EBC-CCS - Working Group Reports

## WORKING GROUP REPORTS

### Importance of Mesoscale Physical Features to Ecological Processes in the California Current System

Cochairs: C.B. Miller and L. Washburn

Participants: K. Bailey, L. Botsford, A. Bucklin, R. Francis, W. Graham, P. Hsueh, J. Jaffe, B. Jones, D. Mackas, J. Paduan, T. Powell, L. Rosenfeld, and E. Woehler

Oceanographers have known for decades that the physical and biological characteristics of eastern boundary current ecosystems vary intensely in space, but until recently have lacked the observational tools for resolving the pattern of this variance. The discovery of filaments, squirts, and persistent eddies through satellite observation of eastern boundary current systems is one of the greatest viewpoint shifts since modern oceanography began after World War II. It ranks with the discovery of continuous midocean ridges and with finding submarine thermal vents and their associated communities. The existence of a massive, churning vortex system with (it now appears) close spatial association of physical and biological pattern was unexpected from all previous observation or theory. Exploration of interactions within this system has theoretical, practical, and public appeal. In every respect the recurring mesoscale-to-subregional flow features deserve extended examination. Within the California Current system, these features are most prominent off the central and northern California coast, and we recommend that their study be concentrated there. However, results from this region promise valuable insight for ecology in other eastern boundary current systems with comparable flow features.

#### HYPOTHESES: THE ECOLOGICAL SIGNIFICANCE OF MESOSCALE FEATURES

The mere existence of central California filament features does not establish their importance to pelagic populations. We therefore pose the following null hypothesis:

Ho: Squirts, jets, and filaments have no significance to the life of planktonic or other organisms.

This null hypothesis cannot be disproved by inspection from space. Rather, a program of direct testing is required. The test of the basic null hypothesis could be rather simple pair-wise sampling of populations within and outside of features. A more sophisticated test would require time series sampling following drift tracers, preferably without tethered surface floats. Sampling in the tracers' vicinity would allow comparison of the level and rate of change of demographic parameters (developmental progress, fecundity, condition factors, enzymatic capacity) for water parcels affected by and external to major mesoscale flow features.

Regardless of final complexity, the test(s) of the null hypothesis should be designed to provide maximum information about each of a number of alternative (and nearly a priori) hypotheses regarding ways in which the features are likely to have significant impact. We recommend that this list of alternative hypotheses include the following.

H1: Eddy features are retention/aggregation sites for meroplanktonic and holoplanktonic populations, and the demographic parameters of individuals inside and outside eddies will differ.

This can be examined by repeated sampling in an eddy feature identified by satellite imagery and marked with drifters interrogated in real time during sampling. The effort should last weeks to a month--long enough for significant progression of developmental stages in contained populations. Both cyclonic and anticyclonic eddies should be investigated to determine whether either has a greater tendency to retain material and persist longer, and whether biological interactions differ in the two types of eddy. In addition to demographic information, we recommend collecting information on within-species genetic resemblance of organisms inside and outside the eddies.

H2: Inshore stocks suffer major losses from seaward advection in streamers and filaments.

We are interested both in the magnitude of loss, and in the sensitivity of this loss to changing climatic conditions. There is already evidence that many populations have compensating behaviors that minimize such losses. Most benthic invertebrates with pelagic phases are winter spawners. Their larvae are at risk only during the season of minimal offshore transport. But there is increasing evidence that alongshore and temporal variation in settlement is linked to variation in nearshore upwelling circulation (Roughgarden et al. 1988; Ebert and Russell 1988). There are coastal holoplankton off southern California, where jet/eddy activity appears to be reduced compared with central California. It is not known to what extent the holoplankton are transported offshore during the season of most active upwelling. The data gathered to date (CTZ project) indicate that very few coastal holoplankton move seaward in the offshore flow fields associated with large filaments (Mackas et al. l991). Although good information is available from the Oregon coast (Peterson et al. 1979), cross-shore exchange of biota within the immediate coastal zone (0-20 km from shore) has not been studied off central California. Much more extensive sampling is also needed to establish the large-scale correspondence, if any, between the alongshore zone of extended upwelling features and distributional boundaries of populations.

H3: Marine organisms actively exploit intense local gradients at the boundaries of mesoscale features.

Jets, filaments, and eddies exhibit strong, convergent secondary flows which produce sharp, persistent fronts. They may also carry localized pulses of dissolved nutrients and particulate food from onshore bands of high concentration into oligotrophic offshore waters. Thus squirts, filaments, and eddies that form off central California can accelerate food chain transfers in several ways: species may aggregate at sharp boundaries; predators may migrate there to eat them; and oceanic plankton mixed into extensions of onshore conditions may find luxuriant food and respond strongly through enhanced reproduction, higher growth rates, and improved condition factors (Mackas et al. 1991; Smith and Lane 1991). Some data suggest that this may particularly involve thaliacians (salps and doliolids), which have extremely rapid population growth responses. All of these trophic enhancement effects are amenable to study by classical and modern techniques of planktology (e.g., acoustical biomass estimation, enzymatic condition indices).

H4: For at least some species, squirts, filaments, and their associated surface and deep return flows provide beneficial transfers at key life stages.

These transports might be either onshore or offshore. On this topic, sampling should be based on detailed hypotheses about the life history and requirements of particular species. Both the timing and the spatial distribution of sampling will have to match the developmental and transport schedules of species of interest.

