Motivation
Research Questions
Approach
Research Findings

Research Activities:
Physical Oceanography at UC Santa Cruz
Deployment History for OSU Nearshore Moorings

Motivation

Understanding coastal marine ecosystems requires detailed knowledge of oceanographic conditions, particularly the inner-shelf region, which extends from the shoreline to about 5-10 km offshore. Most shallow-water, rocky-shore organisms have open-water larvae that disperse beyond the inner shelf and later return to settle into adult habitats.

A principal hypothesis of the PISCO project is that water temperature and coastal currents control larval transport, larval settlement, retention, and species range boundaries. We are particularly interested in the role of coastal circulation processes, which differ in terms of importance depending on where you are along the coast.

Research Questions

1. What are the nearshore oceanographic patterns and processes along the U.S. West Coast?
2. How do nearshore oceanographic conditions affect the dynamics of shallow subtidal and rocky intertidal communities?

Approach

All of the PISCO units use similar methods of physical data to observe the processes believed to be important in nearshore larval transport and settlement. These include coastal currents, eddies, water mass variability, and internal waves. We use moored instruments, small-boat operations and land-based high frequency radars (called CODARs) to study nearshore physical oceanography. Moored instruments over the inner shelf measure temperatures at three depths and water velocities throughout the water column. These observations are supplemented by vertical profiles of water properties (temperature, salinity, and density) taken during small-boat surveys.

Research Findings

PISCO has developed an extensive research program to characterize patterns of the coastal ocean. From north to south, there are several major oceanographic discontinuities along the California Current System. In Oregon, efforts have initially focused on the discontinuity represented by the Heceta-Stonewall Banks, which force the California Current offshore and generate a large eddy over these banks. We suspect that this discontinuity is linked to differences observed in rocky intertidal communities along the central coast of Oregon. As a result, we have focused our efforts on this region and the region to the north where the California Current is closer to shore and exhibits a simpler north-south flow. In California, Point Conception is a major biogeographic boundary. Traditionally oceanographers have assumed that the warmer waters of the Santa Barbara Channel to the south and colder waters of the Santa Maria Basin to the north provide suitable habitats for different species. PISCO is investigating whether coastal currents control larval transport and retention, thereby determining settlement locations and species range boundaries.

Distinct regions within the California Current System

Substantial new insights have resulted from the first three years of PISCO research. Specifically, we have developed an emerging working model for the California Current ecosystem: The West Coast may be divided into three distinct regions that are dynamically quite different:

• North: Washington/Oregon border to Cape Blanco
• Central: Cape Blanco to Point Conception
• South: Point Conception south

The North region is characterized by intermittent, weak summer upwelling with periods of relaxation. Within this region we have found that: 1) Recruitment of invertebrates is very high; 2) Growth rates of sessile filter feeder are high but variable, depending on nearshore productivity; and 3) Predation rates on mussels are high. In contrast, the Central region experiences longer, more persistent, and stronger upwelling than the North region. Within this Central region, we have found that: 1) Recruitment is very low (relative to the north); 2) Growth of filter-feeders is low; and 3) Predation rates are lower but variable. We hypothesize that the distinct oceanographic regimes bear directly on the biological differences between these two sections of coast. The South region has strong offshore upwelling, weak nearshore upwelling, and a gyre between the mainland and the Channel Islands. Here we have found that: 1) Recruitment is very low; 2) Growth of sessile invertebrates is relatively high; and 3) Predation rates are very low. Thus, we predict that the onshore dynamics that drive much of the patterns and distribution of species link directly to the very different nearshore dynamics.

Critical transition zones

In addition to these distinct regions, we are learning that the transition points are more biologically abrupt than previously thought. PISCO scientists are studying the connections between water transport mechanisms and larval recruitment along the coast of Oregon and California. Combined with PISCO's nearshore oceanographic measurements, recruitment data have shed new light on recruitment patterns and processes along the west coast. For example, comparisons across the PISCO study range have shown that recruitment rates of invertebrates are much higher in Oregon than in central and southern California. Large-scale shifts in recruitment rates coincide with oceanographic discontinuities at prominent topographic features like Cape Blanco in Oregon and Point Conception in California. These discontinuities also mark the northern or southern range limits for some marine species suggesting that these features inhibit the north-south movement of larvae. Support for this hypothesis comes from research at UCSB that shows abrupt changes in the recruitment rates of some species at Point Conception and modeling studies that demonstrate the potential for ocean currents to generate these patterns.

A new understanding of nearshore oceanography

PISCO's oceanographic sampling of the inner shelf is documenting important oceanographic patterns that run contrary to conventional wisdom. Specifically, we have observed that: 1) Recently upwelled water is carried further inshore than previously documented; 2) The nearshore water column is often strongly stratified; and 3) Internal waves propagate inshore to depths as shallow as 4 meters. Our measurements indicate that offshore transport in shallow water due to upwelling is much less than predicted by existing theories and that current speeds over the inner shelf are much lower than those offshore. PISCO scientists have also observed nearshore responses to upwelling relaxation, including current reversals, that may be important for delivering invertebrate and fish larvae to subtidal and intertidal habitats.

Food inputs and community structure

A primary goal of PISCO research is to link the dynamics of the nearshore ocean with those in intertidal and subtidal communities. In 2001, PISCO pioneered the deployment of an array of oceanographic instruments in Oregon to measure chlorophyll fluorescence (an indicator of phytoplankton concentration) and light in the intertidal zone. We can now quantify the delivery of phytoplankton and light, two fundamental resources for benthic animals and plants, to intertidal communities. Through continuous, high frequency measurements we are able to make both from moorings and on shore provide us with a new tool for opening the black box of nearshore processes that deliver resources and larvae to the shore.

By Libe Washburn, Research Fellow, Cynthia Cudaback, Postdoctoral Researcher, and Renee Davis-Born, and Lydia Bergen, Policy Coordinators