CCE-LTER : A Focus on Processes Causing Ecosystem Transitions
Overview: The California Current System is a coastal upwelling biome, as found along the eastern margins of all major ocean basins. These coastal upwelling biomes are among the most productive coastal ecosystems in the world ocean. The California Current sustains active fisheries for a variety of finfish and marine invertebrates, modulates weather patterns and the hydrologic cycle of much of the western United States, and plays a vital role in the economy of myriad coastal communities. Observations from the remarkable California Cooperative Oceanic Fisheries Investigations (CalCOFI) coastal ocean time series demonstrate the effects of external factors in forcing alterations to this ecosystem on multiple time scales, including:
- a warming trend that has been documented over the past 6 decades,
- the long term warming and cooling cycles (ca. 20-30 years) represented by the Pacific Decadal Oscillation,
- year-to-year temperature fluctuations dominated by El Niño,
- and marine heatwaves that can form locally or act across the Pacific Ocean.
Combinations of these processes, together with interactions among living organisms, can lead to ecosystem responses that may be manifested as relatively abrupt transitions. Research at the CCE-LTER site focuses on mechanisms leading to transitions over time between different states of the pelagic ecosystem.
Questions and Hypotheses
El Niño, the Pacific Decadal Oscillation, extratropical marine heatwaves and a secular warming trend are known to alter the structure and dynamics of the CCE, leading to the following central questions:
- What are the mechanisms leading to different ecosystem states in a coastal pelagic ecosystem?
- What is the interplay between changing ocean climate, community structure and ecosystem dynamics?
We focus on four principal hypotheses generating changes in this ecosystem:
- In situ food web changes in response to altered vertical stratification and nutrient supply
- Alongshore advection of different assemblages
- Changes in cross-shore transport and loss/retention of organisms
- Altered predation pressure
Interdisciplinary Research Approach
Our site addresses these hypotheses with an integrated research program having three primary elements:
1. Experimental Process Studies allow us to conduct focused experiments to test hypotheses arising from our long-term datasets. During our Process studies, we often employ a Lagrangian sampling scheme that allows us to follow communities of organisms as they are transported by the ocean currents. This allows us time for detailed examination of drifting plankton assemblages. In Phase I we used a space-for-time substitution approach and exploited the strong spatial variability in our study region to investigate how bottom-up changes in local nutrient supply impact food-web structure. In Phase II, we further investigated physical-biological coupling at mesoscale fronts – regions where different water masses meet – to understand how changes in the frequency of such fronts may disproportionately affect organisms. Phase III saw an evolution of our approach as we investigated cross-shore filaments – rivers of nutrient- and plankton-rich water moving from the coast to offshore – and their role in cross-shore transport of living and non-living components of the ecosystem.
More recently, we are using our Process studies to investigate the impacts of marine heatwaves on both short time-scales (through manipulative experiments that investigate organismal responses) and longer time-scales (using a space-for-time substitution approach to investigate community changes). We are also using state-of-the-art autonomous imaging approaches to investigate the fine-scale distributions of organisms to investigate top-down forcing in the ecosystem. A common theme through all of our Process studies has been a focus on measuring ecosystem rates to elucidate the drivers of processes like photosynthesis and grazing and develop functions to predict these rates. These functions are then used in our coupled bio-physical models to simulate the ecosystem over larger spatiotemporal scales.
2. Time Series Studies evaluate our alternative hypotheses, using time series measurements from a variety of CCE LTER research stations. These measurements include (a) a quarterly measurement program at sea that capitalizes on and significantly enhances the CalCOFI time series by also assessing the microbial and microplankton communities, and dissolved and particulate organic matter; (b) satellite remote sensing observations, including phytoplankton pigments and sea surface temperature; (c) Spray ocean glider transects across the California Current at two locations (website); (d) continuous observations at interdisciplinary ocean moorings (website); and (e) frequent temporal measurements at different nearshore locations through collaborations with coastal observing systems. These collaborations include the Scripps pier time series (website), the SCCOOS (Southern California Coastal Ocean Observing System) program (website), and our Education and Outreach partner, the Ocean Institute in Dana Point (data). Our time series span from ocean physics and chemistry to population studies of trophic levels from phytoplankton to fish, seabirds, and whales. We utilize both traditional approaches – e.g., measurements of chlorophyll – that link to the 75+-year CalCOFI time-series and advanced “-omics” and high-resolution optical imaging approaches that allow us to document long-term trends at high taxonomic resolution.
3. Modeling Studies are used to help interpret and understand the dynamics underlying observations; to provide a platform for hypothesis testing through numerical experiments; and to provide a means for dynamic interpolation between observations in space and time. Four different types of models are employed:
- coupled 4-D, eddy-resolving bio-physical models of the California Current ecosystem based on ROMS (the Regional Ocean Modeling System);
- mass-balance models that quantify energy and mass transfer between different trophic levels and abiotic reservoirs,
- statistical models that explore interactions between organisms in a pelagic food web;
- and control volume property flux models that enable us to estimate net fluxes of properties such as heat, salt, nutrients, oxygen and phytoplankton biomass
4. Information Management that provides information and data management services to make well-documented research data publicly available
5. An Education, Outreach and Capacity Building program that communicates the scientific process, as well as scientific understanding, of CCE to a K-through-grey audience.
Phase IV Focus
While the overarching goals of our site have not changed, our research foci have evolved over time as our understanding of the ecosystem has grown and the ecosystem itself has experienced novel changes. In Phase IV, we are especially focused on (1) investigation of marine heatwaves and resultant multiple stressors on organisms and communities, (2) elucidation of ecological stoichiometry and the response of multiple trophic levels to altered elemental ratios of source nutrients, and (3) expanded analysis of top-down pressures mediated by a diverse suite of organisms.
Our focus on marine heatwaves is a direct response to a distinct disturbance to the ecosystem first noted during the 2014-2015 “Blob” North Pacific marine heatwaves. Although surface warming in the CCE associated with the “Blob” was superficially similar to El Ninos and many taxa experienced commensurate shifts, the physical drivers of the “Blob” were distinct, the vertical structure of the warming was quite different, and the mechanisms driving changes in communities appeared to be different. Our research is investigating the impacts of these marine heatwaves, with an emphasis on understanding how more frequent and intense heatwaves may alter the ecosystem in a multi-stressor framework.
A greater focus on ecological stoichiometry – the ratios of different nutrients in the environment or elements and molecules within organisms – grew out of previous CCE research and ecological theory. Our research has shown distinct patterns of light, nitrogen, and iron limitation of phytoplankton in different regions and depths within the ecosystem. Theory suggests that these shifts may reverberate across trophic levels – and we hope to test this theory through experimental manipulation, modeling studies, and analysis of archived time-series samples.
Our increased focus on top-down pressure in the ecosystem – one of our four hypothesized mechanisms of ecosystem change – results in part from CCE research elucidating the “conditional top-down hypothesis”. This hypothesis suggests that during normal conditions the ecosystem is primarily bottom-up, meaning that increases in lower trophic levels lead to increased abundances of organisms up the food chain and vice versa. However, during periods of low nutrient supply (e.g., El Nino) top-down ecosystem forcing becomes more important and we see an alternate pattern in which increased abundance of predators drives a decrease in prey abundances. Our renewed focus on this hypothesis is also driven by methodological advances that now allow us to investigate predator-prey covariance at the sub-meter scales that are relevant to planktonic organisms.