Abstract-The relationship of lake water color (mg liter-' Pt) to watershed and lake morphometry, derived from topographical, hydrological, and bathymetric maps, was examined by log-linear multiple regression on a data set obtained from 337 lakes from the northern United States and Canada. Color was positively related to the drainage ratio, and negatively related to watershed slope, mean lake depth, and lake area. Lakes that receive significant indirect drainage from other lakes were less colored than headwater lakes. The best regression model explained 60% of the variance in lake water color. Regression models were calculated separately for four geographic regions (the Laurentians of Quebec, eastem Quebec-Maine, northeastern Wisconsin, and the Experimental Lakes Area of western Ontario). The regression coefficients for the most significant variables, drainage ratio, watershed slope, and lake area were similar over these four areas, but the regional models still had significantly smaller standard errors than the general model based on the whole data set.The color of lake water is a useful index of dissolved humic matter (Gorham et al. 1983). This "gelbstofl" is made up of refractory acidic polymers that dissolve in soil water or in water percolating through peat and sedge bogs (Shapiro 1957; Aiken et al. 198 5). In addition to imparting weak acidity (Shapiro 1957;Gorham et al. 1986) humic matter reduces transparency and thereby the depth of the euphotic zone and the extent of the littoral, thus reducing the abundance and depth range of aquatic macrophytes (Spence 1982). Humic matter can directly reduce phytoplankton productivity (Jackson and Hecky 1980) and phytoplankton AcknowledgmentsWe thank Martin Scott, Lesley Pope, and Serge Ouimet for help with map measurements. Eville Gorham and three anonymous reviewers made suggestions.This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada to J. Rasmussen, the FCAR (Quebec Government) equipe grant to J. Kalff, W. Leggett, R. H. Peters, and J. Rasmussen.A contribution to the Lake Memphremagog Project, McGill Limnology Research Center.

Intermittent saline intrusions are a common feature of many coastal lakes and wetlands. These ecosystems are often important sites of biodiversity, biological productivity, and ecosystem services such as the removal of sediment, nutrients, and contaminants from inflowing rivers. Predicted effects of global climate change, including sea level rise, are likely to intensify saline intrusions into such ecosystems. Analyses of taxonomic diversity and abundance of zooplankton at different salinities in Lake Waihola, South Island, New Zealand, are supported by results of laboratory studies of salinity tolerances of 3 crustacean taxa Gladioferens pectinatus, Boeckella hamata and Daphnia carinata obtained from the lake. The field and laboratory analyses show that severe perturbations of zooplankton community structure and abundance are caused by even minor saline intrusions into Lake Waihola that raise the salinity to >1.2 psu. Our analyses of Lake Waihola, and data from brackish ecosystems around the world, show that even relatively small increases in salinity levels can drive such systems to a state of depleted biodiversity and abundance, altering ecosystem functioning.KEY WORDS: Zooplankton diversity · Zooplankton abundance · Climate change · Community structure · Shallow lake · Salinity · Lake Waihola · Saline intrusion Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 251: [181][182][183][184][185][186][187][188][189] 2003 a medium sized (surface area = 5.4 km 2 ), shallow (mean depth = 1.15 m), tidal (mean tidal range ca. 0.40 m) lake, connected to the sea via a 10 km reach of the Taieri River, ca. 30 km southwest of the city of Dunedin. The lake has a diverse fish community and a high catch per unit effort (CPUE) relative to other New Zealand lakes, though not relative to shallow Danish lakes (Jeppesen et al. 2000). Lake Waihola has a hydraulic residence time of 153 d, based on non-tidal, freshwater inflows (Schallenberg & Burns 2003). During drought conditions, when water levels in the Taieri River are low and when other freshwater inputs are small, intrusions of saline water enter the lake, as occurred in the austral summers 1997/98 and 1998/99. Global climate change is expected to affect New Zealand in ways similar to the El Niño climatic pattern, in which westerly and southerly airflows dominate and the east coast of New Zealand experiences dry (drought) conditions more frequently (Mullan 1996, NZMfE 2001. During dry summers, Lake Waihola often experiences saline intrusions which create strong temporal and spatial salinity gradients within the lake. Results of a calibrated hydrological model of the Taieri catchment, which was run using meteorological inputs based on downscaled global circulation models and 2 global climate change scenarios (NZMfE 2001), indicated that runoff in the Taieri catchment will decrease during summer months under both scenarios (B. Fitzharris unpubl. data). In addition to decreased summer freshwater inputs, Lake Waihola will ...

