Search

Under‐ice photoautotrophs in lakes are generally considered to be limited by light rather than nutrients. Despite reduced light intensity under the ice, there is increasing evidence that suggests some lakes support high levels of photoautotrophs. We explored how snow cover (i.e., light) and nutrients (i.e., nitrogen and phosphorus) influence ice‐associated photoautotroph growth in a Minnesota, USA lake. Using a novel under‐ice approach, we deployed nutrient diffusing substrates (single or combined nutrient amendments) under two different light scenarios (snow covered, reduced light; snow removed, increased light) near the water‐ice interface to mimic a range of conditions ice‐associated photoautotrophs may be exposed to. Natural snow cover reduced light compared with snow removal, particularly early in the experiment before snow began to melt. When comparing photoautotroph chlorophyll a (Chl a) between snow treatments, we found a significant snow effect with higher concentrations in the snow removed treatment. We also found a significant nutrient effect, for all nutrient treatments, on Chl a concentrations in both snow conditions. The effect of any nutrient treatment on Chl a concentrations was similar. Our results suggest that ice‐associated photoautotrophs were able to grow in all snow conditions, but snow removal resulted in higher growth and nutrient availability also mediated responses. Thus, both light and nutrient conditions in the winter may strongly affect ice‐associated photoautotroph dynamics. [ABSTRACT FROM AUTHOR]

In lakes, the rates of gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP) are often controlled by resource availability. Herein, we explore how catchment vs. within lake predictors of metabolism compare using data from 16 lakes spanning 39°N to 64°N, a range of inflowing streams, and trophic status. For each lake, we combined stream loads of dissolved organic carbon (DOC), total nitrogen (TN), and total phosphorus (TP) with lake DOC, TN, and TP concentrations and high frequency in situ monitoring of dissolved oxygen. We found that stream load stoichiometry indicated lake stoichiometry for C : N and C : P (r2 = 0.74 and r2 = 0.84, respectively), but not for N : P (r2 = 0.04). As we found a strong positive correlation between TN and TP, we only used TP in our statistical models. For the catchment model, GPP and R were best predicted by DOC load, TP load, and load N : P (R2 = 0.85 and R2 = 0.82, respectively). For the lake model, GPP and R were best predicted by TP concentrations (R2 = 0.86 and R2 = 0.67, respectively). The inclusion of N : P in the catchment model, but not the lake model, suggests that both N and P regulate metabolism and that organisms may be responding more strongly to catchment inputs than lake resources. Our models predicted NEP poorly, though it is unclear why. Overall, our work stresses the importance of characterizing lake catchment loads to predict metabolic rates, a result that may be particularly important in catchments experiencing changing hydrologic regimes related to global environmental change. [ABSTRACT FROM AUTHOR]

We report a reversal in the sign of the herbivore-phytoplankton interaction between the zebra mussel (Dreissena polymorpha) and Microcystis aeruginosa, a harmful planktonic cyanobacterium. A pair of large-scale manipulations of mussel density in the same lake in consecutive years showed that when phosphorus concentrations were very low (mean total phosphorus [TP] ∼ 3 µg L-1), the effect of Dreissena on the biomass of M. aeruginosa was monotonically negative across the full range of sustainable mussel densities. When the enclosures were fertilized with phosphorus (mean TP ∼ 9 µg L-1), there was a monotonically positive effect of Dreissena on M. aeruginosa across the same mussel gradient. These contrasting results indicate that D. polymorpha feeds on M. aeruginosa, as shown in previous laboratory feeding experiments, but that the positive effects of D. polymorpha on M. aeruginosa can be larger than the negative effects of consumption. A sign reversal in the interaction between these two species is congruent with highly variable patterns in the response of M. aeruginosa to D. polymorpha invasion across lake and river systems in North America. [ABSTRACT FROM AUTHOR]

Presents a study that investigated how watershed land use relates to phytoplankton primary production and photosynthetic parameters in 12 reservoirs in Ohio and examined spatial variation in these parameters. Materials and methods; Seasonal variation in photosynthetic parameters at inflow and outflow sites; Relationships between watershed land use and primary production, chlorophyll and total phosphorus.

As the lowest point in the surrounding landscape, lakes act as sensors in the landscape to provide insights into the response of both terrestrial and aquatic ecosystems to climate change. Here a novel suite of climate forcing optical indices (CFOI) from lakes across North America is found to respond to changes in air temperature, precipitation, and solar radiation at timescales ranging from a single storm event to seasonal changes to longer-term interdecadal trends with regression r² values ranging from 0.73 to 0.89. These indices are based on two optical metrics of dissolved organic carbon (DOC) quality: DOC specific absorbance (a*320) and spectral slope (S275-295), where the ratio a*320 to S275-295 gives a composite climate forcing index. These indices of DOC quality are more responsive to climate forcing than is DOC concentration. A similar relationship between the component indices a*320 and S275-295 is observed across a wide range of lake types. A conceptual model is used to examine the similarities and differences in DOC-related mechanisms and ecological consequences due to increased temperature vs. precipitation. While both warmer and wetter conditions increase thermal stratification, these two types of climate forcing will have opposite effects on water transparency as well as many ecological consequences, including oxygen depletion, the balance between autotrophy and heterotrophy, and depth distributions of phytoplankton and zooplankton. [ABSTRACT FROM AUTHOR]

We asked if intraspecific variation within a population of the toxic, bloom-forming phytoplankter, Microcystis aeruginosa, leads to differential vulnerability to grazing by the invasive, filter-feeding zebra mussel (Dreissena polymorpha). We performed two series of laboratory feeding experiments in which D. polymorpha were presented with several different sympatric M. aeruginosa clones in a two-species mixture with the high-quality, nutritious alga, Ankistrodesmus falcatus. Mean selectivity across M. aeruginosa clones ranged from near zero (i.e., no feeding on M. aeruginosa) to near one (equal feeding on the two algal species), evidence of maximal variation in grazing vulnerability across twenty clones of M. aeruginosa--a species assumed to be largely 'inedible' to grazers--from the same population. This range of vulnerability is essentially equal to that typically measured across all phytoplankton species. Large intraspecific variation in grazing vulnerability, and its ecological consequences, could influence the promotion or control of noxious blooms of toxigenic cyanobacteria. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]