PC-Mesoscale

Mesoscale Spatial Studies

       Goals and Objectives
       Relevance of Mesoscale Spatial Studies
       Questions and Hypotheses
       Methods

Interactions between coastal topography, winds and a seasonally intensified CCS jet produce a complex system of along- and cross-shelf circulation along the west coast of North America that includes mesoscale (10-100s of km) features whose locations, in some cases, may be predictable (e.g., regions of persistent offshore or onshore transport, creation of quasi-stable eddies, fronts). Although these features sometimes appear to be spatially predictable, the observed variability between seasons and years indicate that their exact location, intensity and vertical structure may be affected strongly by winds, buoyancy flux, and larger-scale currents. This dependence imposes temporal variability ranging from seasonal-to-interannual, which suggests that mesoscale features may be altered significantly by climate change. In Section IV the argument is presented that mesoscale activity may increase due to future greenhouse-gas mediated climate change. Among other properties, local temperature, current strength and stratification will depend on the nature and strength of these mesoscale features. Considerable population variability, and the evolution of life history strategies, is related to mesoscale physical features and processes (e.g., timing of spawning in relation to the timing of the spring transition). Therefore a suite of important biological properties and processes should be affected by and predictably linked to these wind-topography-current interactions. It is important to determine whether zooplankton (including ichthyo- and meroplankton) modify their behavior (e.g., vertical migration) in response to their location within the different types of mesoscale structure in ways that affect their transport and ultimate population persistence.

Mesoscale studies are important components of the large-scale comparisons described above. Given the U.S. GLOBEC focus on interactions between zooplankton and their environment, emphasis is placed on (1) processes involving transport, retention, aggregation, and vital rates as functions of location within mesoscale features, or at their boundaries; (2) timing of the appearance of life history stages with respect to seasonal mesoscale evolution; and (3) spatial variability in the distribution and abundance of species that settle or recruit in shallow water ("life cycle closure"). Spatial and temporal variability in fish recruitment and benthic organism settlement will be a key measurement in mesoscale studies. Mesoscale field studies are envisioned to include a broad spectrum of measurements and platforms, including moorings, fast surveys (electronic sensors on undulating vehicles), slower surveys (water-collecting CTDs and animal collections for abundance estimates and shipboard experiments), surface drifters and neutral floats, and land- and satellite-based remote sensing.

Goals and Objectives

The overall goal of the mesoscale spatial studies is to understand the extent to which the present spatial and temporal variability in mesoscale circulation in the CCS impacts the abundances, species composition, growth, mortality, genetic composition, transport and recruitment of holoplankton, meroplankton and ichthyoplankton, and to relate this present variability to differences in mesoscale circulation expected as a result of climate change.

Many specific relationships between planktonic organisms or populations and mesoscale features must be determined, including but not limited to:

The degree to which the amplitude, position and timing of mesoscale features may be altered by climate change.
The role of various mesoscale features in determining cross-shelf and along-shore transport of meroplankton, holoplankton and icthyoplankton.
The role of mesoscale features in driving active or passive aggregations and maintaining the spatial and temporal coherence or fidelity of populations.
The role of frontal dynamics in producing or maintaining gradients in vital rates, and the exchange between or genetic isolation of populations.
The role of mesoscale features in determining the links between plankton dynamics and the ultimate spatial and temporal variability in recruitment of fish and benthic organisms.
The extent to which zooplankton within mesoscale features use behavior to mediate their net transport.
Links between life histories of target species and the timing of seasonal changes in atmospheric forcing and mesoscale circulation features.

Relevance of Mesoscale Spatial Studies

The life histories of key marine populations are affected by mesoscale circulation, which, in turn, is controlled by processes that are highly susceptible to changes in large-scale forcing due to climate change. This may result in shifts in physical and biological boundaries, as well as changes in the major physical processes within a region (e.g., changes in the intensity and timing of upwelling, seasonality and intensity of mesoscale jets and eddies, degree of vertical mixing, etc.). Changes in mesoscale processes lead to changes in mesoscale features, resulting in changes in ecosystem dynamics. Understanding the mechanisms by which populations respond to present differences in forcing and circulation will allow us to predict how those same populations will respond to climatic changes in forcing.

Mesoscale circulation features may be altered strongly by interannual variations in winds, buoyancy flux and basin-scale circulation, especially during strong ENSO events, and should be affected significantly over longer time scales by climate change. Recent evidence (Huyer et al. 1991; Washburn et al. 1993) suggests that much of the advection within Region II of the CCS occurs in mesoscale features, which also may affect the local intensities of upwelling, downwelling, mixing and primary productivity. This evidence suggests that mesoscale features may be important to plankton dynamics, but this link has not been rigorously evaluated. The intensity and location of mesoscale circulation features should strongly impact the degree of cross-shelf transport as well. Consequently, these features should be important in the retention or loss of regional populations of zooplankton, fishes and benthic animals, and may play a significant role in determining their spatial and temporal variability, and therefore the maintenance of populations may be sensitive to changes in mesoscale circulation associated with a changing climate.

Questions and Hypotheses

Many of the issues raised in Section V will be addressed in the mesoscale studies. Specifically, the following general hypotheses are formulated:

Spatial and temporal variability in mesoscale circulation is determined by local wind forcing and basin-scale currents, both of which will be affected by climate change.
Mesoscale features significantly affect zooplankton biomass and production, and their retention or loss from coastal regions.
Variability of mesoscale dynamics in the CCS plays a critical role in the "life-cycle closure" of juvenile fish and larvae of benthic invertebrates, both by determining their survival at sea and the intensity of their recruitment.

