecosystem Functioning

The population of Ceratium hirundinella in a freshwater lake was studied both by traditional limnological methods and X-ray microanalysis during the final phase of stratification. Correlation and Factor analysis of intracellular elemental concentrations revealed a clear pattern of statistical relationships mainly determined by three factors, with K, P and Mg demonstrating higher loadings on the first, Ca and Si on the second, and S, Cl and Na on the third factor. S also showed significant correlations with K and P, elements belonging to the first association. The results of the statistical analysis are summarised in a structural model of intracellular elemental associations. Consideration of the latter in the mathematical description of nutrient uptake and plankton population dynamics should prove useful in aquatic modelling. Development of the population resulted in a dramatic decrease in transparency and nutrient concentrations in the epilimnion water. The recorded peak of the population coincided with minimum levels of ambient PO4.P and DIN and maximum intracellular levels of a number of important chemicals (Mg, K, P, S). The decrease in the epilimnion concentrations of orthophosphate did not match the simultaneous increase in the total amount of P contained within the population of Ceratium. Ambient nutrient scarcity, therefore, could have been alleviated by Ceratium's capacity for vertical movements, heterotrophic consumption of organic materials and/or partial entrainment of the hypolimnion water during periods of colder weather. Following the demise of the population, chemicals accumulated inside the cells were removed from the epilimnion with easily sedimented biomass. The results clearly showed the importance of Ceratium in transforming nutrients between different levels of the Rostherne ecosystem. Considering the high lake water retention time, the geochemical consequences of Ceratium's ecochemistry and population dynamics may considerably delay changes in the lake's trophic status expected after sewage diversion.

A multi-proxy trophic state reconstruction for shallow Orange Lake, Florida, USA: Possible influence of macrophytes on limnetic nutrient concentrations.

We retrieved four sediment cores from shallow, eutrophic, macrophyte-dominated Orange Lake (A = 51.4 km2, zmax <5 m, zmean < 2 m), north-central Florida, USA. The 210Pb-dated profiles were used to evaluate spatial and temporal patterns of bulk sediment and nutrient accumulation in the limnetic zone and to infer historical changes in lake trophic state. Bulk density, organic matter, total carbon, total nitrogen, total phosphorus and non-apatite inorganic phosphorus (NAIP) concentrations displayed stratigraphic similarities among three of four cores, as did accumulation rates of bulk sediment, organic matter and nutrients. Accumulation rates were slower at the fourth site. Nutrients showed generally increasing rates of accumulation since the turn of the century. Percentages of periphytic diatom taxa increased progressively in the cores after ~ 1930. Diatom-inferred limnetic total P trends were similar among profiles. Eutrophic conditions were inferred for the period prior to the turn of the century. The lake was hypereutrophic in the early decades of the 1900s, but inferred limnetic total P values declined after ~ 1930. Declining inferred limnetic total P trends for the last 60--70 years were accompanied by concomitant increases in accumulation rates of total P and NAIP on the lake bottom. Several lines of evidence suggest that after ~ 1930, phosphorus entering Orange Lake was increasingly utilized by submersed macrophytes. Paleolimnological records from Orange Lake highlight the importance of using multiple sediment variables to infer past trophic state and suggest that aquatic macrophytes can play a role in regulating water-column nutrient concentrations in shallow, warm-temperate lakes.

Stable isotope (δ13C and δ15N) signatures of sedimented organic matter as indicators of historic lake trophic state

