Key Findings

Long term changes in zooplankton abundance and biovolume were documented prior to the funding of NES LTER. The distributions of many fish species in the Mid-Atlantic Bight are shifting northward in the warming ocean. Dominant species of zooplanktivorous forage fishes have interannual, seasonal, and species-specific diet preferences. It remains unresolved how decadal changes in zooplankton influence this higher trophic level.

Co-authors Rob Olson and Heidi Sosik have worked on development of the FlowCytobot series of automated submersible plankton sensors at WHOI since the late 1990s
Credit: Tom Kleindinst

Thanks to automated imaging approaches developed by NES LTER researchers, unprecedented insight has been gained into variations in microzooplankton biomass and diversity across a broad range of space and time scales. In addition, studies in Narragansett Bay documented strong microzooplankton grazing pressure on phytoplankton throughout the year, irrespective of season.

Co-author Alexi Shalapyonok secures FlowCytobot (FCB) underwater at the Martha’s Vineyard Coastal Observatory (MVCO) tower. FCB, an automated submersible flow cytometer, operates unattended at MVCO for 6 months or longer, measuring thousands of individual microscopic plankton every hour.
Credit: Sean Whelan

Phytoplankton bloom dynamics at Martha’s Vineyard Coastal Observatory (MVCO) are sensitive to temperature variability on both seasonal and decadal scales. Multi-year sampling has shown that the genetic background of phytoplankton is diverse and changes rapidly in coastal shelf waters. Ongoing NES LTER observations emphasize the complementary nature of multiple approaches (sequencing, imaging, and flow cytometry) to better document and understand changes in plankton diversity and how it impacts the ecosystem.

We are directly testing how climate mean and variance interacts by building the first field experiments that cross long-term drought with increased rainfall variability.
Credit: Jennifer Rudgers

Pulse-reserve theory has been a dominant conceptual framework for drylands since the 1970s. Detailed long term observations and experiments at the SEV LTER revealed that individual rainfall pulses rarely produce significant reserves and that many ecosystem processes do not “pulse” on the same time scales. SEV LTER researchers have improved pulse-reserve theory with the Threshold Delay Nutrient Dynamics model, which incorporated microbial processes.

We characterized the sensitivity of biological soil crusts to drought (drought plot in photo) in dry grassland ecosystems (Fernandes et al. 2018). We also documented plant evolutionary responses to drought (Whitney et al. 2019)
Credit: Lauren Baur

The climate of SEV LTER ecosystems has become drier and more variable during the past 100 years. SEV LTER research is gaining new insight into the biological consequences of these dual climate changes. For instance, increased climate variability has benefitted desert grassland during dry periods but reduced its productivity in wet periods, while plains grassland has been more sensitive to variability during droughts.

Researchers at SEV LTER led efforts to characterize fungi and bacteria in drylands and document their responses to environmental change. SEV LTER pioneered new assays of microbial function, including carbon use efficiency and ecoenzymatic stoichiometry. They quantified how microbes in roots maintain plant species coexistence and temporal stability in plant communities and how biological soil crusts affect community and ecosystem dynamics.

Field technician Samuel Matta collecting water samples atop a climate tower in lower elevation forest
Credit: Monique Picon

Luquillo LTER uses an elevation gradient as a proxy for studying certain aspects of climate change. High elevation cloud forests on mountain summits harbor many endemic species likely to be threatened by the changes in precipitation and temperature projected to impact these areas within 20 years. Recording changes in biota and critical ecosystem function along the elevational gradient through the year 2100 will capture key aspects of the changing climate and disturbance regime.

Student Celimar Rodriguez working on a seedling drought experiment
Credit: Jess Zimmerman

Drought in tropical wet forest alters greenhouse gas production by soils, affects key nutrient dynamics, and reduces forest productivity. Downscaling studies at LUQ LTER support global models that predict declining precipitation through the end of the century. Current ecosystem drying and warming model projections predict that net forest ecosystem productivity may fall to zero by 2036. A long term streamflow reduction experiment will determine impacts of long term drought on stream functioning.

Luquilla field station, El Verde, after Hurricane Irma (just a few weeks before Hurricane Maria)
Credit: Melissa Salva

Luquilla field station, El Verde, after Hurricane María
Credit: Melissa Salva

Luquilla field station, El Verde, in 2020
Credit: Melissa Salva

The long term Canopy Trimming Experiment revealed many important aspects of hurricane disturbance, particularly that canopy opening caused more change in biota and biogeochemistry than debris deposition. More frequent disturbance led to canopy opening but less debris deposition, and changed forest species composition, which may alter resilience in the face of future disturbances. Frequent hurricane disturbance causes forest ecosystems to retain less carbon and export more nutrients.

