Lawrence F. Small

Food selection capabilities of the estuarine copepod Acartia clausi

Existing viewpoints and theories of selective grazing by copepods are briefly reviewed in order to formulate explicit hypotheses to be tested experimentally. Based on these hypotheses, a series of grazing experiments was run to determine (1) the extent of the selective ingestion capabilities of Acartia clausi and (2) how these capabilities were affected by previous feeding histories. Groups of copepods were separately preconditioned on a small diatom (Thalassiosira pseudonana), a large diatom (T. fluviatilis), or a plastic sphere. The ingestive behavior was then examined on various combinations of spheres and food particles. Spheres offered alone were not ingested. In mixtures of diatoms and spheres, the copepods avoided ingesting spheres intermediate in size between the sizes of the diatoms. The copepods either ingested particles on either side of the spheres, or ignored all particles less than the size of the largest spheres. The pattern observed depended upon the size of the preconditioning food. However, if the spheres were larger than the largest food particles, the copepods still selectively ingested the food particles. The above results demonstrate that A. clausi has a complex grazing behavior consisting of (1) more efficient grazing on larger particles within its particle-size ingestion range; (2) the ability to alter “effective” setal spacing to optimize feeding behavior (i.e., the ability to increase efficiency of capture of food particles, and to avoid non-food particles); and (3) the ability for post-capture rejection of non-food particles when they interfere with the ingestion of food particles on which the copepod has been preconditioned. The behavioral patterns observed depend heavily on the food preconditioning and the presence or absence of non-food particles. These results clearly indicate that a simple “mechanistic” explanation of selective grazing is insufficient.

Patterns of primary productivity and biomass in a coastal upwelling region

Abstract Average distributions of chlorophyll α during upwelling in areas of smooth bathymetry off Oregon have been computed from historical data. Chlorophyll concentrations in cross-shelf sections over the Oregon continental shelf (44°40′N) were similar to those in cross-shelf sections off northwest Africa during JOINT-1 studies but differed from those in similar sections over an adjacent narrow region of the Oregon shelf (44°55′N to 45°12′N). The fact that larger concentration differences can occur along short sections of one coastline than between upwelling regions half a world apart bears on the time and space scales of sampling and might have bearing on the support and distributions of other trophic levels. Relationships between the local winds and the broader-scale Bakun (1975) upwelling indices were used to classify various phytoplankton biomass and primary productivity distributions according to whether they were in strong upwelling steady state, weak upwelling steady state, or one of two transition states. The upwelling steady state conforms to the ‘upwelling event’ scale (about 3 to 10 days) of Walsh, Whitledge, Kelley, Huntsman and Pillsbury (1977) and the transition state of approximately one-day duration might specify the critical scale for driving the upwelling off Oregon. Under strong steady-state upwelling in early summer a single band of high primary productivity and biomass develops in the surface layer parallel to the bottom contours, but under similar upwelling conditions in later summer a two-celled zonal circulation occurs and two parallel bands develop. Our strong upwelling distributions are discussed in light of current models of the Oregon upwelling system. Maintenance of biological properties through time in the upwelling bands is also discussed. Under weak steady-state upwelling the primary productivity and biomass bands are farther inshore or immediately against the coast. Productivity in the weak upwelling bands can be twice that of the strong upwelling bands and often 20 times that in surrounding water. Under transient conditions in which the local winds are favorable for strong upwelling but the slower-responding Bakun index indicates weak upwelling, chlorophyll distributions change within a day to resemble distributions during strong steady-state upwelling. Under transient conditions in which the local winds weaken dramatically but the Bakun index still indicates strong upwelling, chlorophyll concentrations are high and widely distributed throughout the upwelling region.

Sedimentation dynamics in the Santa Monica-San Pedro Basin off Los Angeles: radiochemical, sediment trap and transmissometer studies