We explored the use of carbon and nitrogen isotopes (δ13C and δ15N) in sedimented organic matter (OM) as proxy indicators of trophic state change in Florida lakes. Stable isotope data from four 210Pb-dated sediment cores were compared stratigraphically with established proxies for historical trophic state (diatom-inferred limnetic total phosphorus, sediment C/N ratio) and indicators of cultural disturbance (sediment total P and 226Ra activity). Diatom-based limnetic total P inferences indicate a transition from oligo-mesotrophy to meso-eutrophy in Clear Lake, and from eutrophy to hypereutrophy in Lakes Parker, Hollingsworth and Griffin. In cores from all four lakes, the carbon isotopic signature of accumulated OM generally tracks trophic state inferences and cultural impact assessments based on other variables. Oldest sediments in the records yield lower diatom-inferred total limnetic P concentrations and display relatively low δ13C values. In the Clear, Hollingsworth and Parker records, diatom-inferred nutrient concentrations increase after ca. AD 1900, and are associated stratigraphically with higher δ13C values in sediment OM. In the Lake Griffin core, both proxies display slight increases before ~1900, but highest values occur over the last ~100 years. As Lakes Clear, Hollingsworth and Parker became increasingly nutrient-enriched over the past century, the δ15N of sedimented organic matter decreased. This reflects, in part, the increasing relative contribution of nitrogen-fixing cyanobacteria to sedimented organic matter as primary productivity increased in these waterbodies. The Lake Griffin core displays a narrow range of both δ13C and δ15N values. Despite the complexity of carbon and nitrogen cycles in lakes, stratigraphic agreement between diatom-inferred changes in limnetic total P and the stable isotope signatures of sedimented OM suggests that δ13C and δ15N reflect shifts in historic lake trophic state.

ENVIRONMENTAL AUDITING: Regional Lake Trophic Patterns in the Northeastern United States: Three Approaches

/ During the summers of 1991-1994, the Environmental Monitoring and Assessment Program (EMAP) conducted variable probability sampling on 344 lakes throughout the northeastern United States. Trophic state data were analyzed for the Northeast as a whole and for each of its three major ecoregions-the Adirondacks (ADI), the New England Uplands (NEU), and the Coastal Lowland and Plateau (CLP)-and inferred to the entire population of lakes >/=1 ha (N = 11,076). Results were compared to a large, nonrandomly sampled data set for the same area compiled by Rohm and others and contrasted with lake trophic state information published in the National Water Quality Inventory: 1994 Report to Congress [305(b) report. Lakes across the entire Northeast were identified by EMAP data as 37.9% (+/-8.4%) oligotrophic, 40.1% (+/-9.7%) mesotrophic, 12.6% (+/-7.9%) eutrophic, and 9.3% (+/-6.3%) hypereutrophic. Lakes in the ADI and NEU generally are at a low, nearly identical trophic state (96% oligotrophic/mesotrophic), while those in the CLP are much richer (45% eutrophic). EMAP results are similar to results of the Rohm data set across the entire region. In the CLP, however, EMAP identified approximately 45% of the lakes as eutrophic/hypereutrophic, while the Rohm data set identified only 21% in these categories. Across the entire Northeast, the 305(b) report identified a much higher proportion (32.2%) of lakes in eutrophic condition and a much smaller proportion (19.8%) in oligotrophic condition than did the EMAP survey data (12.5% +/- 7.9% and 37.9% +/- 8.5%, respectively). Probability sampling has several advantages over nonrandom sampling when regional resource condition assessment is the goal.KEY WORDS: Lake trophic state; Phosphorus; Probability survey; Regional condition

Bioaccumulation of radionuclides in fertilized Canadian Shield lake basins

Radionuclide tracers of heavy metals ( 59Fe, 60Co, 65Zn, 75Se 85Sr, 134Cs and 203Hg) representing potential contamination from nuclear power plants, industry and agriculture were added to separate basins of Lake 226, Experimental Lakes Area, northwestern Ontario. The two basins were part of a eutrophication experiment and differed in their trophic status; the north basin (L226N) was eutrophic whereas the south basin (L226S) was mesotrophic. Our objective was to determine the uptake of the radionuclides by biota and the effect of lake trophic status on their bioaccumulation. The trophic status of the lakes did not appear to have a marked effect on the accumulation of radionuclides by the biota. This may have been because of a mid-summer leakage of nutrients between the basins which enhanced primary production in L226S, because there is a time lag between primary production and the availability of the radionuclides to the fishes or because trophic status does not affect the uptake of at least some of these radionuclides. However, there was a tendency for faster uptake of the radionuclides in L226N by fish than L226S, but the differences were not significant. Concentrations in the biota generally decreased in the order: fathead minnow>pearl dace>tadpoles>slimy sculpin>leeches. Concentrations in biota generally decreased in the order: 65Zn> 203Hg> 75Se> 134Cs> 60Co> 85Sr= 59Fe. Cobalt-60 concentrations in tadpoles were greater than in the other biota. Radionuclide concentrations in the tissues of lake whitefish indicated that uptake was predominantly from food. Radionuclide concentrations were usually higher in the posterior gut, liver and kidney than in other tissues, whereas body burdens were generally high in the muscle for 75Se, 134Cs and 203Hg; kidney and gut for 60Co; and bone for 65Zn and 75Se. Mercury-203 burdens were also high in the bone and gut.

