Oceanographic Background

## Biophysical Interactions

           Overview of Biophysical Interactions
           Biophysical Interactions Involving Phytoplankton
           Biophysical Interactions Involving Zooplankton
           Biophysical Interactions Involving Benthic Invertebrates
           Biophysical Interactions Involving Marine Fish and Shellfish
           Biophysical Interactions Involving Bird Populations
           Summary of Biophysical Interactions-Hypothesis Formulation

### Overview of Biophysical Interactions

Physical factors may influence biological systems directly or indirectly. Direct physical influences include changes in transport pathways in both the horizontal and vertical planes, alterations in physiological rates and shifts in the spatial boundaries of acceptable habitat. Indirect pathways include secondary responses of production to influences on nutrient concentrations, food availability, the distribution of predators, growth rates, and reproductive capabilities.

In the CCS the underlying importance of large scale physical forcing on marine organisms is evidenced in several comparative studies. Hollowed et al. (1987) noted that extreme year classes of several marine fish tend to occur synchronously even when species are separated by large geographical distances. Synchronicity over large geographic areas would not be expected if the principal processes influencing survival were linked to local ocean conditions. These synchronous extreme year classes tend to occur during periods of unusual environmental conditions (Hollowed and Wooster 1992, Beamish 1993). Further evidence of the importance of large scale physical forcing on marine organisms is found in numerous single species studies that demonstrate an apparent relationship between survival and physical forcing (Tables 7-8). Many of these studies show apparent linkages between biological response variables and temperature, wind driven transport or ENSO. Long-term trends in the abundance of marine fish stocks also show coherent patterns of association with large scale physical forcing. Francis and Sibley (1991) and Beamish and Boullion (1993) found coherent long term population trends in many stocks of Pacific salmon, with the southern (CCS) and northern (Canadian and Alaskan) stocks being out of phase. On an even broader scale, time series of Pacific sardine catch from populations in the southeast, northeast, and northwest Pacific all show remarkably similar trends (Kawasaki 1992). These comparative studies suggest that production of marine fish stocks is linked to large scale (but unknown) ocean processes.

A notable feature of time series of physical and biological measurements in the eastern boundary currents are that they both exhibit abrupt changes in state. The most recent shift occurred in the mid 1970s when coastal sea surface temperatures abruptly became warmer than average in the California Current (Hollowed and Wooster 1992, Trenberth 1990, Royer 1993, Figure 8). This shift was accompanied by a drop in zooplankton abundance, recovery of the depleted Pacific sardine population in the Southern California, and reduced size of many salmon runs in Oregon and Washington. Evidence of similar shifts at earlier times are found in the paleoclimatic records (Baumgartner et al. 1992). Together the paleoclimate and historic records suggest that the CCS response to gradually increasing concentrations of greenhouse gasses may be abrupt changes in qualitative states of the physical and biological system, rather than gradual change.

A principal goal of the U.S. GLOBEC CCS project is to identify the processes linking physical change to observed biological responses at the population level. Quantifying the impact of environmental change on survival rates of marine organisms may be difficult because the responses are often non-linear and may involve a sequence of events rather than a single factor. Detailed studies of the processes underlying survival have been conducted for only a few fish/shellfish species (English sole, northern anchovy, salmon, Dungeness crab, sea urchins, shortbelly and widow rockfish (underway)), the common barnacle, the copepod Calanus marshallae, and a euphausiid (Euphausia pacifica) in the CCS. These studies have illustrated the importance of environmental processes that occur on the scale of days, weeks and seasons. Sources of short term variability on the scale of days to weeks include upwelling-relaxation events, changes in turbulent mixing of the upper ocean and variability in upwelling jets and California Current meanders and eddies. Variations on seasonal scales include changes in storm tracks and frequency, frequency and intensity of upwelling, turbulent mixing, poleward flow (Davidson Current), California Current position and strength, air-sea heat exchange and freshwater runoff.

This overview illustrates that while there appears to be coherence in time series of abundance on the long time and large space scale, there is also considerable variability on the short time and small space scales. Predicting the consequences of climate change ultimately rests on accounting for the processes that generate variation in population processes at all time and space scales.