Methods

Mesoscale studies at selected sites in the CCS will take place over several years and may include the following:

Collection of meteorological, physical and biological data by satellite and shore-based remote sensors, drifters and fixed moorings.
Repeated at-sea sampling of plankton, fishes, hydrographic and nutrient data. Both Eulerian-frame (fixed grid and feature oriented) sampling and Lagrangian-frame (following water parcels and/or individual populations) sampling will be employed.
Onshore and nearshore sampling to document the ultimate recruitment of benthic organisms and fish.
Interaction with the modeling activities to parameterize the rates and behaviors in a manner which allows their inclusion in models; comparisons of model-predicted and measured responses to variations in forcing.

Because of the striking differences among the various regions of the CCS in the patterns of wind (seasonality, direction and strength), coastal orography and nearshore bathymetry, it would be especially illuminating to carry out mesoscale studies in two or more regions simultaneously and contrast their results (e.g., Latitudinal Studies). Biological and physical oceanographic field measurements should be closely coordinated with modeling programs that address CCS circulation and biological responses to physical forcings and climatic fluctuations. Coordination will ensure that key parameters are identified, targeted for in situ study and measured with adequate spatial and temporal resolution.

Mesoscale spatial physical and biological oceanographic measurements are needed within two broad time frames. High horizontal and temporal resolution measurements will be made over several-week periods that define time scales of many mesoscale features. These measurements will permit resolution of important, shorter period events as well, such as diurnal migrations of plankton. This time frame also will ensure that significant progression of developmental stages of planktonic populations has been followed. These high resolution programs should be carried out two or more times during a season to encompass the staggered reproductive periods of the key fauna in the CCS. For example, planktonic stages of Dungeness crab (Cancer magister) and the red urchin (Strongylocentrotus franciscanus) typically occur during winter and early spring in the CCS, whereas development of many key zooplankton species, as well as certain fish and benthic invertebrates, are more closely tied to the onset of spring blooms. High resolution programs should occur in several consecutive years so that interannual variability in biological responses to key physical processes can be evaluated.

Superimposed upon these high-resolution measurement programs will be a series of coarser-resolution, fixed-point measurements (moored, shore-based, and monitoring cruise measurements) that will extend continuously over longer periods of time (multiple years). Winter and summer conditions may demand different fixed mooring locations or measurement technologies/strategies. These measurements are necessary to evaluate seasonal as well as interannual variability in key physical forcings in the CCS, as well as some basic biological responses on this scale of variability.

High-resolution measurements of biological oceanographic variables and processes should be coordinated closely with near real-time measurements of mesoscale features. Satellite data, including AVHRR and color images, or data acquired from aircraft fly-overs, could identify locations of particular mesoscale features and be used to track a feature's location and strength through time. These data will be used to guide the field sampling program that includes acquisition of diverse physical and biological oceanographic data. Ultimately, satellite or aircraft images should be related to measured fields of surface currents and wind fields obtained at high resolution in regions extending 10-20 km offshore, using shore-based Doppler radar and Lidar.

High-resolution field studies of biological responses to mesoscale features could include both fixed mooring and ship-based sampling. Moorings also might be an important component of the longer-term, continuous measurement program. Moorings located within predictable mesoscale features (e.g., regions of persistent offshore transport near headlands) could provide good temporal resolution measurements of (1) cross-shore variability in transport, and (2) key physical and biological variables. Measurements obtained from moorings should include temperature, salinity, ADCP currents, and bio-optical data (PAR, pigments). Vertical arrays of settling plates placed on separate moorings (so that they can be easily exchanged) could be used to measure short-term settlement patterns of some benthic invertebrates relative to other variables measured using moorings. Moored sediment traps can provide information on the temporal and spatial variability in the rate and composition of sinking material.

A key aspect of the ship-based mesoscale program is pairwise sampling of zooplankton populations within and outside of specific mesoscale features. This should be augmented by a more sophisticated sampling program wherein populations inside and outside of features are followed over time. To accomplish this, sampling should occur over time in the vicinity of drifters that are released inside and outside of specific features. This would permit documentation of the time rate of change of demographic and physiological properties of zooplankton populations as driven by sub-mesoscale features.

The use of Lagrangian drifters that can follow water at specified densities could provide a unique and important data set. In addition to tracing water movement, the drifters could be equipped with GPS receivers and internal data storage so that temperature and velocity measurements over short time scales can be acquired continuously within individual mesoscale features. Such Lagrangian-frame data can be especially useful in documenting the importance of various frontal and mesoscale features to the retention or loss of populations of plankton.

Biomass in mesoscale features can be mapped rapidly using acoustics or optics. In particular, spatial surveys of biomass could settle some of the important questions about the ecological importance of mesoscale features. Collection of these data will be driven by satellite and aircraft-derived images of the location of mesoscale features. Biomass determinations are by themselves insufficient to evaluate the importance of mesoscale features to the population biology of zooplankton. Detailed measurements of species and stage abundances, mortality rates, genetic composition and population vital rates also must be determined relative to specific mesoscale features. Practical constraints demand that these measurements must be limited in time and spatial coverage; thus they should be focused on especially important locations and times of interest.

The high resolution, ship-based sampling program will provide data linking the dynamics of planktonic organisms and populations to mesoscale variability in circulation within the CCS. To complete the analysis of life-cycle closure these measurements should be augmented with a sampling program that will document spatial and temporal variability in the ultimate recruitment of key benthic organisms and fish, relative to plankton dynamics. These studies should aim to resolve early recruitment (of fish) or settlement (of benthic organisms) soon after the planktonic stage ends, so that the impact of post-recruitment events (e.g., fishing, predation) on populations is minimized-this will provide the best opportunity to link biological and physical influences on planktonic stages to recruitment.