Chrysophycean stomatocyst flora from south-central Ontario lakes

Chrysophycean stomatocyst morphotypes were identified and enumerated from the sediments of 55 Muskoka-Haliburton south-central Ontario lakes. Principal components analysis (PCA) and TWINSPAN cluster analysis showed that the distribution of cyst assemblages was most closely related to lake water pH. Cyst abundance varied with lake trophic status, decreasing in highly productive lakes. The diversity of the observed cyst assemblages was similar to that seen in other stomatocyst studies from temperate regions, with a total of 246 morphotypes noted. The Muskoka-Haliburton cyst flora contained a high proportion (22.8%) of as yet undescribed morphotypes and an abundance of cysts ornamented with circuli and small ridges. Fifty-six previously unidentified morphotypes were observed, 29 of which were studied in detail using scanning electron microscopy, and described using International Statospore Working Group guidelines. Six variants of common morphotypes, characterized by the presence of a siliceous envelope, are also described. Twelve new distinct morphotypes observed only using light microscopy are documented as unidentified forms.Key words: Chrysophyceae, stomatocyst, taxonomy, Muskoka-Haliburton, Ontario, lakes, paleolimnology.

The fauna in the upper stony littoral of Danish lakes: macroinvertebrates as trophic indicators

1. The macroinvertebrate fauna living on stones in the exposed stony littorals of thirty‐nine Danish lakes were examined by multivariate numerical methods. The data were derived from 125 semi‐quantitative samples and a species list of 126 taxa. The mean number of individuals per sample was 960, and among the most common taxa were Asellus aquaticus, Gammarus, Oulimnius, Tinodes, Cricotopus and Dicrotendipes.2. The total number of species and fourteen individual taxa were positively correlated to mean depth of the lakes and eleven taxa were correlated to the total phosphorus concentration. The Shannon diversity was negatively correlated to the chlorophyll a concentration ([Chl a]).3. Community patterns were examined by detrended correspondence analysis (DCA), and the relationship between species data and selected environmental variables was analysed by canonical correspondence analysis (CCA). Mean lake depth was found to be the strongest environmental variable in explaining the species data. The [Chl a] and Secchi depth also explained significant variation in the distribution of the stony littoral invertebrates. Wind fetch and relative exposure did not explain any variation in the faunal composition among sites.4. The abilities of the macroinvertebrates to predict the lake trophic state, expressed as log ([Chl a]), were explored by means of weighted averaging (WA) regression and calibration. Two tolerance‐weighted WA models using inverse and classical regression for deshrinking are presented. The models were assessed by the root mean square error (RMSE) of prediction, using bootstrapping as cross validation, and by the correlation between observed and inferred log ([Chl a]). The model using inverse deshrinking had a RMSEboot = 0.41 and r2 = 0.63. By using classical regression, the predictability in the ends of the gradient was improved but the RMSE increased: RMSEboot = 0.46.5. Although the factors determining faunal distribution patterns in the Danish lowland lakes were highly multivariate and difficult to disentangle, it seems reasonable to use the WA estimated species optima and tolerances to [Chl a] in a bio‐assessment model.