The following section summarizes our current understanding of the major biophysical interactions in each region. This background may assist in the development of research initiatives designed to enhance or understanding of processes influencing production of our nations marine resources in the CCS.

### Biophysical Interactions Involving Phytoplankton

Studies of the response of phytoplankton to physical forcing have been conducted in all four regions. In Regions I and II upwelling is the dominant control of primary production, supplies "new" nutrients to euphotic zone, leads to high primary production rates and high phytoplankton standing stocks (Small and Menzies 1981; Dugdale and Wilkerson 1989). These regions experience strong seasonal changes in rates of primary production due to seasonal shifts in the intensity of prevailing winds. The predominant equatorward wind stress in Regions I and II results in high rates of coastal upwelling during spring and summer and subsequent high levels of primary production. The positive wind curl leads to Ekman pumping and increased primary production in areas of surface divergence (Chelton et al. 1982; Strub et al. 1991), and an offshore maximum in zooplankton biomass (Chelton et al. 1982). Variations in the wind field are believed to play a major role in determining the distribution of phytoplankton and zooplankton (Abbott and Barksdale 1991).

There appears to be a close link between mesoscale features (eddies, jets) and the distribution of marine organisms in Region II. Satellite imagery suggests high concentrations of chlorophyll are associated with jets, eddies and other mesoscale features that occur as a result of coastal upwelling and the advection of this water (Flament et al. 1985). Chavez et al. (1991) found that high-nutrient and phytoplankton regions occur south and inshore of the Pt. Arena upwelling jet. Two distinct phytoplankton communities (coastal diatom and oceanic single-cell) are found in Region II and occur as separate populations on either side of upwelling fronts or divided by jets (Chavez et al. 1991).

In Region III, dense dinoflagellate blooms, often with very restricted vertical and horizontal distributions, are common features of the Southern California Bight during the summer (reviewed in Beers, 1986). Many of these dinoflagellates are important prey for larval anchovies (e.g. Lasker et al., 1970). Lasker (1981) noted differences in the abundance and nutritional content of dinoflagellates and diatoms in Region III as related to the food requirements of first feeding northern anchovy. Lasker's work demonstrated the importance of identifying not only the amount of production, but the type of production that occurs in a given year.

### Biophysical Interactions Involving Zooplankton

Large scale circulation may influence CCS species through its influence on zooplankton production. The biomass and abundance of zooplankton has a distinct seasonal cycle, with a two- to five-fold difference between winter and summer. Seasonality is most pronounced in inshore waters, less so offshore. Cycles are similar from Oregon to Baja California, Mexico, with high abundance and biomass in spring-summer, and low in autumn-winter (Brinton 1976; Peterson and Miller 1977; Chelton et al. 1982; McGowan 1989). These patterns are thought to be related to the monsoonal pattern of coastal upwelling (off Oregon) and to seasonal variations in southward transport of the California Current (central to Baja California). Species that depend on zooplankton for prey may have adapted to the timing of seasonal changes (Cushing's (1972; 1990) match-mismatch hypothesis). If the timing of key seasonal changes is altered these may influence subsequent survival of organisms at higher trophic levels.

Within a season, several studies indicate climate change could influence zooplankton by influencing the location and persistence of mesoscale features such as semipermanent eddies, or upwelling jets. In Region I, consistent maintenance of some copepod species within a nearshore band has been explained on the basis of two-celled upwelling ( Peterson, et al. 1979) and intermittent upwelling (Wroblewski 1980).

Zooplankton in Region II appear to be influenced by mesoscale features. Smith and Lane (1991) hypothesize that upwelling jets or eddies may transport the copepod, Eucalanus californicus, to the surface and onshore where egg laying occurs. Mackas et al. (1991) summarized the distribution of zooplankton near a jet in Region II. This study showed zooplankton species composition can change dramatically in response to fronts, resulting in distinct coastal and oceanic zooplankton communities. More recently, Laurs, Lynn and Peterson (unpublished) conducted a CTD, chlorophyll and zooplankton survey of a portion of the California Current jet and two associated eddies (34 deg -37 deg N, 123 deg -126 deg W, several hundred kilometers off Monterey and Point Sur) in July 1991. The Mackas survey and the Laurs et al. survey demonstrated that the California Current was enriched in zooplankton hundreds of kilometers seaward of the coast, in contrast to Washington and Oregon where zooplankton biomass is far higher on the shelf than over the slope. The zone of enrichment is separated from the warm offshore water by the meandering California Current "jet", a narrow (50 km wide) zone of low salinity water (32.7-32.9 ppt) that winds its way southward from the Oregon shelf to as far as southern California.