#### TIME-SCALE INTERACTIONS IN THE CALIFORNIA CURRENT SYSTEM

Physical and biological processes in the CCS have multiple time scales. Physical processes have at least interannual, annual, mesoscale event, weather, diel, inertial, and turbulent time scales. Pelagic organisms respond to habitat variations on all of these scales. They do so by different, scale-appropriate shifts in location and activity. Table 1 shows some of the correspondences among these time scales, physical process variations, and biological responses. Each physical process varies over a range of frequencies in the vicinity of those named. Likewise, some biological responses can adapt to physical processes on several time (and space) scales. Understanding this time-scale matching is a goal of all ecology; for GLOBEC, time-scale matching is an essential design consideration for CCS studies.

Table 1. Time-Scale Relationships Among Physical and Biological Processes in the CCS, Emphasizing Mesoscale Processes

Frequency | Physical Events | Biological Responses
--- | --- | ---
Annual | Alongshore jet formation/dissipation; high-low cycle in mesoscale activity. | Phenological responses (ontogenetic migrations, diapause, spawning migrations, selection of spawning timing, large scale horizontal migrations).
Semi-seasonal | Filament formation, extension, decay; eddy persistence. | Production and stock responses in zooplankton and small nekton (through spawning and growth variations); food storage, use of stored nutriment.
Weeks | Rotation period within features; feature translation alongshore. | "Vulnerability scale" - changes in life-stage maturation, reproductive activity. (Vulnerability to predators varies sharply with size, stage, and related escape capability. It may either decrease or increase with developmental progress).
Days | Weather, upwelling-downwelling cycle. | Production and stock responses in phytoplankton and microzooplankton; some mesoscale migratory responses?
Diel | Illumination cycle; local heating and cooling; convective turbulence cycling. | Daily vertical migrations; daily activity cycles including feeding, metabolism, mating, molting, etc.
Short Scales | Surface waves; internal waves; light flicker (hours to msec); cloud shifts; turbulent rotations in all axes. | Feeding bout cycling; swimming search patterns; near-field collision and predator avoidance; body orientation.

#### APPROACHES TO MESOSCALE ECOLOGICAL PROCESSES VIA BIOMASS ESTIMATION

Biomass distributions, which may be characterized much more rapidly than species or stage distributions, can be used to test the importance and significance of mesoscale features. In particular, spatial surveys of acoustic biomass should rapidly settle the most basic questions of the ecological importance of mesoscale features. If mesoscale features transport large amounts of biomass, they are important to processes both upstream and downstream. The transport will be important to all the species making up that biomass, whatever they may happen to be.

The objectives of a survey of mesoscale variability will be (1) to determine the covariance of zooplankton distribution and flow features, (2) to determine the covariance of biomass size distribution and flow features, and (3) to measure the flow.

Coincident physical and biomass observations are required to meet these objectives. Deployment of acoustic biomass evaluation must be guided by recent satellite images of flow features. Sampling should be persistent enough to demonstrate the biota's delayed responses to flow features. This may well be possible if, for example, thaliacian population bursts are a major system response to translations of nearshore phytoplankton stocks and nutrients into the offshore zone. Spatial sampling should be fine enough to prevent aliasing; more than a few paired observations inside and outside features will be required. This requirement should not be overly stringent, given the data rates of emerging acoustical systems. The sampling distribution must also be more extensive than the flow features for adequate comparisons to emerge. The large size of these features creates a fairly heavy work load for the project. Sampling should resolve diurnal variation in biomass, since in the oceanic reaches into which filaments extend, major fractions of zooplankton biomass migrate vertically. In the CCS this includes some of the species most likely to be of interest to general GLOBEC goals (e.g., Euphausia pacifica and Metridia pacifica).

Equipment required for high-resolution biomass studies is under development, some of it with GLOBEC support. It should be noted that there are fundamental limitations to acoustic techniques. It will not be possible, because of physical limitations, to simultaneously and synoptically survey a large number of separate size classes in three dimensions. But we can develop systems at appropriate frequencies to map two-dimensional distributions of biomass with some size-class resolution. Physical data can be gathered at comparable density along these sections. Devices like the Pieper-Holliday towed, multi-frequency sonar will be appropriate. Suitably oriented sections will allow powerful tests of our hypotheses.

#### SPECIES OF INTEREST

Not all of the pelagic biota of the CCS can be addressed by GLOBEC. A modest number of species must be selected for intense biological study. The selection criteria should include the likelihood that an organism will significantly interact with mesoscale features. The working group accepted the GLOBEC focus on mesoplankton and larger pelagic animals, and developed a list of candidate species. These were evaluated under a list of suitability criteria, which were:

- Distribution - Does the species live in the central California Current and have potential for interaction with mesoscale features?

- Larval Period - Does this match or complement mesoscale activity?

- Data Base - Is information about spatial and temporal pattern for the species already available?

- Culture Potential - Rearable species are more readily studied.

- Biotechnological Potential - Biotechnical methods promise great insight, and species adaptable to such methods should be chosen.

- Paleontological Record - We can extend our understanding to time scales of global change by selecting species with fossil records, particularly high-resolution recent records.

- Commercial Value - Commercial value secures the sampling power of fisheries and assures the interest of the public and mission-oriented government agencies.

- Known Ecological Interactions - Certainty of interaction with other species of interest will be an advantage.

In accordance with the above criteria, workshop participants suggested the following list of species:

Fish: Northern anchovy, Pacific hake, California sardine, the rockfish complex, Dover sole, English sole, chinook salmon.

Benthos: Cancer magister (and various congeners as comparative cases); pink shrimp; Emerita analoga; barnacles (presumably Balanus spp. will be favored); sea urchins (Strongylocentrotus franciscanus, S. purpuratus); kelp.

Holoplankton:

Copepods: Calanus pacificus, Metridia pacifica, Paracalanus parvus

Euphausiids: Euphausia pacifica, Nyctiphanes simplex, Thysanoessa spinifera

Thaliacians: Salpa spp., Thalia democratica, Dolioletta spp. (study of gelatinous herbivores will have to be as opportunistic as they are.)