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O_LIThe larval stage in fish development is characterized as a critical period with unusually high mortality rates. Additional stress may be imposed on fish larvae as fresh waters brown in color due to increased inputs of dissolved organic matter from the terrestrial watershed. Both energy transfer and foraging efficacy are predicted to decrease as light availability declines with browning.nC_LIO_LIHere, we investigated the balance between indirect versus direct effects of browning on zooplankton-larval fish interactions. Experimental tanks were established in the laboratory where water color was altered with increasing concentrations of SuperHume (water color measured as absorbance at 440 nm = 1.6 - 10.8 m-1). Phytoplankton and zooplankton densities were monitored across treatments for one month to assess possible changes in community structure leading up to fish spawning (i.e., late Spring in central Virginia, USA). We then introduced larval largemouth bass to the tanks to assess indirect effects on larval fish feeding rate and growth. Additionally, direct effects on the foraging efficiency and selectivity of largemouth bass and bluegill were determined with separate short-term feeding experiments using the same experimental setup.nC_LIO_LIBrowning did not directly alter the ability of larval fish to see and capture prey across our brown water treatments. However, significant indirect effects on larval fish foraging, growth, and survival were observed as phytoplankton and zooplankton densities decreased with increasing water color.nC_LIO_LIOur data suggest lake browning will reduce energy transfer to larval fish, during a critical stage of development, due to a reduction in prey availability but not visual foraging.nC_LI

Elevated allochthonous inputs of organic matter are increasingly recognized as a driver of ecosystem change in lakes, particularly when concurrent with eutrophication. Evaluation of lakes in a nutrient-color paradigm (i.e., based on total phosphorus and true color) enables a more robust approach to research and management. To assess temporal and spatial patterns in nutrient-color status for U.S. lakes and associated food web attributes, we analyzed the U.S. Environmental Protection Agency's National Lakes Assessment (NLA) data. With 1000+ lakes sampled in 2007 and 2012 in a stratified random sampling design, the NLA enables rigorous assessment of lake condition across the continental U.S. We demonstrate that many U.S. lakes are simultaneously experiencing eutrophication and brownification to produce an abundance of "murky" lakes. Overall, "blue" lakes decreased by ~ 18% (46% of lakes in 2007 to 28% in 2012) while "murky" lakes increased by almost 12% (24% of lakes in 2007 to 35.4% in 2012). No statistical differences were observed in the proportions of "green" or "brown" lakes. Regionally, murky lakes significantly increased in the Northern Appalachian, Southern Plains, and Xeric ecoregions. Murky lakes exhibited the highest epilimnetic chlorophyll a concentrations, cyanobacterial densities, and microcystin concentrations. Total zooplankton biomass was also highest in murky lakes, primarily due to increased rotifer and copepod biomass. However, zooplankton : phytoplankton biomass ratios were low, suggesting reduced energy transfer to higher trophic levels. These results emphasize that many lakes in the U.S. are simultaneously "greening" and "browning", with potentially negative consequences for water quality and food web structure.

We examined seasonal and spatial patterns in dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) in the Chowan River watershed, North Carolina, a blackwater river which discharges into the second largest estuary in the United States, the Albemarle–Pamlico Estuarine System. From April 2008 to May 2010, DOC concentration did not significantly vary across seasons (range 7.69–30.39 mg L−1); however, CDOM molecular size and aromaticity increased throughout the spring, decreased during the summer and fall, and remained relatively low in the winter. Spectral slope ratios suggested microbial processing of CDOM in the spring and photodegradation of CDOM in the summer and fall. Spatially, DOC and CDOM concentrations were similar in the mainstem and at the mouths of two tributaries, Bennetts Creek and Wiccacon River, but were significantly higher upstream on the tributaries. DOC concentration was positively correlated with CDOM absorbance coefficients at 254 and 350 nm; however, these optical proxies explained only ~60 % of the variance. DOC and CDOM absorption loads to the Albemarle Sound ranged from 2.63 × 1010 g year−1 and 9.84 × 1010 m2 year−1, respectively, in a dry year and 7.9 × 1010 g year−1 and 2.2 × 1011 m2 year−1, respectively, in a wet year, which are comparable to non-blackwater rivers with larger watersheds. Blackwater rivers may therefore represent “hotspots” in coastal carbon chemistry, with seasonal variations in the quality and quantity of DOC and CDOM influencing estuarine food web dynamics and net ecosystem metabolism.

