Janet M. Fischer

The dual nature of community variability

Author(s): Micheli, Fiorenza; Cottingham, Kathryn L.; Bascompte, Jordi; Bjornstad, Ottar N.; Eckert, Ginny L.; Fischer, Janet M.; Keitt, Timothy H.; Kendall, Bruce E.; Klug, Jennifer L.; Rusak, James A | Abstract: Community variability has a dual nature. On the one hand, there is compositional variability, changes in the relative abundance of component species. On the other hand, there is aggregate variability, changes in summary properties such as total abundance, biomass, or production. Although these two aspects of variability have received much individual attention, few studies have explicitly? related the compositional and aggregate variability of natural communities. In this paper, we show how simultaneous consideration of both aspects of community variability might advance our understanding of ecological communities.We use the distinction between compositional and aggregate variability to develop an organizational framework for describing patterns of community variability. At their extremes, compositional and aggregate variability combine in four different ways: (I) stasis, low compositional and low aggregate variability; (2) synchrony, low compositional and high aggregate variability; (3) asynchrony, high compositional and high aggregate variability; and (4) compensation, high compositional and low aggregate variability. Each of these patterns has been observed in natural communities, and can be linked to a suite of abiotic and biotic mechanisms. We give examples of the potential relevance of variability patterns to applied ecology, and describe the methodological developments needed to make meaningful comparisons of aggregate and compositional variability across communities. Finally, we provide two numerical examples of how our approach can be applied to natural communities.

Detection of scale-specific community dynamics using wavelets.

The response of ecological communities to anthropogenic disturbance is of both scientific and practical interest. Communities where all species respond to disturbance in a similar fashion (synchrony) will exhibit large fluctuations in total biomass and dramatic changes in ecosystem function. Communities where some species increase in abundance while others decrease after disturbance (compensation) can maintain total biomass and ecosystem function in the face of anthropogenic change. We examined dynamics of the Little Rock Lake (Wisconsin, USA) zooplankton community in the context of an experimental pH manipulation conducted in one basin of the lake. A novel application of wavelets was used to partition patterns of synchrony and compensation by time scale. We find interestingly that some time series show both patterns of synchrony and compensation depending on the scale of analysis. Within the unmanipulated basin, we found subtle patterns of synchrony and compensation within the community, largely at a one-year time scale corresponding to seasonal variation. Within the acidified lake basin, dynamics shifted to longer time scales corresponding to the pattern of pH manipulation. Comparisons between pairs of species in different functional groups showed both strong compensatory and synchronous responses to disturbance. The strongest compensatory signal was observed for two species of Daphnia whose life history traits lead to synchrony at annual time scales, but whose differential sensitivity to acidification led to compensation at multiannual time scales. The separation of time scales inherent in the wavelet method greatly facilitated interpretation as patterns resulting from seasonal drivers could be separated from patterns driven by pH manipulation.

Differences in UV transparency and thermal structure between alpine and subalpine lakes: implications for organisms

Ultraviolet (UV) radiation is a globally important abiotic factor influencing ecosystem structure and function in multiple ways. While UV radiation can be damaging to most organisms, several factors act to reduce UV exposure of organisms in aquatic ecosystems, the most important of which is dissolved organic carbon (DOC). In alpine lakes, very low concentrations of DOC and a thinner atmosphere lead to unusually high UV exposure levels. These high UV levels combine with low temperatures to provide a fundamentally different vertical structure to alpine lake ecosystems in comparison to most lowland lakes. Here, we discuss the importance of water temperature and UV transparency in structuring alpine lake ecosystems and the consequences for aquatic organisms that inhabit them. We present transparency data on a global data set of alpine lakes and nearby analogous subalpine lakes for comparison. We also present seasonal transparency data on a suite of alpine and subalpine lakes that demonstrate important differences in UV and photosynthetically active radiation (PAR, 400-700 nm) transparency patterns even within a single region. These data are used to explore factors regulating transparency in alpine lakes, to discuss implications of future environmental change on the structure and function of alpine lakes, and ways in which the UV transparency of these lakes can be used as a sentinel of environmental change.