Abstract A large number of sediment cores and sediment trap samples collected from different parts of the Santa Monica-San Pedro (SM-SP) Basin during 1985–1988 were studied for radionuclides, trace metals and other sedimentary components. The radiochemical data are presented here to give a basin-wide view of the sedimentation dynamics. 210 Pb stratigraphy indicated that sedimentation rates were higher and more variable (30–80 mg cm −2 y −1 ) in the more dynamic slope region, but were uniformly low (13.4–18.8 mg cm −2 y −1 ) in the flat, deep basin. From the sediment record, it was suggested that sedimentation rates were decreasing and the area of anoxia had been expanding, at least during the past one to two centuries. Turbidite layers found in the sediment cores suggested higher frequency and more recent occurrence toward the basin margins. 210 Pb geochronologies indicated that the recent turbidites might be related to major storms which occurred during the past two decades. Sediment traps deployed in the basin recorded very large short-term spatial and temporal variabilities of mass flux, with unusually high fluxes corresponding to recorded large wave events. Trap-measured near-bottom mass fluxes averaged over all collection periods (622 days) were consistent with 210 Pb-based sediment accumulation rates. With few exceptions, trap-measured fluxes decreased offshore but increased with depth at any location, strongly suggesting lateral input of materials. Transmissometer data demonstrated the existence of nepheloid plumes off the eastern slope of the SM-SP Basin. The offshore decrease of sedimentation rate in the eastern part of the SM Basin was consistent with the fact that nepheloid plumes were confined to the basin slopes and that the major transport pathway of suspended particles, as indicated via progressive vector analysis, was alongshore from the SP Basin toward the SM Basin. Concentrations and fluxes of radionuclides measured in the near-bottom trap in the deep basin compared favorably with those registered in bottom sediments. Based on water column, sediment trap and sediment core data, self-consistent flux balances can be constructed for 228 Th and 210 Pb. Flux balances for 234 Th were less well defined. The cyclic pattern of uranium profiles in deep basin sediments appeared to be in phase with the sedimentary record of CaCO 3 and the historical record of primary production and anchovy biomass. It is suggested that the removal of uranium from the water might be regulated by longterm regional changes in biological processes and sedimentation environments.

Seasonality of phytoplankton production in the Columbia River: A natural or anthropogenic pattern?

Abstract This study evaluated seasonal variability in the quantity and quality of particulate organic matter exported by the Columbia River into its estuary. Samples of suspended particulate material (SPM), collected monthly over a 1 year period (November 1995–October 1996) from a freshwater site in the Columbia River, near the head of its estuary, were analyzed for total aluminum, particulate organic carbon and nitrogen (POC and PN, respectively) and algal pigments (chlorophyll and carotenoids). High Al content, averaging 7.9% by weight throughout the year, indicated that detrital minerals accounted for the majority of SPM mass at all times. Organic matter (approximately 2 × POC) varied on a seasonal basis both in terms of its mass contribution to SPM (5–26% by weight) and its chemical composition. In winter, organic matter originated mainly from allochthonous sources, most likely from erosion of soils. Diatoms, the major phytoplankton group present as inferred from pigment analysis, “bloomed” in April–June 1996, resulting in increased autochthonous contribution to POC during spring and summer as well as increased organic matter content of SPM introduced to the estuary. Comparison with results for the same site in the mainstem Columbia obtained between 1990 and 1999 as part of a land-margin ecosystem research project shows that enhanced algal production in springtime is not a feature unique to our data set, at least in the Columbia River’s present highly regulated state of hydrographic operation. The possibility is discussed that recent human influence, principally dam construction, has significantly altered the historical SPM concentration and the chemical composition of its associated organic matter.

Nitrogen isotopic ratios in fecal pellets produced by marine zooplankton

At each site and in every season studied, zooplankton in the upper ocean produced fecal pellets that were 1.3% lower in {delta}{sup 15}N than their body tissue but 2.2% higher than their apparent food source. {sup 14}N-containing molecules are evidently preferentially assimilated in zooplankton intestinal tracts, though other isotopic effects must account for the enrichment in {sup 15}N of these organisms relative to their food. These results also show zooplankton to be important modifiers of nitrogen isotopic ratios for marine particulate matter. {delta}{sup 15}N values for sinking particles and fecal pellets are similar, supporting the perspective that macrozooplankton are important factors in producing and processing particles that sink into the oceans interior and sediments. In contrast, the relationship in {delta}{sup 15}N between fecal pellets and suspended particles in the euphotic zone is more variable. It appears that zooplankton select food particles of varying {delta}{sup 15}N from the suspended particle pool. These results suggest that both zooplankton feeding behavior and their digestive chemistry are important in determining the composition of sinking particulate matter in the ocean with respect to the suspended particle source in the euphotic zone.

The role of sinking fecal pellets in stratified euphotic zones

Abstract The euphotic zone at the VERTEX II oligotrophic sediment trap station (18°N, 108°W) can be viewed for simplicity a as two-layered system with (1) an upper mixed layer (0–40 m) of relatively high primary production, low chlorophyll concentration, and low zooplankton biomass, and (2) a lower layer (40–100 m) containing the chlorophyll and particulate organic carbon maxima, higher zooplankton biomass, and relatively low primary production. Particle traps at 30 m yielded no particulate carbon flux, while traps at 120 m registered 38 mg C m−2 d−1, with many visible fecal pellets. Zooplankton in the upper layer apparently produced small pellets which were recycled in situ. Fecal pellet carbon production by 200–2000 μm zooplankton in the lower layer, however, was 30% of trap-measured C flux. Tiny pellets produced by 53–200 μm zooplankton were mostly recycled within the euphotic zone, regardless of which euphotic layer the animals were inhabiting when collected. Comparisons are made between the relative contributions of sinking fecal pellets when the VERTEX II euphotic zone is considered as a single uniform layer rather than as a two-layered system. In addition, comparison of the VERTEX II data with data from the eutrophi VERTEX Vc site (35°50′N, 122°30′W) suggests that zooplankton can be more significantly coupled to carbon flux out of the bottom layers of oligotrophic euphotic zones than to carbon flux out of unlayered eutrophic euphotic zones.