Climate-related shifts in forest composition and chemistry will likely affect the quantity and quality of dissolved organic matter (DOM) entering inland waters, and consequently, carbon dioxide (CO 2 ) evasion. We examined the photodegradation of DOM derived from 2 common riparian plant species ( Populus tremuloides and Salix alba ) grown at ambient (360 ppm) and elevated (720 ppm) atmospheric CO 2 concentrations. Rates and total photolytic CO 2 production were determined for sterilized leachates ranging from 5 to 100 mg L −1 dissolved organic carbon (DOC). Based on multiple regression analysis, DOC concentration, followed by plant species, best predicted the rate and total flux of CO 2 . Photolytic CO2 production increased linearly with DOC concentration; however, the 5 mg L −1 treatment had the greatest rate per unit carbon, suggesting a self-shading effect of increasing DOC. The atmospheric CO2 conditions under which the plants were grown had no statistically significant effect, despite observed differences in CO 2 fluxes between ambient and elevated Populus leachates. Fluorescence data suggest differences in photolytic CO 2 production among treatments are related to differences in plant chemistry within the humic fraction. Thus, the magnitude of the photolytic CO 2 flux from fresh waters in the future will depend primarily on climate-related changes in the quantity of terrestrial DOM inputs and secondarily by DOM source.

The intensity and spectral composition of visible light are known to influence fish predation on zooplankton. However, in clear-water systems, ultraviolet (UV) radiation (UVR) may also influence fish predation, directly through UV-enhanced foraging, indirectly through alterations in predator-prey overlap, or in a combination of the two. Here we test the hypothesis that UVR facilitates fish predation on zooplankton in an oligotrophic lake. Experiments were conducted in 2.2-m-long UV-transparent or UV-blocking columns suspended in the epilimnion. Zooplankton consumption by young-of-the-year (YOY) largemouth bass (Micropterus salmoides) was compared in the presence and absence of UVR, and the vertical distributions of prey were quantified across light and fish treatments. In addition, a separate experiment was conducted to examine zooplankton vertical responses to UV exposure vs. fish kairomones. Overall, YOY predation on zooplankton was higher in the presence than in the absence of UVR, particularly on diaptomid copepods. This result was only partially explained by zooplankton migratory behaviors, suggesting UV-enhanced searching capabilities in YOY bass. Furthermore, diaptomids displayed a stronger vertical behavioral response to fish kairomones than to UVR whereas Daphnia exhibited a stronger response to UVR.

In transparent, thermally stratified lakes, ultraviolet radiation (UV) and temperature are among the primary factors influencing diel vertical migration (DVM) of zooplankton. It is not well known how behavioral responses will vary across a wide range of depths with steep gradients in UV, photosynthetically active radiation (PAR), and temperature. Our objective was to determine the relative importance of UV and temperature on DVM of Daphnia catawba and Leptodiaptomus minutus in a high UV, thermally stratified lake. Using UV-transparent (+UV) and UV-opaque (–UV) columns suspended within the mixed layer and metalimnion, we found that both species were generally shallower in the –UV than in the +UV columns. Daphnia catawba responded negatively to UV, even below the 1% depth for 320 nm, whereas L. minutus responded to UV only within the mixed layer and upper metalimnion. Daphnia catawba did not migrate in the –UV in the mixed layer, but migrated upwards in the –UV in the deep part of the metalimnion, indicating a temperature-induced rather than phototactic response. At night, both species migrated upwards in the metalimnion and were evenly distributed in the mixed layer. These results indicate that in transparent lakes, UV may constrain some zooplankton to cooler, suboptimal temperatures, which may compromise fitness.

