The effect of a large cape on distribution patterns of coastal and oceanic copepods off Oregon and northern California during the 1998–1999 El Niño–La Niña

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Abstract

Hydrographic and ocean drifter measurements made along the Oregon coast indicate that the spatial structure of the coastal upwelling system differs in waters to the north and the south of Cape Blanco, Oregon. North of the Cape, a 10–30 km wide zone of coastal upwelling parallels the coast, but south of the Cape, increased wind stress leads to a seaward expansion of the upwelling system and cold upwelled water extends 50–100 km offshore. Because the hydrography and the transport differ, we hypothesize that zooplankton distributions will differ as well. In this paper we investigate differences in copepod distributions and copepod community composition between the waters north and south of Cape Blanco. Five cruises were conducted in 1998 and 1999, which were years of contrasting ocean conditions; there was a strong El Niño in 1998, which was followed by a strong La Niña in 1999. Copepod biomass did not differ between the El Niño and La Niña periods; however, species composition of the copepod assemblages differed vastly. During the 1998 El Niño, the copepod community was dominated by subtropical neritic and warm-water offshore species. During the 1999 La Niña, the zooplankton community was dominated by cold water boreal neritic species. The warm water species were widely distributed in shelf and slope waters in 1998, whereas in 1999, they were found primarily offshore of central Oregon, but over the shelf off northern California. During the summer upwelling season of both years, copepod community composition in shelf waters differed significantly from slope waters in the region to the north of the Cape, however, community composition was the same in shelf and slope waters in the region south of the Cape. These results lead us to suggest that offshore transport by the upwelling jet may be an important mechanism controlling copepod community structure south of Cape Blanco. When we examined these patterns in community composition on a species-by-species basis, among the dominant boreal copepod species, Pseudocalanus mimus and Acartia longiremis were displaced offshore and maintained high population densities in the waters south of Cape Blanco whereas densities of Calanus marshallae and Centropages abdominalis declined in the waters south of the Cape. Thus, the interaction between the boreal copepods and the waters north versus south of Blanco is species-specific. Species may be either lost or retained depending upon interactions between vertical current shear and their vertical distributions. Alternatively, there may be a differential ability among species to survive and reproduce in waters offshore and south of Cape Blanco.

Introduction

Zooplankton are not distributed uniformly in the cross-shelf and alongshore directions in continental shelf waters. Rather, they often form distinct patterns of zonation in both biomass and species composition. For the waters off Newport Oregon, Peterson and Miller (1975) and Peterson, Miller and Hutchinson (1979) described patterns in cross-shelf distributions. There is a nearshore group of species composed of all life stages of the copepods Acartia hudsonica and Centropages abdominalis, eggs, nauplii and adults of Calanus marshallae, all stages of cladocerans and larvae of benthic invertebrates; a midshelf group dominated by the copepods Pseudocalanus mimus, Acartia longiremis, juveniles of Calanus marshallae, and the euphausiid Thysanoessa spinifera. There is also an outershelf/slope group composed of the preceding three species along with the copepods Eucalanus californicus, Metridia pacifica and the euphausiid Euphausia pacifica. Landry and Lorenzen (1989) reported similar patterns for the coastal waters off Washington as did Mackas (1992) for the shelf/slope waters off southwestern Vancouver Island, Canada. We do not know if these same patterns exist in waters off southern Oregon or California.

One of the key hypotheses driving research in the US GLOBEC Northeast Pacific program is that intensification of the north winds in the vicinity of Cape Blanco, a cape off southern Oregon at 42°50′N (Fig. 1), results in a seaward expansion of the upwelling zone south of the Cape, which in turn results in a wider area of high primary and secondary production. A corollary is that growth and survival of juvenile salmonids and other small pelagic fishes is higher in waters south of Cape Blanco because of the expansion of the region of high secondary production. As a step towards testing these hypotheses, we examine in this paper whether Cape Blanco influences alongshore and cross-shelf distributions of California Current zooplankton resulting in differences in zooplankton biomass and species composition in waters off southern Oregon and northern California as compared to southern British Columbia, Washington and northern/central Oregon. We expect that distribution patterns may be altered by Cape Blanco because large capes in other upwelling systems have been shown to displace shelf-species offshore and, in some cases, even to act as faunal boundaries. The effect of capes in redistributing species has been shown by Shannon and Pillar (1986) and Shillington, Peterson, Hutchings, Probyn and Waldron (1990) for capes in the northern and southern Benguela region, by Mittelstaedt (1983) and Weikert (1983) for Cape Blanc off northwest Africa, and by Valentin and Monteiro-Ribas (1993) for Cabo Frio, southern Brazil.

