Research

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.

 

 

Research Focus

The CCE site is evaluating four hypothesized mechanisms that could explain these kinds of rapid ecosystem transitions:

 

 

  1. Alongshore transport of different assemblages of organisms
  2. Localized food web changes in response to changes in ocean temperature, vertical stratification, and nutrient supply
  3. Changes in cross-shore transport and loss/retention of organisms
  4. Altered predation pressure

 

 

Interdisciplinary Research Approach

Our site addresses these hypotheses with an integrated research program having three primary elements:

 

Experimental Process Studies 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.

 

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).

 

Modeling Studies 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.

 

Project Hypothesis and Abstract

 

Nonlinear Transitions in the California Current Coastal Pelagic Ecosystem

We have created an LTER site focusing on the California Current pelagic Ecosystem (CCE). This new site builds on what has been learned from the unparalleled suite of coastal observations developed by CalCOFI (California Cooperative Oceanic Fisheries Investigations) since its inception in 1949, but moves far beyond that program. The CCE site focuses on the mechanisms leading to transitions between ecosystem states. The proposed research program is coordinated with three interwoven components:

 

  1. Experimental Process Studies focused on hypothesized mechanisms leading to system transitions
  2. Space-resolving Time Series that explore alternate hypotheses through characterization of ecosystem responses on time scales from hours to decades
  3. Integrated Modeling studies that synthesize experimental and observational results, provide a platform for hypothesis-testing and eventually ecosystem forecasting, and help to optimize the sampling program

 

The California Current System (CCS) is a coastal upwelling biome, as found along the eastern margins of all major ocean basins. These are among the most productive coastal ecosystems in the world ocean. The CCS 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. El Niño, the Pacific Decadal Oscillation, 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:

  1. In situ food web changes in response to altered vertical stratification and nutrient supply
  2. Alongshore advection of different assemblages
  3. Changes in cross-shore transport and loss/retention of organisms
  4. Altered predation pressure

 

We have developed research approaches that address these hypotheses, while concurrently addressing the five core research themes that are held in common across all LTER sites. We are also conducting comparative studies with sites having interests in alternate stable states, El Niño and lower-frequency climate forcing, and the role of top-down impacts on ecosystem dynamics. A long term goal for this site is to develop a mechanistic, coupled bio-physical model for understanding and forecasting the consequences of high-frequency (e.g., El Niño) and low-frequency climate forcing on pelagic ecosystems of the California Current and similar biomes.