CCE-LTER: A Focus on Ecosystem Transitions
Overview: Research at the CCE-LTER site focuses on mechanisms leading to transitions over time between different states of the pelagic ecosystem. Observations from the remarkable California Cooperative Oceanic Fisheries Investigations (CalCOFI) coastal ocean time series — now in its 7th decade of sampling — demonstrate the effects of external factors in forcing alterations to this ecosystem on multiple time scales. These factors include 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, and the year-to-year temperature fluctuations dominated by El Niño. Combinations of these processes, together with interactions among living organisms, can lead to ecosystem responses that may be manifested as relatively abrupt transitions.
The CCE site is evaluating four hypothesized mechanisms that could explain these kinds of rapid ecosystem transitions:
- Localized food web changes in response to changes in ocean temperature, vertical stratification, and nutrient supply
- Alongshore transport of different assemblages of organisms
- Changes in cross-shore transport and loss/retention of organisms
- Altered predation pressure
Interdisciplinary Research Approach
have initially focused on the hypothesis of localized food web changes in response to changes in water column stratification. Here we use space as a substitute for time, since many of the temporal changes that are observed in this region have clear spatial analogs. For example, the nitracline depth (depth where nitrate first exceeds 1µM) deepened dramatically during the 1997-98 El Niño, after which it returned to a shallower average depth. At a single point in time we find spatial variations in nitracline depth within our LTER region that encompass these temporal variations. Variations in nitracline depths over this range are associated with changes in composition of the food web's primary producers, in this case tiny unicellular cyanobacteria, which show highest abundances at intermediate nitracline depths. We exploit such spatial differences to develop continuous functions that describe growth and loss rates of different members of the plankton assemblage in relation to nitracline depth. These functions are being used in our coupled bio-physical models to simulate the ecosystem effects of changes in nitracline depth over time.
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).
are an integral part of the CCE site. Computer models 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. Three 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); models that explore interactions between organisms in a pelagic food web; and control volume property flux models. Control volume models enable us to estimate net fluxes of properties such as heat, salt, nutrients, oxygen and phytoplankton biomass through the 3D box defined by the stations and the coast, by assuming that the convergence of mass into the box created by horizontal currents is balanced by upwelling-related divergence of mass out of the box, and solving for the net flux.