Holly A. Ewing

Cyanobacteria as biological drivers of lake nitrogen and phosphorus cycling

Here we draw attention to the potential for pelagic bloom-forming cyanobacteria to have substantial effects on nutrient cycling and ecosystem resilience across a wide range of lakes. Specifically, we hypothesize that cyanobacterial blooms can influence lake nutrient cycling, resilience, and regime shifts by tapping into pools of nitrogen (N) and phosphorus (P) not usually accessible to phytoplankton. The ability of many cyanobacterial taxa to fix dissolved N2 gas is a well-known potential source of N, but some taxa can also access pools of P in sediments and bottom waters. Both of these nutrients can be released to the water column via leakage or mortality, thereby increasing nutrient availability for other phytoplankton and microbes. Moreover, cyanobacterial blooms are not restricted to high nutrient (eutrophic) lakes: blooms also occur in lakes with low nutrient concentrations, suggesting that changes in nutrient cycling and ecosystem resilience mediated by cyanobacteria could affect lakes across a gradient of nutrient concentrations. We used a simple model of coupled N and P cycles to explore the effects of cyanobacteria on nutrient dynamics and resilience. Consistent with our hypothesis, parameters reflecting cyanobacterial modification of N and P cycling alter the number, location, and/or stability of model equilibria. In particular, the model demonstrates that blooms of cyanobacteria in low-nutrient conditions can facilitate a shift to the high-nutrient state by reducing the resilience of the low-nutrient state. This suggests that cyanobacterial blooms warrant attention as potential drivers of the transition from a low-nutrient, clear-water regime to a high-nutrient, turbid-water regime, a prediction of particular concern given that such blooms are reported to be increasing in many regions of the world due in part to global climate change.

Fog water and ecosystem function: heterogeneity in a California redwood forest.

Fog is thought to influence ecological function in coastal forests worldwide, yet few data are available that illuminate the mechanisms underlying this influence. In a California redwood forest we measured water and nitrogen (N) fluxes from horizontally moving fog and vertically delivered rain as well as redwood tree function. The spatial heterogeneity of water and N fluxes, water availability, tree water use, and canopy N processing varied greatly across seasons. Water and N fluxes to soil (annual average of 98% and 89%, respectively) across the whole forest occurred primarily in the rain season and was relatively even across the whole forest. In contrast, below-canopy flux of fog water and N declined exponentially from the windward edge to the forest interior. Following large fog events, soil moisture was greater at the windward edge than anywhere else in the forest. Physiological activity in redwoods reflected these differences in inputs across seasons: tree physiological responses did not vary spatially in the rain season, but in the fog season, water use was greater, yet water stress was less, in trees at the windward edge of the forest versus the interior. In both seasons, vertical passage through the forest changed the amount of water and form and concentration of N, revealing the role of the tree canopy in processing atmospheric inputs. Although total fog water inputs were comparatively small, they may have important ecosystem functions, including relief of canopy water stress and, where there is fog drip, functional coupling of above- and belowground processes.

Ecosystem Effects of a Tropical Cyclone on a Network of Lakes in Northeastern North America

Here we document the regional effects of Tropical Cyclone Irene on thermal structure and ecosystem metabolism in nine lakes and reservoirs in northeastern North America using a network of high-frequency, in situ, automated sensors. Thermal stability declined within hours in all systems following passage of Irene, and the magnitude of change was related to the volume of water falling on the lake and catchment relative to lake volume. Across systems, temperature change predicted the change in primary production, but changes in mixed-layer thickness did not affect metabolism. Instead, respiration became a driver of ecosystem metabolism that was decoupled from in-lake primary production, likely due to addition of terrestrially derived carbon. Regionally, energetic disturbance of thermal structure was shorter-lived than disturbance from inflows of terrestrial materials. Given predicted regional increases in intense rain events with climate change, the magnitude and longevity of ecological impacts of these storms will be greater in systems with large catchments relative to lake volume, particularly when significant material is available for transport from the catchment. This case illustrates the power of automated sensor networks and associated human networks in assessing both system response and the characteristics that mediate physical and ecological responses to extreme events.