Numerous studies have been conducted on biophysical interactions of zooplankton in Region III. The distributions of many zooplankton taxa in Region III are influenced by a recurrent, deep offshore eddy southwest of Point Conception. The distributions of euphausiids, both those species with cool water, more northerly affinities and those with warm, more southerly affinities have been shown to be markedly affected by this offshore eddy (Haury et al. 1986), as have several other zooplankton species (Haury 1984).

Spatial variations in primary production and microplankton food influence different taxa in different ways. For example, in Region III, Checkley (1980) found that the copepod Paracalanus parvus became increasingly more food limited with distance from shore (to 50 km), while Calanus pacificus and Rhincalanus nasutus were found to have higher lipid contents and presumably better nutritional conditions in offshore waters of the CCS (Hakanson 1984). On smaller spatial scales, multi-species patches of copepods occur (Star and Mullin 1979), suggesting some common patterns of response to patch-generating processes.

### Biophysical Interactions Involving Benthic Invertebrates

Research on the influence of water column physical processes on benthic invertebrate recruitment has been conducted in Region II. These studies suggest that recruitment in some benthic invertebrates is influenced by mesoscale transport processes. Roughgarden et al. (1988) noted that both the landward edge of the distribution of larvae of a pelagic barnacle and the seaward edge of the distribution of larvae of a coastal barnacle moved further from shore during upwelling. From observations near Monterey that coastal barnacles settled during relaxation events (Farrell, et al. 1991), Roughgarden, et al. (1991) postulated that larvae were entrained in the upwelling front and settled only when it moved closer to shore during relaxation in upwelling. Based on settlement data, Wing et al. (in press) showed that crab larvae are transported northward during upwelling relaxation events. Sea urchin settlement in northern California is episodic on both annual and intra-annual scales (Pearse and Hines 1987, Wing, et al. in press; Ebert et al., in press; CDF&G). Shifts in the frequency and intensity of short term storm events may be important to plankton. Recruitment of meroplanktonic larvae to the intertidal zone may also be influenced by internal tidal bores (Pineda 1991).

### Biophysical Interactions Involving Marine Fish and Shellfish

Many studies of biophysical interactions involving marine fish and shellfish populations have been conducted in the CCS (Tables 7-8). Most of these studies demonstrated significant relationships between key biological processes and variations in temperature, upwelling and ENSO. These physical factors relate to large-scale circulation processes, the type of process that would be strongly influenced by large scale climate change (see Section IV).

Earlier (Section IV) we developed hypothetical climate change scenarios based on our current understanding of the processes linking climate and oceanography. Sixteen key physical changes were identified (Table 1). In this section we extend our hypotheses to include potential direct impacts of these changes to major fish and shellfish communities. This exercise is not intended to suggest that we are certain of the biological responses. The importance of the exercise is to identify the most important biophysical interactions and to point out areas where we lack knowledge about specific biological responses to physical changes.

Based on the background information found in Tables 7-8 and the predicted influence of climate change (Table 1) we identified the expected biophysical responses of marine fish and shellfish (Tables 9-11[Region I fish and shellfish; Region III fish]). An important outcome of this exercise was that it revealed areas where the expected biological responses were unknown. Very few studies have been conducted to identify the impact of changes in turbulent mixing (variable 9), transport in eddies and meanders (variable 10), changes in the depth of the thermocline (variable 13), or changes in the intensity or frequency of storms (variables 14 and 15).

