While there is a general sense that lakes can act as sentinels of climate change, their efficacy has not been thoroughly analyzed. We identified the key response variables within a lake that act as indicators of the effects of climate change on both the lake and the catchment. These variables reflect a wide range of physical, chemical, and biological responses to climate. However, the efficacy of the different indicators is affected by regional response to climate change, characteristics of the catchment, and lake mixing regimes. Thus, particular indicators or combinations of indicators are more effective for different lake types and geographic regions. The extraction of climate signals can be further complicated by the influence of other environmental changes, such as eutrophication or acidification, and the equivalent reverse phenomena, in addition to other land-use influences. In many cases, however, confounding factors can be addressed through analytical tools such as detrending or filtering. Lakes are effective sentinels for climate change because they are sensitive to climate, respond rapidly to change, and integrate information about changes in the catchment.

Trace metals are required for numerous processes in phytoplankton and can influence the growth and structure of natural phytoplankton communities. The metal contents of phytoplankton reflect biochemical demands as well as environmental availability and influence the distribution of metals in the ocean. Metal quotas of natural populations can be assessed from analyses of individual cells or bulk particle assemblages or inferred from ratios of dissolved metals and macronutrients in the water column. Here, we review the available data from these approaches for temperate, equatorial, and Antarctic waters in the Pacific and Atlantic Oceans. The data show a generalized metal abundance ranking of Fe≈Zn>Mn≈Ni≈Cu≫Co≈Cd; however, there are notable differences between taxa and regions that inform our understanding of ocean metal biogeochemistry. Differences in the quotas estimated by the various techniques also provide information on metal behavior. Therefore, valuable information is lost when a single metal stoichiometry is assumed for all phytoplankton.

We analyzed published rates of extracellular release (ER) of organic carbon to determine the primary constraints on this process and its importance to bacteria. From 16 studies we extracted observations of ER, particulate primary production (PP), and phytoplankton biomass. In a regression model based on 225 observations, PP explained 69% of the variance in ER. From this model we estimate the average percent extracellular release (PER) to be 13% of total fixation. The slope of this relationship does not support the hypothesis that the PER declines with increasing productivity. Differences exist between marine and freshwater systems. In lakes, ER increases nonlinearly with productivity, resulting in very low PER in very eutrophic systems. In coastal marine and estuarine systems, ER increases linearly with productivity and the PER does not vary systematically. ER is not primarily related to phytoplankton biomass as predicted by passive diffusion models. Instead, ER appears to be constrained by the total availability of photosynthates. By comparing our model to an existing model of bacterial production and assuming a 50% growth efficiency, we estimate that ER amounts to less than half the C required for bacterial growth in most pelagic systems.The release of dissolved organic substances by phytoplankton has long been recognized as an important source of high yuality carbon to bacteria (Cole et al. 1982) as well as a frequently significant loss of photosynthate from pelagic algae (Fogg et al. 1965). As a consequence, this extracellular release (ER) has been studied widely, both within systems over depth and time and across production gradients. Unfortunately, results have often conflicted, making generalization concerning ER difficult.