Sublethal Exposure to UV Radiation Affects Respiration Rates of the Freshwater Cladoceran Daphnia catawba

Abstract We examined the effects of UV radiation (UVR) on metabolic rates of the freshwater cladoceran Daphnia catawba. We exposed D. catawba to UVB for 12 h in a lamp phototron at levels of 2.08 and 4.16 kJ m−2 both with and without concomitant exposure to UVA and visible photorepair radiation (PRR). We also included a group that received PRR only and a dark control group. Respiration rates were measured for 6 h following exposure. Respiration rates increased by 31.8% relative to the dark control at the lowest level of UVB stress (2.08 kJ m−2 UVB with PRR), whereas respiration was inhibited by 70.3% at the highest stress level (4.16 kJ m−2 UVB without PRR). Survival rates in the group that received PRR only and the group exposed to 2.08 kJ m−2 and PRR were not significantly different from that in the control group; however, the survival rate was reduced for all other UVR exposures. We hypothesize that enhanced respiration rates reflect energetic costs related to repair of cellular components damaged by sublethal levels of UVR. Increases in respiration rate of the magnitude we found in our experiment could significantly reduce energetic reserves available for growth and reproduction, especially in cases where these costs are incurred repeatedly during a series of days with high levels of UVR.

Implications of climate change for Daphnia in alpine lakes: predictions from long-term dynamics, spatial distribution, and a short-term experiment

Alpine lakes may be particularly useful as sentinels of climate change because they are highly sensitive to environmental conditions. To explore the potential biotic consequences of climate change in these systems, we conducted paleo- and neoecological observational studies, as well as a short-term experiment to examine Daphnia responses to changing environmental conditions in Rocky Mountain alpine lakes. Our analysis of a sediment core from Emerald Lake representing two periods from the Holocene revealed a significant positive relationship between the abundance of Daphnia remains and fossil Aulacoseira lirata, a diatom associated with deeper mixing depths. In addition, we detected a significant increase in mean Daphnia density in the long-term record (1991–2005) from Pipit Lake, a trend that correlated well with increases in mean surface temperature. In our survey of Daphnia in 10 lakes in the Canadian Rocky Mountains, Daphnia abundance was positively correlated with both dissolved organic carbon concentration and temperature. Finally, our short-term incubation experiment demonstrated significant effects of physical conditions (i.e., temperature and/or UV radiation) and water chemistry on the juvenile growth rate of Daphnia. Overall, our findings highlight the sensitivity of Daphnia to changes in mixing depth, water temperature, and dissolved organic matter, three limnological variables that are highly sensitive to changes not only in air temperature, but also to precipitation and location of the treeline in alpine catchments. Thus, we conclude that Daphnia abundance could serve as a powerful sentinel response to climate change in alpine lakes of the Rocky Mountains.

Light attenuation characteristics of glacially-fed lakes

Transparency is a fundamental characteristic of aquatic ecosystems and is highly responsive to changes in climate and land use. The transparency of glacially-fed lakes may be a particularly sensitive sentinel characteristic of these changes. However, little is known about the relative contributions of glacial flour versus other factors affecting light attenuation in these lakes. We sampled 18 glacially-fed lakes in Chile, New Zealand, and the U.S. and Canadian Rocky Mountains to characterize how dissolved absorption, algal biomass (approximated by chlorophyll a), water, and glacial flour contributed to attenuation of ultraviolet radiation (UVR) and photosynthetically active radiation (PAR, 400–700 nm). Variation in attenuation across lakes was related to turbidity, which we used as a proxy for the concentration of glacial flour. Turbidity-specific diffuse attenuation coefficients increased with decreasing wavelength and distance from glaciers. Regional differences in turbidity-specific diffuse attenuation coefficients were observed in short UVR wavelengths (305 and 320 nm) but not at longer UVR wavelengths (380 nm) or PAR. Dissolved absorption coefficients, which are closely correlated with diffuse attenuation coefficients in most non-glacially-fed lakes, represented only about one quarter of diffuse attenuation coefficients in study lakes here, whereas glacial flour contributed about two thirds across UVR and PAR. Understanding the optical characteristics of substances that regulate light attenuation in glacially-fed lakes will help elucidate the signals that these systems provide of broader environmental changes and forecast the effects of climate change on these aquatic ecosystems.