Many species exhibited mixed responses to the climate change scenario. The most common case of mixed responses was the expected biological response to upwelling and temperature. This mixed response may be a product of our hypothesized physical scenario, where both upwelling-favorable winds (based on the Bakun hypothesis) and temperatures increase (based on general warming in GCMs). Increased upwelling may still produce cooler temperatures at the coast than farther offshore, but the mean temperature will be everywhere warmer than at present. If the hypothesized increase in stratification is strong enough or if the thermocline is deeper, the upwelled water may come from above the thermocline and nutricline and decrease both the cooling and fertilizing effect of upwelling, as in the warm phase of ENSO events off South America. Since most of the biophysical studies were conducted under present ocean conditions, an inverse correlation between upwelling and temperature is built into our present thinking. The presence of mixed reponses to upwelling and surface temperatures illustrates the need for an understanding of the process underlying the biological responses, particularly when the relationship between physical variables is expected to change. Use of comparative studies between latitudinal regions, between several EBCs, and measurements during unusual periods (such as warm ENSO events) will help broaden our experience and understanding.

The predicted response of marine fish and shellfish to our hypothetical climate change scenario is consistent with the ideas of Bailey and Incze (1985) who expected that species at the northern (southern) end of their range would increase (decrease) in response to El Nino like ocean conditions. The majority of the species that spawn in Region III showed positive responses to our global warming scenario (Table 11). The notable exception to this trend is the northern anchovy stock that is expected to decline (Table 11). Predicting the response of species that spawn in Region I is more difficult because of the large number of unknown responses (Table 9-10). However, the tables suggest that coho and chinook salmon, Pacific herring and dungeness crab stocks will all decline.

The importance of developing an understanding of processes underlying production of marine fish and shellfish has been recognized and process oriented research has been initiated for a few key species. In Region I, process oriented research has focused on larval transport mechanisms. In Region I, direct observations of spatial distribution of some planktonic organisms, and the temporal course of settlement give some indication of potential transport mechanisms. In observations of the cross-shore distribution off the Oregon coast, pelagic larvae (primarily fish) could be separated into 3 zones: coastal (<30 km), transitional (approximately 30 km), and offshore (> 30 km) (Richardson and Pearcy 1977, Richardson, et al. 1980). These were thought to be due to the distance from shore of spawning locations and the fact that currents were primarily alongshore. Some crabs and fish which migrate into the neuston were exceptions, being spawned nearshore but appearing offshore. During the 1982-83 ENSO, most of the ichthyoplankton found nearshore were "offshore" species (Brodeur et al. 1985). Hobbs et al. (1992) showed that the cross shelf distribution of Dungeness crab megalopae over the California to Washington coast matched that expected on the basis of a wind driven transport mechanism.

In Region II strong offshore flows (jets) may lead to short nearshore residence times. Species that rely on advection to return to nearshore locales may not survive in this environment. Eggs and larvae will be carried offshore in regions where the coastal jet first separates from the coast (flowing from shelf to deep ocean) or meanders back onto or near the shelf and then offshore again. At present, species that successfully spawn in Region II brood their eggs and larvae or extrude live larvae (e.g., rockfishes), or use protected embayments as spawning and nursery grounds (e.g., Pacific herring) to reduce the probability of offshore transport of pelagic larval stages (Parrish et al. 1981). Wing et al. (in press) examined event scale processes and found that crab larvae (primarily Cancer sp.) settled reliably in response to upwelling relaxation. Concentration of larvae in upwelling shadows south of promontories, then subsequent northward transport may provide a retention mechanism in the face of strong southward currents during upwelling. Offshore transport also may affect larvae by the removal of their prey.