Evaluation of Data Generated from Lake Samples Collected by Volunteers

ABSTRACT The goals of the Lakes of Missouri Volunteer Program are to involve citizens in the collection of water samples to monitor lake trophic state and to provide outreach education about lake water quality. Results indicate data generated with volunteer help are of the same quality as that generated by a research laboratory. This conclusion is based on three different methods of evaluation: (1) Comparisons of volunteer and University collected samples showed trophic state classifications were the same for 74% of lakes based on total phosphorus, 84% for total nitrogen and 89% for chlorophyll; (2) Agreement between paired chlorophyll filters was assessed to gauge volunteer processing technique; 88% of the filter pairs was considered good or excellent based on a rating scale developed for this program; (3) Split sampling showed no significant differences for total suspended solids, chlorophyll or total nitrogen. Total phosphorus analysis showed a significant difference with volunteer samples being consistently less than University samples. Prior to analysis, volunteer samples for total phosphorus were stored frozen in high density polyethylene bottles while University samples were refrigerated in glass tubes. This difference in storage method may have caused the irregularity in our results.

Bythotrephes longimanus in the Commonwealth of Independent States: variability, distribution and ecolog

Bythotrephes is presently extending its distribution to basins throughout Europe and North America. We used long term data (1880–1997) on Bythotrephes longimanus in Eurasian waterbodies to characterize its ecology and tolerance to different ecological factors. Depending on season and environmental conditions of the waterbody, Eurasian B. longimanus may undergo cyclomorphosis and can exist as two distinct forms ( B. longimanus or B. cederstroemi). Regional forms of the species ( B. l. var. balticus, B. l. var. it transcaucasicus, and B. l. var. arcticus) have also been described. Occurrence and density of Bythotrephes populations varies across geographical scales, and are apparently affected primarily by water temperature and salinity. Bythotrephes is limited to regions where water temperature ranges between 4 and 30 ^C, although the species prefers waters between 10 and 24 °C. Bythotrephes tolerates salinity values between 0.04 and 8.0i, though it prefers water between 0.04 and 0.4i. These factors strongly restrict Bythotrephes' distribution in warmer and more saline waters of the Commonwealth of Independent States (CIS), particularly in southern Russia and Kazakhstan. Local abundance and distribution of Bythotrephes is affected by abiotic factors, including basin morphometry and flushing rate, temperature, oxygen concentration and pH, as well as by lake trophic status and fish predation. Planktivorous fish may restrict habitat use by Bythotrephes to regions of basins where overlap between these groups is minimized. Rivers, canals, and ephemeral waterbodies serve as important modes of Bythotrephes' dispersal throughout the CIS and western Europe. Morphological adaptations (e.g. a long caudal process, cyclic parthenogenesis, and production of resting eggs) permit Bythotrephes to tolerate seasonally inhospitable conditions, assist in its dispersal and range extension, and minimize fish predation.

A Test Of A Morphometric Index To Predict Vertical Phosphorus Transport in Lakes

ABSTRACT Osgood (1988) proposed a morphometric index, mean depth over square root of surface area ([zbar] / √ A0), to estimate the probability of partial or complete mixing of lakes during summer storms. Such mixing could introduce phosphorus (P)-rich waters to the entire water column (polymictic lakes) or to the epilimnion, and could delay lake recovery following abatement of allochthonous P loading. Cooke et al. (1993) hypothesized that aluminum sulfate (alum) applications to control internal P loading would be most effective in lakes with an Osgood Index less than 6 and significant diversion of external loading. We tested the hypothesis that vertical P transport is higher in lakes with the lowest Osgood Index numbers by estimating transport in four northeastern Ohio lakes with Osgood numbers of 2.9, 7.4, 9.7, and 26.3 during Summer 1992. The hypothesis was rejected. Although the lake with the lowest number most often had the highest vertical P transport rate, transport was not significantly different between lakes. The qualitative trends among lakes corresponded with their relative differences in P concentration gradients. Lakes with a high Osgood Index could have significant vertical P transport if there is a steep P gradient in the water column. The results suggest that alum applications have the potential to decrease trophic state in lakes with a high Osgood Index value.