Ultraviolet Radiation and Daphnia Respiration in Context: The Facts

In their letter, Lucia Fidhiany and Klaus Winckler (1) raise several concerns with our recent article (2) in Photochemistry and Photobiology. Several of the comments questioned our treatment of the literature, and others challenged our experimental design and results. We strongly disagree with these concerns for the reasons we outline below and remain confident in the conclusions of our study. We do not agree with Fidhiany and Winckler’s assertion that an extensive review of the last 80–200 years of literature on respiratory responses to various types of stressors is necessary to provide proper context for the research results we report. In the introduction to our paper we chose to cite several recent studies in which other authors used oximetry to document metabolic responses of freshwater fish to sublethal UV radiation (UVR) stress. In the first paragraph of the introduction, we state ‘‘In addition to studies that have shown behavioral changes associated with UVR exposure, several recent studies have documented a metabolic response to UV exposure (3– 4). However, the nature of the metabolic response to UVR is inconsistent among studies. Respiration rates of juvenile rainbow trout increase with UV exposure (3), whereas maximum routine respiration rates and metabolic scope of vendace and whitefish larvae decrease in response to UVR (4).’’ These specific examples were chosen for two reasons: (1) the experimental protocols of these studies are parallel to our own because they involve manipulation of UVB radiation and (2) they reinforced the point that respiratory responses to sublethal doses of UVR are inconsistent. Fidhiany and Winckler also cautioned against ‘‘using second-level literature for citation.’’ In our paper, 17 of 20 citations are from the primary literature. The three citations from the secondary literature included two chapters from highly regarded edited books (5–6) to support broad statements in the introduction about general effects of UVR on aquatic organisms and a reference to the World Meteorological Organization report (7) on the international UV index scale. Fidhiany and Winckler’s assertions that our paper lacked an adequate treatment of previous studies on UVR effects on Daphnia and was missing appropriate references about effects of environmental stress on general Daphnia metabolism are unfounded. In the introduction and discussion, we included references to previous studies on Daphnia responses to UVR, including both laboratory and field studies (8–14). In addition, we make a clear statement in the introduction that our work is an extension of a larger literature on UVR and Daphnia by stating ‘‘Most research has focused on the effects of UVR on the survival of D. catawba and, consequently, UVR exposure levels have been relatively high (e.g. .26 kJ m 2 UVR) (11). In this study, we extend these previous studies by exposing D. catawba to lower levels of UVR and monitoring respiration and survival rates.’’ The list of references that Fidhiany and Winckler cite in their letter includes a wealth of excellent information about Daphnia; however, we do not believe that specific reference needs be made to these papers in order to correctly interpret our results. In particular, none of the listed papers address the effects of sublethal levels of UVR exposure on Daphnia metabolism. Furthermore, some of the papers have only a peripheral relationship to our work. For example, the work by Scott et al. (15) on effects of UVB irradiated food on survival and fecundity of Daphnia pulex is not directly relevant to our work because it was the Daphnia, not their food source, that were differentially irradiated in our experiment. Similarly, we did not consider the role of hemoglobin in our discussion because previous studies indicate that lacustrine Daphnia species do not synthesize hemoglobin under well-oxygenated conditions such as those in our oligotrophic study site and our experiment (16,17). We recognize the valuable contributions of Fidhiany and Winckler to the field of UVR research, in particular their previous work on sublethal effects of UVA on the metabolism of the convict cichlid fish (18,19). However, we disagree strongly with their statement that we have ignored the effects of UVA on Daphnia respiration. Our experimental design explicitly included a photorepair radiation treatment (referred to as PRR) to examine effects of UVA and photosynthetically available radiation (PAR) on Daphnia respiration. We found that respiration rates in the PRR treatment did not differ from the dark control, indicating that neither UVA nor PAR was driving the significant metabolic responses that we observed in other treatments in our experiment. The first paragraph of our discussion addresses the lack of a UVA effect in our results in detail. In addition to the behavioral mechanisms that we review, it is possible that UVA could affect Daphnia respiration through other pathways such as UVA-induced DNA damage or oxidative stress (20,21). However, our results are not consistent with these hypothesized UVA effects. Rather, the

Macroecological drivers of zooplankton communities across the mountains of western North America

Disentangling the environmental and spatial drivers of biological communities across large scales increasingly challenges modern ecology in a rapidly changing world. Here, we investigate the hierarchical and trait-based organization of regional and local factors of zooplankton communities at a macroscale of 1240 mountain lakes and ponds spanning western North America (California, USA, to Yukon Territory, Canada). Variation partitioning was used to test the hypothesized importance of climate, connectivity, catchment features, and exotic sportfish to zooplankton betadiversity in the context of key functional traits (body size and reproductive dispersal potential) given the pronounced environmental heterogeneity (e.g. thermal gradients), topographic barriers, and legacy of stocked fish in mountainous regions. Dispersal limitation was inferred from multispecies patch connectivity estimates based on nearest and average distances to occupied patches. Environmental heterogeneity best explained community composition as catchment/lake features (morphometry, land cover, and lithology) collectively captured greater variation than did climate (temperature, precipitation, and solar radiation), local stocking, or connectivity; however, single climatic variables captured the most variation individually. Macrospatial variation by larger obligate sexual species was better explained than that by smaller cyclically parthenogenetic asexual species. Our results provide several novel insights into the macroecology of zooplankton of the North American Cordillera, demonstrating their stronger associations to climatically driven aquatic-terrestrial habitat coupling than dynamics arising from introduced salmonids, human land-use, or species dispersal. These findings highlight the clear and important role of these communities as bioindicators of the limnological impacts of accelerating rates of climate change, as their responses appear relatively not confounded by local human perturbations or dispersal limitation.