Constructing a Broader and More Inclusive Value System in Science

ABSTRACT A scientific culture that welcomes a diversity of participants and addresses a broad range of questions is critical to the success of the scientific enterprise and essential for engaging the public in science. By favoring behaviors and practices that result in a narrow set of outcomes, our current scientific culture may lower the diversity of the scientific workforce, limit the range and relevance of scientific pursuits, and restrict the scope of interdisciplinary collaboration and public engagement. The scientific community will reach its full intellectual potential and secure public support through thorough, multitiered initiatives that aim to change individual and institutional behaviors, shift current reward structures to reflect a wider set of values, and explicitly consider societal benefits in the establishment of research agendas. We discuss some shortcomings and costs of the current value system and provide some guidelines for the development of initiatives that transcend such limitations.

Spatial and temporal variability in recruitment of the cyanobacterium Gloeotrichia echinulata in an oligotrophic lake

Abstract Recruitment from dormant stages in the benthos can provide a critically important inoculum for surface populations of phytoplankton, including bloom-forming cyanobacteria. For example, water-column populations of the large (1–3-mm diameter) colonial cyanobacterium Gloeotrichia echinulata (Smith) P. Richter can be strongly subsidized by benthic recruitment. Therefore, understanding controls on recruitment is essential to an investigation of the factors controlling Gloeotrichia blooms, which are increasing in low-nutrient lakes across northeastern North America. We quantified surface abundances and recruitment from littoral sediments at multiple near-shore sampling sites in oligotrophic Lake Sunapee, New Hampshire, USA, during the summers of 2005–2012 and used this data set—the longest known record of cyanobacterial recruitment—to investigate potential drivers of interannual differences in Gloeotrichia recruitment. We found extensive spatiotemporal variability in recruitment. Recruitment was higher at some sites than others, and within seasons, recruitment into replicate traps at the same site was generally more similar than recruitment at different sites. These data suggest that local factors, such as substrate quality or the size of the seed bank, may be important controls on recruitment. Benthic recruitment probably accounted for <4%, but possibly up to 8%, of pelagic populations, within the range observed in previous studies. Across years, higher seasonal recruitment rates were associated with greater lake mixing during August, including deeper thermoclines, lower Schmidt stability, lower minimum air temperatures, and greater variability in water temperature. Taken together, our data suggest that interannual variation in Gloeotrichia recruitment may be related to regional climatic variability.

Fog as a source of nitrogen for redwood trees: evidence from fluxes and stable isotopes

Author(s): Templer, PH; Weathers, KC; Ewing, HA; Dawson, TE; Mambelli, S; Lindsey, AM; Webb, J; Boukili, VK; Firestone, MK | Abstract:

Earthworms Reduce Biotic 15-Nitrogen Retention in Northern Hardwood Forests

Invasive exotic earthworms are significantly influencing understory community composition, soil, and ecosystem processes in northern hardwood forests in North America, but their effect on the retention of nitrogen (N) has been inconclusive. We examined this in two northern hardwood forest sites in New York state, USA through a tracer study. In both spring and fall, we added tracer amounts of 15N as nitrate—to simulate atmospheric deposition—with the biologically less active tracer bromide (Br−) to areas both with and without large populations of invasive earthworms. Total recovery of 15N was lower in earthworm-invaded plots, largely due to less retention in litter and upper soil horizons. Although the strong relationship between retention in the upper soil horizons and total 15N recovery suggests that earthworm destruction of the forest floor may be one mechanism reducing the capacity for N retention, in some cases the mineral soil in earthworm-invaded plots retained substantial N. Biotic pools, particularly litter and microbial biomass, retained significantly less 15N in earthworm-invaded plots than in their uninvaded counterparts. In plots invaded by earthworms, negative effects of earthworms on trees were revealed through root-uptake assays suggesting somewhat greater plant demand for ammonium in the spring and in lower 15N recovery in maple seedlings the year following tracer addition. Although similar patterns of Br− movement across treatments suggested that earthworms had smaller effects on hydrologic tracer movement than expected, they appear to have significant effects on the biological processes that underlie N retention.