Credit: Andrew Robison

Despite expanded suburban development, nitrogen fluxes to the estuary have remained steady since the early 1990s. River flow, which is becoming more variable along with climate, largely determines nitrogen retention. Imbalances between nutrient supply and demand reduce nutrient regulation during higher flows. Work at PIE LTER helped lead to a generalized framework for modeling material fluxes at river network scales – the River Network Saturation framework. Knowing when and where river networks become saturated for different constituents allows scientists and managers to better extrapolate to broader spatial scales, clarify the role of rivers in continental element cycles, and identify policy priorities.

Credit: Ashley N. Bulseco et al

A decade of nutrient enrichment significantly increased rates of oxygen uptake and nitrate reduction in sediment. Surprisingly, the proportion of the dormant microbial population increased (overall composition of the microbial community remained unchanged). This response to a perturbation may reflect the microbial community’s strategy for maintaining diversity in a highly dynamic environment.

For marshes where rates of sea level rise exceed about 3 mm/year, external sediment supply is critical to marsh survival. Although riverine sediment inputs to the Great Marsh are low, PIE LTER research has shown that marsh edge erosion during moderate intensity storms currently supplies enough sediment to maintain the marsh platform. However, with accelerating sea level rise, this will not be the case. Landscape scale studies of spatial and temporal changes (rather than relying on point measurements of platform accretion) provide more reliable information and allow better predictions to be made about future changes. Plum Island LTER is developing GIS methods to make more statistically robust comparisons between historical and current maps.

Long-term nutrient enrichment causes S. alterniflora habitat to collapse into the creek, limiting access to high-marsh invertebrates. Fish use more energy to access the high marsh which reduces trophic efficiency. In addition, these cracks also create crevasses where fish predators, such as American eel, can hide and feed on mummichog.
Courtesy of the Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/symbols/).
Credit: Figure 7 in: Nelson, J.A., Johnson, D.S., Deegan, L.A. et al. Feedbacks Between Nutrient Enrichment and Geomorphology Alter Bottom-Up Control on Food Webs. Ecosystems 22, 229–242 (2019). https://doi.org/10.1007/s10021-018-0265-x

For the first six years of an ongoing 13-year nitrate addition experiment in tidal creeks, benthic algae, invertebrate prey, and a small fish, the mummichog, showed a classic positive bottom-up response to added nutrients. However, after six years, creek banks began to collapse and mummichog abundance in fertilized creeks declined relative to reference sites, likely because the changing shape of creek channels cut off access to food resources on the marsh platform. Amphipods in fertilized creeks also developed a much higher incidence of trematode parasites, which made them more vulnerable to predation.

Konza Prairie Biological Station features a replicated watershed-scale experiment with contrasting fire frequency and grazing treatments. Fire frequency affects plant composition and ecosystem state (i.e. whether an ecosystem is grassland, shrubland, or woodland). Fire also affects nutritional quality and quantity of vegetation, which influences foraging decisions by large herbivores at multiple scales. Herbivore choices cascade to impact grassland biodiversity via changes in dominance, a mechanism which KNZ LTER researchers found to be consistent with grasslands worldwide.

Credit: Barb Van Slyke

Climate change forecasts for mesic grasslands include increased climate variability and extremes. Experimental climate manipulations at Konza Prairie reveal a spectrum of responses to climate change, ranging from a lack of resistance to extreme drought, to great resilience to increased precipitation and heat wave variability. Although community composition changes with climate extremes, tallgrass prairie resilience is promoted by compensatory responses by dominant plant species.

Experiments at KNZ LTER have identified significant time lags between treatment initiation and sustained community effects. At a minimum, these times lags are 3-6 years for water and nutrient manipulations, but can be decades according to fire suppression and woody plant expansion studies. Decreases in plant diversity evident in the first few years after water and nutrient enrichment did not necessarily persist long term due to stochastic influences on community assembly. In streams, communities reassembled and ecosystem processes recovered over weeks to months following flood or drought. These observations represent a paradigm shift in understanding grassland assembly and spatial and temporal responses to changing external drivers.

Decades of research based on carbon stable isotope analyses supported the idea that macroalgal detritus, especially that of kelp, is a major source of food to coastal marine ecosystems, particularly suspension feeders. Comparative and experimental research from SBC LTER has overturned this paradigm, showing that phytoplankton, not kelp, are the main food resource for coastal benthic suspension feeders.

