California Current Upwelling System Research Articles

Micronekton community structure in the epipelagic zone of the northern California Current upwelling system

Spatial and temporal variability in the micronekton community and in oceanographic conditions were evaluated from nighttime midwater trawl samples collected between Heceta Head, Oregon (44.0°N) and Willapa Bay, Washington (46.6°N). Collections from 13 cruises (176 trawls) from 2004 to 2006 yielded over 17,000,000 micronekton individuals (350,000 excluding euphausiids), representing 76 taxa and 43 families. The community was numerically dominated by euphausiids, followed in decreasing order by midwater shrimp ( Sergestes similis), lanternfishes (Myctophidae), late larval/juvenile rockfishes ( Sebastes spp.), age-0 Pacific hake ( Merluccius productus), and pelagic squid ( Abraliopsis felis). We used cluster analysis, ordinations, multi-response permutation procedures (MRPP), and indicator species analysis (ISA) to examine community structure of the 28 dominant taxa. Ordination and cluster results indicated that distance from shore and sea-floor depth best characterized habitats used by different assemblages of the micronekton community. Temperature and salinity at various depths influenced community structure to a lesser extent, along with Ekman transport. MRPP and ISA results indicated that nearly all dominant taxa were associated with cross-shelf gradients. Based upon a comparison between historical samples collected in 1976 and 1981 and comparable trawls from this survey, distinct decadal differences among micronektonic fish assemblages were observed, including more juvenile flatfishes and rockfishes but a lower diversity of mesopelagic fishes, which may be related to interdecadal environmental changes between the two time periods. This study represents the first examination of the relationships between both vertebrate and invertebrate members of the epipelagic nekton community.

Influences of upwelling and downwelling winds on red tide bloom dynamics in Monterey Bay, California

It has recently been shown that inner shelf waters of NE Monterey Bay, California function as an “extreme bloom incubator”, frequently developing dense “red tide” blooms that can rapidly spread. Located within the California Current upwelling system, this open bay is strongly influenced by oceanographic dynamics resulting from cycles of upwelling favorable winds and their relaxation and/or reversal. Different wind forcing causes influx of different water types that originate outside the bay: cold nutrient-rich waters during upwelling and warm nutrient-poor waters during relaxation. In this study, we examine how the bay's bloom incubation area can interact with highly variable circulation to cause red tide spreading, dispersal and retention. This examination of processes is supported by satellite, airborne and in situ observations of a major dinoflagellate bloom during August and September of 2004. Remote sensing of high spatial, temporal and spectral resolution shows that the bloom originated in the NE bay, where it was highly concentrated in a narrow band along a thermal front. Upwelling circulation rapidly spread part of the bloom, mixing cool waters of an upwelling filament with warm bloom source waters as they spread. Vertical migration of the dinoflagellate populations was mapped by autonomous underwater vehicle surveys through the spreading bloom. Following bloom expansion, a two-day wind reversal forced intrusion of warm offshore waters that dispersed much of the bloom. Upwelling winds then resumed, and the bloom was further dispersed by an influx of cold water. Throughout these oceanographic responses to changing winds, an intense bloom persisted in sheltered waters of the NE bay, where extreme blooms are most frequent and intense. Microscopic examination of surface phytoplankton samples from the central bay showed that spreading of the bloom from the NE bay and mixing with regional water masses resulted in significantly increased abundance of dinoflagellates and decreased abundance of diatoms. Similar dinoflagellate bloom incubation sites are indicated in other areas of the California Current system and other coastal upwelling systems. Through frequent bloom development and along-coast transports, relatively small incubation sites may significantly influence larger regions of the coastal marine ecosystems in which they reside.

Inorganic geochemical indicators of glacial-interglacial changes in productivity and anoxia on the California continental margin

Evidence from sediments in cores collected from within the present oxygen-minimum zone (OMZ; 600–1200 m) on the central and northern California margins record several episodes during the last interstadial (OIS-3, ca. 60-24 ka) of deposition of laminated sediments containing elevated concentrations of several trace elements indicative of anoxic conditions (e.g., Mo, Ni, Zn, and Cu). The presence of abundant well-preserved organic matter, as well as lack of bioturbation and the presence of elevated concentrations of Mo and other trace elements, all support the theory that the OMZ in the northeastern Pacific Ocean was more intense, possibly anoxic, at several times during the late Pleistocene. Sediments of all ages in cores from the southern California margin contain elevated concentrations of Mo, suggesting that this area has always had higher rates of sulfate reduction than either the central or northern California areas. Most of the Ba in sediments in all cores collected on the upper continental slope (200–2700 m) off California and southern Oregon is derived from detrital clastic material, and this source did not change much in time. However, the amount of biogenic Ba did vary with time, and these variations closely follow the temporal variations in organic C (Corg) mass accumulation rate. Using Ba and Corg mass accumulation rates as proxy variables for productivity, all cores show that organic productivity under the California Current upwelling system was highest during OIS-3 and the Holocene, and lowest during the last glacial interval (LGI, ca. 24-10 ka). All paleoproductivity proxy variables indicate that the southern California area has always experienced higher productivity than other areas under the California Current, at least over the last 50 ky.

Geochemical evidence for enhanced preservation of organic matter in the oxygen minimum zone of the continental margin of northern California during the Late Pleistocene

The present upper water mass of the northeastern Pacific Ocean off California has a well‐developed oxygen minimum zone between 600 and 1200 m wherein concentrations of dissolved oxygen are less than 0.5 mL/L. Even at such low concentrations of dissolved oxygen, benthic burrowing organisms are abundant enough to thoroughly bioturbate the surface and near‐surface sediments. These macro organisms, together with micro organisms, also consume large quantities of organic carbon produced by large seasonal stocks of plankton in the overlying surface waters, which are supported by high concentrations of nutrients within the California Current upwelling system. In contrast to modern conditions of bioturbation, laminated sediments are preserved in upper Pleistocene sections of cores collected on the continental slope at water depths within the present oxygen minimum zone from at least as far north as the California‐Oregon border and as far south as Point Conception. Comparison of sediment components in the laminae with those delivered to sediment traps as pelagic marine “snow” demonstrates that the dark‐light lamination couplets are indeed annual (varves). These upper Pleistocene varved sediments contain more abundant lipid‐rich “sapropelic” (type II) organic matter than the overlying bioturbated, oxidized Holocene sediments. The baseline of stable carbon isotopic composition of the organic matter in these slope cores does not change with time, indicating that the higher concentrations of type II organic matter in the varved sediments represent better preservation of organic matter rather than any change in the source of organic matter.