Acidic atmospheric deposition interacts with tree type and impacts the cryptogamic epiphytes in Acadia National Park, Maine, USA

Abstract We examined plot-level interactions among nitrogen and sulfur deposition, tree type, and epiphytic diversity, biomass, and abundance in Acadia National Park (ACAD), Maine, that receives pollutant deposition and is of particular concern for biological conservation. Spruce appeared to acidify incoming deposition resulting in more concentrated inputs of nitrogen and sulfur within the canopy as well as to the soil relative to maple. This resulted in more acidic and sulfur-rich substrates and reduced biomass and species richness for cryptogamic epiphytes on spruce relative to maple. Position vertically on the tree bole was more important for epiphytes on maples than on spruces because spruces were uniform in their bark chemistry. Apparent overlap between the bark chemistry of spruce and maple, particularly for samples from higher on maple boles, suggests a reduction in the area of chemically suitable substratum for epiphytes in ACAD. Because tree species vary markedly in habitat variables important to epiphytes, stands with a mix of deciduous and evergreen trees provide the widest gradient of habitats, which appears to be reflected in the highest species richness for epiphytes occurring in mixed stands. Air quality and forest health monitoring studies utilizing epiphytes as indicators should account for the complex effects of stand-level tree species composition to better detect biological effects of deposition at the landscape scale.

Grazers and soil moisture determine the fate of added 15NH4+ in Yellowstone grasslands.

The influence of ungulate grazers on nutrient cycling and ecosystem productivity in grasslands has been shown to differ with moisture, nutrient availability, and feedbacks between above- and belowground activities. We examined the movement of nitrogen (N), applied as (15NH4)2SO4, through both dry and mesic sites in the northern range of Yellowstone National Park to test the hypothesis that plants were more able to acquire added N in grazed relative to ungrazed sites. Previous studies showed enhanced N mineralization in grazed areas, and detritus removal by grazers was predicted to enhance early-season plant growth. Thirteen months after tracer addition, there were no differences in plant 15N as a function of grazing, but historically ungrazed sites retained more 15N in accumulated litter than at grazed sites. This result demonstrated the importance of detritus in regulating redistribution of incoming N and the role of grazers in this process. Site moisture status influenced 15N recovery in all pools—soils, microbial biomass, and plants—and greater plant 15N acquisition occurred in roots at dry relative to mesic sites. Understanding how grazers influence nutrient cycling at the landscape scale requires further investigation of interactions among soil moisture, plant production, litter accumulation, grazing intensity, and belowground processes.

The internal and watershed controls on hypolimnetic sediment phosphorus release in Lake Auburn, Maine, USA

ABSTRACT Doolittle HA, Norton SA, Bacon LC, Ewing HA, Amirbahman A. 2018. The internal and watershed controls on hypolimnetic sediment phosphorus release in Lake Auburn, Maine, USA. Lake Reserv Manage. 00:00–00. Lake Auburn, Maine, USA, is a historically oligotrophic lake serving as a drinking water source. In 2011–2012, epilimnetic total phosphorus (P) concentrations, turbidity, and cyanobacteria blooms increased. This water quality decline was linked to earlier onset, and increased extent, of late-summer hypolimnetic anoxia, causing internal P loading from lake sediment. This study evaluated chemical P speciation in hypolimnetic sediment, using the Psenner method for sequential chemical extractions, and watershed roles in controlling sediment P release. Surficial sediments (0–2 cm) are high in reducible Fe hydroxide (107–1267 µmol/g) and associated P (15–103 µmol/g), and relatively low in Al hydroxide (77–242 µmol/g). Previous research has shown sediment with molar Al:reducible Fe ratios <3 and Al hydroxide:reducible Fe-bound P < 25 to act as a source of P during anoxia. Lake Auburn surface sediment has molar Al hydroxide:reducible Fe hydroxide ratios between 0.2 and 1.7, and molar Al hydroxide:reducible Fe hydroxide-bound P ratios between 2.0 and 14.5, indicating high risk for internal P loading. Al concentration decreases downstream from catchment lakes and along Lake Auburn tributaries, while Fe export from upstream wetlands is high. The decrease in Al and proportionately greater increase in Fe flux to the lake and in-lake deposition of Al and Fe hydroxides are impacting sediment chemistry, increasing the lakes vulnerability to eutrophication following the onset of hypolimnetic anoxia. Understanding the significance of these critical sediment ratios and influences from watershed sources is imperative for lake managers dealing with systems having significant internal recycling of phosphorus.