Previous studies indicate that leachates derived from plants grown under elevated CO 2 are more recalcitrant and consequently reduce bacterial productivity. Here bacteria were cultured on organic leachates derived from trembling aspen (Populus tremuloides) and white willow (Salix alba) grown under ambient (360 ppm) and elevated (720 ppm) CO 2 concentrations and exposed and unexposed to simulated solar radiation. In treatments not exposed to light, determined numbers of colony forming units (CFU mL -1 ) were higher on ambient CO 2 than elevated CO 2 leachates in both plant species. After exposure to simulated sunlight, the number of CFU mL -1 decreased on the ambient CO 2 leachates and increased on the elevated leachates, particularly the aspen. These results demonstrate that environmental parameters, mediated by climate change, may have both positive and negative effects on the growth of bacteria.

Ultraviolet (UV) photoreceptors have been reported in a wide variety of freshwater and marine organisms, suggesting that UV vision is prominent in aquatic ecosystems. However, its adaptive significance remains speculative. The present study tested whether the foraging of juvenile bluegill (Lepomis macrochirus) is enhanced in the presence of UV radiation (UVR). Laboratory feeding trials were conducted in a laminar flow tank in which L. macrochirus juveniles between 2.3 and 3.5 cm standard length were fed the cladoceran Daphnia magna. Sighting and striking distances, as well as capture success, were measured in the presence and absence of UVR. Mean sighting and striking distances and capture success did not differ significantly between the two light treatments. There were also no significant differences in the frequency distributions of sighting and striking distance. These results suggest that UV vision may not be used to enhance foraging in L. macrochirus on Daphnia within the size class tested.

Zooplankton taxa exhibit varying tolerances to ultraviolet radiation (UVR), with rotifers and copepods tending to be more UV tolerant than cladocerans, regardless of lake UV transparency. On the basis of these differences, we hypothesized that UV avoidance behavior would be greatest in zooplankton with low UV tolerance, particularly in high-UV systems. Both downward and upward movements of zooplankton were examined in cylindrical acrylic columns that either transmitted full sunlight or blocked UV-B and short wavelength UV-A radiation. Columns were suspended vertically in the surface waters of a high- and low-UV lake at varying times of day and cloud cover. In the high-UV lake, the cladoceran Daphnia catawba displayed a distinct avoidance of the surface waters in the UV+ treatment but often showed a preference for the surface in the UV– treatment. Copepods were less responsive to UVR with calanoids displaying a somewhat greater UV avoidance than cyclopoids. In the low-UV lake, no behavioral differences were observed among UV treatments. These results suggest that UVR may influence the vertical distribution and habitat partitioning of certain zooplankton taxa in high-UV lakes while other factors, such as predation and food limitation, may be more important in low-UV lakes.

Recently, small-scale field experiments have demonstrated that exposure to UV radiation can invoke downward migrations in UV-sensitive zooplankton; however, questions remain as to zooplankton responses at the whole-lake scale. Here a three-year database on three lakes of varying UV transparency was used to examine the relationship between UV exposure and the seasonal vertical distribution and abundance of several zooplankton species. We hypothesized that if UV radiation is an important ecological force influencing the vertical distribution and seasonal abundance of zooplankton, the strength of a given species' response to UV would be inversely proportional to its UV tolerance. UV habitat did have a significant effect on zooplankton distribution in taxa with low UV tolerance compared to taxa with high UV tolerance. Cladocerans, which are the least UV-tolerant of three major groups, tended to avoid the surface waters of the highest UV lake during periods of high UV. More UV-tolerant groups, rotifers and copepods, showed little or no evidence for avoidance of high UV habitats. Among lakes, the relationship between UV avoidance and taxon was evident only in the highest UV lake. These data are consistent with the hypothesis that UV influences zooplankton community dynamics in low DOC, UV-transparent lakes.

