We analyzed a diverse set of 1,646 north-temperate lakes to evaluate the nutrient-color paradigm that integrates total phosphorus (TP) and colored dissolved organic carbon to define lake trophic status. Our objectives were to quantify the combined influence of TP and color (Col) on lake trophic status, to determine if TP and Col had similar relationships with hydrogeomorphic (HGM) variables, and to examine how TP and Col affected the balance of heterotrophic and autotrophic processes. For the latter we examined the Col to chlorophyll a ratio (Col : Chl a), an index of allochthonous contributions of carbon to pelagic consumers, and deviations of lake pCO 2 from atmospheric, an index of net heterotrophy. Both Col and TP had strong effects on Chl a (positive) and Secchi transparency (negative), suggesting that ignoring Col would lead to misinterpretation of these widely used trophic status indicators. Lakes with high TP and Col tended to be shallower with large catchment to lake area ratios. Negative correlations with water retention time (WRT) were stronger for Col than for TP. Both TP and Col were related to forest and wetland land cover, although the direction of the relationships were opposite. Only 29% of the lakes had relatively high allochthony according to their Col : Chl a ratios; these were predominately high color, oligotrophic or mesotrophic lakes with short WRT. Over 90% of a subset of 682 lakes were net heterotrophic, with pCO 2 exceeding atmospheric levels. The positive relationship between pCO 2 and Col : Chl a suggests that only in very heterotrophic systems was the transfer of allochthonous carbon to pelagic consumers appreciable. Our results provide strong empirical support for the nutrient-color paradigm and highlight its importance both for management applications and for expanding our understanding of how lakes are influenced by terrestrial subsidies of carbon and nutrients. AcknowledgementsWe thank the many dedicated state agency professionals who contributed to the lake database through sample collection, laboratory analysis, and data management and Michigan State University's Remote Sensing and Geographic Information Science Research and Outreach Services for quantification of the landscape data. T. Wagner, B. A. Lake, R. H. Foy, M. Hoyer, and one anonymous reviewer provided helpful reviews.

Nutrient limitation of primary production was experimentally assessed using an in situ bioassay technique in the Quebrada Salto, a third-order tropical stream draining the northern foothills of the Cordillera Centrall in Costa Rica . Bioassays employed artificial substrata enriched with nutrients that slowly diffuse through an agar-sand matrix (Pringle & Bowers, 1984) . Multiple comparisons of regression coefficients, describing chlorophyll-a accrual through time for different nutrient treatments, revealed positive micronutrient effect(s) . Micronutrient treatment combinations (Fe, B, Mn, Zn, Co, Mo, EDTA), supplemented with and without nitrate and phosphate, exhibited significantly greater chlorophyll-a accrual over all other treatments (P < 0.05), supporting over three times that of the control after 14-d of substratum colonization . Neither of the major nutrients (N or P) produced a significant stimulation, although the N treatment displayed =50% more chlorophyll-a than the control after 14-d . Similarly, Si, EDTA, and Si + N + P treatments did not exhibit chlorophyll-a response curves that were significantly different from the control . During the experiment, mean NH4-N and (NO2 + NO 3)-N concentrations in the Salto were 2 .0 µM (28 .6 µg • 1 -') and 7.2 µM (100 .2 µg •1 -'), respectively. High concentrations of P0 4-P (x = 2 .0 µM ; 60 .9 µg •1 -') and TP (x = 3 .0 µM ; 94 .0 µg •1 -1 ) were also found, and consequently low molar N :P ratios (1-c = 4.7). Despite the potential for N limitation in the system, both N and P appear to be at growth saturating levels . This may be due to micronutrient limitation and/or light limitation of periphyton growth in densely shaded upstream portions of the stream .

We evaluated the effectiveness of a portable, backpack electrofishing unit, used from a small boat, for sampling fish assemblage structure in 11 lakes in the northeastern United States. Samples collected with a boat-mounted, 350-W backpack unit (EF-BP) were compared with samples taken by (1) two 5,000-W electrofishing units, (2) gill nets, (3) trap nets and minnow traps, and (4) seines. The EF-BP was more effective than the nonelectrofishing gears, capturing a mean of 79% of a lake's documented fish species pool. Gill nets, beach seines, and trap nets captured, on average, about 50% of the species; minnow traps were least efficient. With identical transect times, the EF-BP was as effective as a standard barge-mounted electrofishing unit for capturing species and numbers of individuals. At three low-conductivity lakes, the EF-BP collected fewer species and individuals than a semiportable 5,000-W unit but was more effective than any of the nonelectrofishing gears. Analysis of species accumulation curves for the EF-BP indicated that three or four 4-min electrofishing of transects were sufficient to collect a reasonably complete sample of the available species pool in lakes smaller than 20 ha; in larger lakes up to 900 ha, as many as nine transects were required to achieve a similar degree of sampling sufficiency.

We quantified potential biases associated with lakes monitored using non-probability based sampling by six state agencies in the USA (Michigan, Wisconsin, Iowa, Ohio, Maine, and New Hampshire). To identify biases, we compared state-monitored lakes to a census population of lakes derived from the National Hydrography Dataset. We then estimated the probability of lakes being sampled using generalized linear mixed models. Our two research questions were: (1) are there systematic differences in lake area and land use/land cover (LULC) surrounding lakes monitored by state agencies when compared to the entire population of lakes? and (2) after controlling for the effects of lake size, does the probability of sampling vary depending on the surrounding LULC features? We examined the biases associated with surrounding LULC because of the established links between LULC and lake water quality. For all states, we found that larger lakes had a higher probability of being sampled compared to smaller lakes. Significant interactions between lake size and LULC prohibit us from drawing conclusions about the main effects of LULC; however, in general lakes that are most likely to be sampled have either high urban use, high agricultural use, high forest cover, or low wetland cover. Our analyses support the assertion that data derived from non-probability-based surveys must be used with caution when attempting to make generalizations to the entire population of interest, and that probability-based surveys are needed to ensure unbiased, accurate estimates of lake status and trends at regional to national scales.

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.. The Association for Tropical Biology and Conservation is collaborating with JSTOR to digitize, preserve and extend access to Biotropica. ABSTRACTGill nets set at three depths at an open-water station in Lake Titicaca during August 1984 captured only two species of endemic killifishes (Orestias ispi and 0. mulleri) and an exotic silverside (Basilichthyes bonariensis). These species segregated strongly by depth, with B. bonariensis and 0. mulleri occurring only in the surface and bottom sets, respectively, and 0. ispi being most abundant in the mid-water set. All three species were feeding on zooplankton, but 0. mulleri also included benthic crustaceans in its diet. Copepods were strongly selected by large (>200 mm SL) B. bonariensis, whereas smaller size classes ate mainly cladocerans.