Summary 1. Wind‐induced sediment resuspension can affect planktonic primary productivity by influencing light penetration and nutrient availability, and by contributing meroplankton (algae resuspended from the lake bed) to the water column. We established relationships between sediment resuspension, light and nutrient availability to phytoplankton in a shallow lake on four occasions. 2. The effects of additions of surficial sediments and nutrients on the productivity of phytoplankton communities were measured in 300 mL gas‐tight bottles attached to rotating plankton wheels and exposed to a light gradient, in 24 h incubations at in situ temperatures. 3. While sediment resuspension always increased primary productivity, resuspension released phytoplankton from nutrient limitation in only two of the four experiments because the amount of available nitrogen and phosphorus entrained from the sediments was small compared with typical baseline levels in the water column. In contrast, chlorophyll a entrainment was substantial compared with baseline water column concentrations and the contribution of meroplankton to primary production was important at times, especially when seasonal irradiance in the lake was high. 4. Comparison of the in situ light climate with the threshold of light‐limitation of the phytoplankton indicated that phytoplankton in the lake were only likely to be light‐limited at times of extreme turbidity (e.g. >200 nephelometric turbidity units), particularly when these occur in winter. Therefore, resuspension influenced phytoplankton production mainly via effects on available nutrients and by entraining algae. The importance of each of these varied in time. 5. The partitioning of primary productivity between the water column and sediments in shallow lakes greatly influences the outcome of resuspension events for water column primary productivity.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 128.235.251.160 on Sun, Abstract. We have analyzed our own data set as well as a number of previously published ones to examine factors related to the numbers and biomass of sediment bacteria in a variety of ecosystems. We have chosen to express sediment variables either on an areal or per unit sediment volume basis to avoid bias due to the more common standardization to sediment dry mass (also known as gram dry weight). Cross-system comparisons show that the amount of organic matter present and the sediment water content are important to sediment bacterial abundance. The C:N ratio of the sediment organic matter, possibly reflecting organic matter quality, was found to explain variation in bacterial numbers in addition to that explained by organic content. Over a broad range, sediment water content is related to sediment bacterial numbers in a unimodal fashion indicating dilution of sediments by water at high (> 80%) water content sites and dilution by inorganic particles at low (< 50%) water content sites. In lakes, the most important factors explaining sediment bacterial biomass were sediment water content and hydraulic flushing rate, a finding supported by analysis of an independent data set of English lakes. Planktonic chlorophyll a and planktonic bacterial numbers (both reflecting autochthonous organic matter levels) were not significantly related to sediment bacterial biomass. We found that sediments overlain by epilimnetic water had significantly higher bacterial biomass than those overlain by hypolimnetic water. A strong positive relationship between macrobenthos and sediment bacterial biomass was found for lakes in Ontario and Quebec. Calculations of annual bacterial production and P:B ratios necessary to sustain the macrobenthic population, based on the observed bacterial and macrobenthic biomass and temperature at our sites, indicate that the bacterial biomass present is more than sufficient to maintain the macrobenthos. In addition, based on the observed ratios of bacterial to detrital carbon at our sites and on a range of published assimilation efficiencies for macrobenthos grazing on these carbon sources, we calculated that the mean percentage of carbon of bacterial origin assimilatable by macrobenthos at our sites is 53% with a maximum of 79% and a minimum of 12%. Our results suggest a tighter trophic linkage between sediment bacteria and macrobenthos in our lakes than in most marine habitats, as indicated by published studies. From these analyses, the general picture emerges of a decoupling between pelagic and benthic bacterial processes in lakes, the pelagic ones being driven mainly by autochthonous production and the benthic ones by a...

There is substantial environmental variance at small spatial scales (1 m or less) in both natural and disturbed environments. We have investigated the spatial structure of physical variables at larger scales (up to 10 m). We analysed surveys of edaphic properties of Wisconsin forest soils, of the water chemistry of lakes in Ontario and Labrador, and of temperature and precipitation in northeastern North America. We found no clear indication that the variance among sites approaches some maximal value as the distance between them increases. We suggest instead that the variance of the physical environment tends to increase continually with distance. The slope of the log-log regression of variance on distance provides a means of comparing the heterogeneity of different environments with respect to a given factor, or of comparing different factors within a given environment. This slope provides a useful measure of environmental structure that can be related to the biodiversity or plasticity of native organisms.