Damaging solar radiation was one of the first factors proposed to explain zooplankton diel vertical migrations (DVM), yet other factors such as temperature, food, and especially predation have typically been more widely studied and identified as the primary factors inducing DVM. This is true in spite of the fact that recent experiments have shown that ambient levels of solar ultraviolet radiation (UVR) are potentially lethal to zooplankton and negative phototactic behavior has been demonstrated in the laboratory. Here we present the first evidence from in situ experiments demonstrating that UVR wavelengths influence the vertical swimming behavior of zooplankton under full spectrum solar radiation. Cylindrical acrylic columns that transmitted or blocked UVR were used to examine the behavioral responses of Daphnia pulicaria to UVR. Experiments were conducted in the epilimnion of a high-UV system at midday and in late afternoon. In both experiments, a large proportion of individuals migrated downward in the presence of UVR, whereas those in the UVR-shielded treatments tended to remain closer to the surface.

Chromatin diminution, the fragmentation and elimination of chromosome regions during early embryogenesis, occurs in some species of free-living, fresh-water copepods. We studied the variation in timing of this developmental trait by examining durations of cell stages in several genera of Cyclopidae. Chromatin diminution varied in timing within and among genera, and ranged from the 8-64-cell stage. The timing of chromatin diminution, as measured by the number of hours post fertilization, differs considerably within and among genera. An unusually long cell-stage duration relative to that of other cell stages is indicative of the stage during which chromatin diminution occurs or the preceding stage. Thus, the relative length of cell-stage durations within a species is a reliable assay for the presence or absence of chromatin diminution.

[1] We generated suspended sediment solutions using river sediments and river water at concentrations similar to those observed during 1.5 year floods (Q1.5) and a dam removal (∼325 mg L−1) on the Deep River, North Carolina. Suspended sediment solutions were exposed to simulated solar radiation, equivalent to one clear, summer day at the study site (35°N). Concentrations of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), soluble reactive phosphorus (SRP) and total dissolved iron (Fed) were measured before and after exposure. Total dissolved carbon (TDC) budgets for each experiment were produced using DOC and DIC data. Sediment suspensions in the presence of simulated solar radiation were significant sources of dissolved C (119 ± 11 μmol C L−1 d−1; ± values indicate 1 standard error) and DON (1.7 ± 0.5 μmol N L−1 d−1) but not DIN or SRP. Extrapolations through the Deep River water column suggest that suspended sediments in the presence of light represent dissolved organic matter fluxes of 3.92 mmol C m−2 d−1 and 40 μmol N m−2 d−1. Additionally, sediment suspensions lowered river water Fed concentrations immediately (∼24%) and progressively (∼40–90%) in both light and dark treatments. Our research suggests suspended sediments in river ecosystems are potential sources of dissolved organic C and dissolved organic N while effectively removing Fed from the water column.

Seasonal increases in the ultraviolet (UV) transparency of the surface waters of an oligotrophic lake in Pennsylvania suggest that clear-water phase (CWP) events similar to those previously observed for visible light also exist for the potentially damaging UV wavelengths. Seasonal increases in zooplankton grazers indicate that they play a role in these changes in UV that is similar to the role that zooplankton play in CWP events involving longer-wavelength visible, or photosynthetically active radiation (PAR, 400¿700 nm). The potential role of zooplankton and incident UV in generating UV CWP events was investigated with a set of in situ microcosm experiments that manipulated UV and zooplankton, and followed changes in particulate and dissolved absorbance in the UV (320 nm) and PAR wavelength ranges over an 8-d period in April. Nutrients were also manipulated independently to examine the potential role of nutrient regeneration by zooplankton grazing in altering water transparency. Photobleaching by incident solar UV led to a strong and significant decrease in dissolved UV and PAR absorbance. The presence of zooplankton grazers also significantly decreased dissolved UV absorbance but increased dissolved PAR absorbance. Neither zooplankton nor UV had any significant effects on UV or PAR absorbance by particulates. In contrast, nutrient additions significantly increased dissolved absorbance in both the UV and PAR wavelength ranges, indicating that regeneration of nutrients by zooplankton offsets decreases in UV absorbance and enhances increases in PAR absorbance due to grazing. While photobleaching by UV radiation is likely to make a consistent strong contribution to UV CWP events in lakes, the net effects of zooplankton on UV transparency in a given lake will depend upon multiple factors including zooplankton density and a balance between the edibility and extent of nutrient limitation of the phytoplankton.

We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.