Region III is the main spawning ground for the major pelagic species of the CCS (Pacific sardine, northern anchovy, Pacific hake, Jack mackerel and Pacific mackerel). While process oriented studies have been conducted on many species in Region III, the most complete study has been conducted on the northern anchovy. NOAA's SWFC conducted process oriented research on the northern anchovy for the last twenty five years. These studies include analysis of the spatial distribution of spawning (MacCall 1984), temperature effects on adult maturation (Fiedler et al. 1986), estimates of the rates of egg production (Stauffer and Picquelle 1981), larval retention through transport or eddies (Power 1986, Hewitt 1981, Fiedler 1986), egg dispersal and incubation rates (Smith and Hewitt 1985, Zwiefel and Lasker 1981), larval condition (O'Connell 1980, Theilacker 1986, Theilacker and Watanabe 1989), larval daily growth and mortality rates (Methot 1983, Butler 1989, Lo 1986), predator distributions (Hunter and Kimbrell 1980, Webb 1981, Alvarino 1980), the influence of turbulence on prey type and availability (Scura and Jerde 1977, Lasker 1975, 1978, Wroblewski 1984) and regional differences in survival (Owen et al. 1989). These studies demonstrate that factors determining year class strength of pelagic spawning marine fish are complicated and can involve several different physical events (or processes).

### Biophysical Interactions Involving Bird Populations

Marine birds have proved to be sensitive to perturbations in marine climate, food-web structure and prey availability (e.g. Crawford & Shelton 1978, Glantz and Thompson 1982, Barber & Chavez 1986, Ainley and Boekelheide 1990, Ribic et al. 1990, Cairns 1992). Seabirds, in fact, have proven useful as indicator species of oceanographic conditions owing to their great mobility (quick response), large numbers and easy visibility (e.g. Schreiber and Schreiber 1989) and sensitivity to variations in food supply.

The most extensive research linking oceangraphic processes to production and survival of marine birds has been conducted in Region II. In this region a unique 22-year time series of data has been collected on the reproductive phenology, demography, diet and at-sea occurrence patterns of several species including the Cassin's Auklet, a planktivorous seabird. This time series shows the auklet to be a sensitive indicator of the effects of coastal upwelling on zooplankton abundance and species composition, especially the euphausiids, Thysanoessa spinifera (a coastal species) and Euphausia pacifica (an oceanic species). The auklets near the Farallon Islands breed successfully in years when T. spinifera is available in great quantity near the surface and poorly or not at all when the euphausiid is not available, as during ENSO warming events (Tables 12 and 13; Ainley and Boekelheide 1990). Egg laying closely follows the "spring transition" to upwelling conditions (Huyer et al. 1979), which often leads rapidly to the offshore spread of cold, upwelled water and surface swarms of T. spinifera. During late summer, when T. spinifera is no longer available as prey (as demonstrated by its disappearance and the appearance of Euphausia pacifica in the auklet diet), breeding ceases and remaining chicks are abandoned.

The importance of the auklet lies both in their use as zooplankton samplers (through diet analysis) and as analogs for other top trophic level species. They are especially useful because their favored prey, T. spinifera, is also important in the diet of other species including targets of major fisheries (hake, rockfish and salmon), other sea birds and baleen whales. For instance, analysis of stomach contents of salmon show that they, like the auklet, switch in spring from their winter diet of cephalopods and other prey to gorge on T. spinifera in the outer waters of Region II (Adams et al., ms).

### Summary of Biophysical Interactions-Hypothesis Formulation

The wide variations of physical and biological conditions observed in the CCS facilitate the formulation and testing of hypotheses. Existing biophysical hypotheses for fish and shellfish populations include the 'match-mismatch' (Cushing 1972), 'stable ocean' (Lasker 1975, 1978), 'basin' (MacCall 1984), and 'habitat retention' (Isles and Sinclair 1982, Sinclair 1988) models. The 'match-mismatch' hypothesis involves the physical and biological coupling at the seasonal or shorter scale of production or bloom phenomena and the reproductive cycle of marine species. The 'stable ocean' hypothesis suggests that periods of low physical mixing which initiates and maintains vertical stability necessary for growth and aggregation of phytoplankton are necessary for organisms that rely on phytoplankton for food. The 'basin' hypothesis links population expansion at the periphery to population density at the core during the portion of the life cycle which is dominated by density-dependent phenomena. The 'habitat retention' model proposes that population structure of resource populations is determined by physical forcing which maintains the dispersive phase of the population in the appropriate range of sites which constitute a population home base. Knowledge of the geographic and seasonal distribution distribution of upwelling, transport, mixing, transitions, productivity and life cycles of key marine species is necessary to test these hypotheses.