During the Southern Ocean Iron Experiment (SOFeX), we analyzed Si, P, S, Mn, Ni, and Zn in individual diatoms, autotrophic flagellates, and heterotrophic flagellates with synchrotron-based X-ray fluorescence (SXRF) and calculated cellular C from measurements of cell size. Element stoichiometries for the different types of protists (normalized to either C, S, or P) were generally in good agreement with prior bulk analyses of natural assemblages but also revealed previously undocumented differences in elemental composition among cell types. Flagellated cells contained 39% more P than diatoms, which in turn contained 79% more Mn, 3-fold more Ni, and 2.6-fold more Zn than flagellates. Heterotroph cells contained approximately 40% more P and twice as much Zn as autotrophs. Manganese and Ni stoichiometries were negatively related to cell volume, while larger cells contained more Zn per mole C. Iron fertilization resulted in an approximate doubling of Mn, Ni, and Zn quotas and smaller increases in cellular P, but the timing of the stoichiometric changes varied between the two patches. Silicon contents of diatoms dropped approximately 40% after the first Fe addition but returned to prefertilization levels as cellular P doubled following the second addition, resulting in 40% lower Si : P ratios in the fertilized waters. The mean elemental stoichiometries calculated for all cells analyzed were comparable with previously published extended Redfield ratios for mixed plankton assemblages, but the observed differences between diatoms and flagellates and between autotrophs and heterotrophs indicate that valuable information is lost when all types of co-occurring plankton are grouped together for analysis.Metal biogeochemistry and ocean ecosystem function are closely linked. Several transition metals, including Mn, Fe, Co, Ni, Cu, and Zn, are required as cofactors for common proteins, and both heterotrophic and autotrophic protists accumulate these elements from the surrounding seawater, lowering dissolved concentrations in surface waters. Remineralization of these metals from sinking cells or zooplankton fecal pellets elevates their concentrations below the euphotic zone, resulting in the nutrient-like profiles commonly observed for these bioactive metals. Further, the availability and accumulation of trace metals can exert control over the biological functioning of the organisms. AcknowledgmentsThe authors gratefully acknowledge Michael Landry for providing the opportunity to participate in the SOFeX project and Chief Scientist Ken Johnson and the Captain and crew of the R/V Roger Revelle for enabling the collection of the samples. Jörg Maser, Stefan Vogt, and Dan Legnini provided invaluable technical assistance with the SXRF microprobe, and the manuscript benefited from discussions with Mark Brzezinski and the comments of two anonymous reviewers.

Determining how ecological and evolutionary processes produce spatial variation in local species richness remains an unresolved challenge. Using mountains as a model system, we outline an integrative research approach to evaluate the influence of ecological and evolutionary mechanisms on the generation and maintenance of patterns of species richness along and among elevational gradients. Biodiversity scientists interested in patterns of species richness typically start by documenting patterns of species richness at regional and local scales, and based on their knowledge of the taxon, and the environmental and historical characteristics of a mountain region, they then ask whether diversity–environment relationships, if they exist, are explained mostly by ecological or evolutionary hypotheses. The final step, and perhaps most challenging one, is to tease apart the relative influence of ecological and evolutionary mechanisms. We propose that elucidating the relative influence of ecological and evolutionary mechanisms can be achieved by taking advantage of the replicated settings afforded by mountains, combined with targeted experiments along elevational gradients. This approach will not only identify potential mechanisms that drive patterns of species richness, but also allow scientists to generate more robust hypotheses about which factors generate and maintain local diversity.

The study of trace metal cycling by aquatic protists is limited by current analytical techniques. Standard "bulk" element analysis techniques that rely on physical separations to concentrate cells for analysis cannot separate cells from co-occurring detrital material or other cells of differing taxonomy or trophic function. Here we demonstrate the ability of a synchrotron-based X-ray fluorescence (SXRF) microprobe to quantify the elements Si, Mn, Fe, Ni, and Zn in individual aquatic protist cells. This technique distinguishes between different types of cells in an assemblage and between cells and other particulate matter. Under typical operating conditions, the minimum detection limits are 7.0 x 10(-16) mol microm(-2) for Si and between 5.0 x 10(-20) and 3.9 x 10(-19) mol microm(-2) for Mn, Fe, Ni, and Zn; this sensitivity is sufficient to detect these elements in cells from even the most pristine waters as demonstrated in phytoplankton cells collected from remote areas of the Southern Ocean. Replicate analyses of single cells produced variations of <5% for Si, Mn, Fe, and Zn and <10% for Ni. Comparative analyses of cultured phytoplankton cells generally show no significant differences in cellular metal concentrations measured with SXRF and standard bulk techniques (spectrophotometry and graphite furnace atomic absorption spectrometry). SXRF also produces two-dimensional maps of element distributions in cells, thereby providing information not available with other analytical approaches. This technique enables the accurate and precise measurement of trace metals in individual aquatic protists collected from natural environments.

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