Results from long term measurements and experiments reveal that climate-driven disturbances that alter giant kelp abundance cascade through the kelp forest community, affecting biodiversity and ecosystem function. These effects are due to kelp’s overwhelming influence on environmental conditions and habitat availability rather than its effects as a food source for fauna.

Fire and land use affect the amount and timing of nutrient organic matter and sediment delivery from watersheds to the ocean. Drought and fire followed by rain causes large fluxes of terrestrial nutrients to the coastal ocean. During storms, runoff plumes containing high concentrations of nutrients remain close to the coast, but are advected offshore and quickly diluted once the storms pass, thereby reducing the contribution of land-derived nutrients to the productivity of coastal ecosystems.

Credit: Sarah Sampson

A marine heat wave of extreme magnitude and duration in 2014-15 allowed SBC LTER researchers to test predictions about the effects of climate change on kelp forests. Although kelp was diminished by the prolonged high temperature and low nitrate conditions, it rebounded quickly, and most other flora and fauna were not greatly affected. Ocean sampling revealed that ammonium and urea persisted during warm periods and experiments showed that kelp can use these recycled nitrogen sources.

A recent shift in the Alaska fire regime has increased the frequency and size of high-severity fires that combust deep organic layers, exposing mineral soil and favoring the invasion of hardwoods. Within the BNZ LTER Regional Site Network, many sites that were dominated by black spruce prior to burning in 2004 have now converted to dense birch stands with very different vegetation and ecosystem characteristics (BD1 site).
Credit: Roger Ruess

Severe late summer fires consume the soil organic layer, allowing deciduous tree species, such as aspen and birch, to establish at high densities. The fast decomposing litter and rapid evapotranspiration of deciduous trees maintain a thinner, drier organic layer that does not sustain spruce forests or insulate permafrost. This ecosystem state change alters an iconic Alaskan ecosystem by modifying productivity and carbon storage, climate regulation, and other ecosystem services to society.

Increases in air temperatures and depth of insulating snow cover during winter are accelerating the rate of permafrost thaw. In areas of ice-rich permafrost, widespread thermokarst is having significant consequences for biogeochemical cycling and surface hydrology.
Credit: Ted Schuur

Measurements across latitudinal gradients, field experiments, and laboratory incubations all point to significant releases of CO2 and CH4 from soils that have been frozen or waterlogged since the last ice age. Over decadal time scales, this carbon release could overwhelm increased plant carbon uptake. In a warmer world, the boreal forest could be transformed into a major carbon emitter, putting the forest on par with global land use change.

Climate-related changes in environmental conditions are challenging rural residents’ ability to traverse the land and access subsistence resources.
Credit: Miki Collins

Local residents observe that warming has changed the timing of freeze up, affected river ice thickness and melt, and has reduced winter travel safety and access to local ecosystem services. Wildfire reduces access to the land, threatens cultural and historic sites, and reduces wildlife densities for one to several decades (e.g. moose and caribou, respectively). Sources of resilience range from oral traditions and cooperative harvesting strategies to new technologies and network sharing.

Browsing by snowshoe hares and moose have dramatic top-down influences on vegetation composition, successional dynamics, and ecosystem function throughout interior Alaska.
Credit: Knut Kielland

Browsing by moose and snowshoe hares affects plant species composition, growth, population dynamics, nutrient cycling, and ecosystem function at both stand and landscape scales, causing effects that can persist for decades. Both species selectively consume willows, leading to the dominance of alder, an important nitrogen-fixing species that is chemically defended against herbivory. Snowshoe hare abundance varies nearly as much on an intra-annual basis as it does across a decadal population cycle, underscoring the complex interaction of biophysical factors. This in turn influences predation intensity and the population abundance of lynx, which is largely controlled by emigration and immigration.

Recent models indicate that lengthening of the snow-free season and associated declines in surface albedo constitutes the largest positive feedback to climate warming in the boreal forest.
Credit: Roger Ruess

Models that assess climate feedbacks over the next century have simulated decreases in albedo due to a shorter snow season, wider extents of deciduous forest due to altered fire regimes, and changes in climate and atmospheric CO2 and CH4 emissions. The strongest climate feedback was positive, derived from lengthening the growing season (reducing the snow-albedo feedback). Increases in young, faster-growing deciduous forests and a net increase in carbon uptake by terrestrial ecosystems only partially counterbalanced this change.