In the vicinity of Cape Blanco Oregon, energetic coastal jets, filaments and current meanders can extend several hundred kilometers from shore (Strub, Kosro, Huyer & CTZ Collaborators, 1991). These mesoscale features originate on the continental shelf, and so they may transport zooplankton off the shelf of southern Oregon and northern California and into offshore waters. Hydrographic sections and high-resolution Seasoar surveys indicate that the spatial structure of the coastal upwelling ecosystem changes significantly near Cape Blanco (Barth and Smith, 1998, Barth, Pierce and Smith, 2000). North of the Cape, the upwelling front and associated coastal jet lie over the mid- or outer-continental shelf, with relatively fresh waters from the Columbia River immediately offshore; south of the Cape the front and jet often lie far offshore, well beyond the shelf-break. Because waters inshore of the jet have higher nutrient and chlorophyll concentrations (Hayward & Mantyla, 1990), the offshore shift of the coastal front and jet south of Cape Blanco may extend the coastal zone of high biological productivity offshore, thus creating an extended area in which coastal zooplankton can flourish.

We report here on five cruises conducted in 1998 and 1999 during which hydrographic properties and zooplankton biomass and species composition were measured along several transects off central Oregon and northern California. Copepods are the focus of this research because they comprise the majority of the zooplankton biomass in our study area. Our goal was to test the hypothesis that copepod community composition differs between the waters to north and to the south of Cape Blanco because of the greater degree of offshore transport of coastal water in the region south of the Cape. The expectation is that coastal copepod species that are common in shelf waters off southern British Columbia, Washington and central Oregon are advected offshore in the water being transported offshore to the south of Cape Blanco, resulting in marked differences in the copepod communities north and south of the Cape. The five cruises coincided with the strong 1997/1998 El Niño and the subsequent 1999 La Niña, and so we will also examine the spatial and temporal response of the zooplankton to those strongly contrasting oceanic conditions.

Section snippets

Methods

Cruises were conducted in spring (April 1998 and 1999) and summer (August 1998, and July & September, 1999). During each of these five cruises, zooplankton and environmental data were collected along a series of latitudinal transect lines off Oregon and northern California, five lines being sampled in 1998, and four in 1999. Lines sampled were the Newport Hydrographic line (44.6°N), Coos Bay line (43.2°N), Crescent City line (41.9°N), Eureka line (40.9°N), and the Point Arena line (38.6°N) in

Winds

Winds prior to and during the April 1998 and 1999 cruises were similar; short periods of alternating strong northerly and southerly winds were seen prior to each cruise (Fig. 2). Winds blew steadily from the north prior to, and throughout, the August 1998 cruise. Winds were strong from the north preceding and during the July 1999 cruise; winds in September 1999 were variable both before and during the cruise.

Temperature/salinity/fluorescence

In April 1998, sea surface temperature over the sampling area ranged from 10.9 to

Discussion

Our estimates of copepod biomass are similar to past measurements in the continental shelf waters of the Pacific Northwest. We found a range of 0.3–2.4 g carbon m−2 for the upper 100 m of the water column. Peterson et al. (1979) reported copepod biomass of 0.4–1.6 g carbon m−2 off Newport, Oregon, in the upper 20 m of the water column, and Landry and Lorenzen (1989) observed copepod biomass of 1.6 and 1.7 g carbon m−2 along transects off the coast of Washington in the upper 100 m in June 1981

Acknowledgments

This work was supported by NSF (OCE-9732386) and NOAA (NA67RJ0151) grants and is contribution number 204 of the US GLOBEC–Northeast Pacific program. We thank A. Huyer (Oregon State University, College of Oceanographic and Atmospheric Sciences) for processing the CTD data and L. Feinberg and T. Loher for comments on the manuscript.

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