Dissolved and fecal pellet carbon and nitrogen release by zooplankton in tropical waters

Abstract Carbon (C) and nitrogen (N) release by tropical zooplankton (mostly copepods) and micronekton (euphausiids, pelagic red crabs, and salps) was investigated near VERTEX particle traps at 18°N, 108°W (in 1981) and 15°40′N, 107°30′W (in 1982). The objective was to assess the significance of fecal pellet release relative to respiratory and dissolved excretory release of C and N and relative to primary production in the same waters. For small ( Daily fecal pellet C egestion represented only 2 to 3% of both large and small zooplankton body C content, and daily fecal pellet N egestion was

Columbia river estuary studies: An introduction to the estuary, a brief history, and prior studies

~Fisheries Research Institute, WH-IO, University of Washington, Seattle, WA 98195, USA ZCollege of Oceanography, Oregon State University, Corvallis, OR 97331, USA SDepartment of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331, USA 4Geophysics Program, AK-50, University of Washington, Seattle, WA 98195, USA 5School of Oceanography, WB-I O, University of Washington, Seattle, WA 98195; current affiliation - Battelle, Pacific Marine Science Laboratories, Sequim, WA 98382, USA

Primary production, plant and detrital biomass, and particle transport in the Columbia River Estuary

Abstract The dynamics of primary production and particulate detritus cycling in the Columbia River Estuary are described, with particular reference to mechanisms that account for patterns within the water column, on the tidal flats, and in the adjacent wetlands. Analysis of patterns in phytoplankton flora and biomass and in distribution of detrital particulate organic matter (DPOC) in the water column indicated that salinities of 1–5 delineated an essentially freshwater flora from a marine or euryhaline flora, and that living phytoplankton was converted to DPOC at the freshwater-brackishwater interface. Similarly, the benthic diatom assemblages on tidal flats reflected either the fresh or the brackish nature of the water inundating the flats. Emergent vascular plants were grouped into six associations by cluster analysis, the associations being separated mainly on the bases of different relative abundances of freshwater, euryhaline or brackishwater species, and on whether samples occurred in high or low marsh areas. Annual rates of net areal 24-hr production averaged 55, 16, and 403gC m −2 y −1 for phytoplankton, benthic algae, and emergent vascular vegetation, respectively. Total production over the whole estuary was 17,667 metric tons C y −1 for phytoplankton, 1,545mt C y −1 for benthic algae, and 11,325mt C y −1 for emergent vascular plants, for a grand total to 30,537mt C y −1 . Phytoplankton biomass turned over approximately 39 times per year on average, while benthic algae turned over about twice and emergent plants once per year. Budgets for phytoplankton carbon (PPOC) and DPOC were developed based on PPOC and DPOC import and export, grazing loss, and in situ production and conversion of PPOC to DPOC. It is suggested that 36,205mt y −1 of PPOC is converted to DPOC in the estuary, principally at the freshwater-brackishwater interface. About 40,560mt y −1 of PPOC is exported to the ocean, and 159,185mt y −1 of DPOC is transported into the marine zone of the estuary (no data are available on DPOC export to the ocean). Thus, the estuary acts principally as a conduit for the transport of particles to the sea, and only secondarily as a converter of viable phytoplankton cells to detrital carbon and as a trap for DPOC.

Budgets and behaviors of uranium and thorium series isotopes in Santa Monica Basin sediments

Abstract Nine natural decay-series isotopes were measured in six box cores collected from a transect across the Santa Monica Basin. The 210 Pb-derived sedimentation rate decreases from ~80 mg/cm 2 -yr at the slope to ~20 mg/cm 2 -yr in the deep central basin. Sediment mixing prevails in sites underlying oxic waters, but is subdued in the anoxic deep basin below the sill depth. Uranium contents in sediments are controlled by levels of authigenic U, which are higher in the more reduced condition in the deep basin. Most of the authigenic U results from precipitation within the sediments. The 232 Th- 228 Th disequilibrium in sediments indicates that 228 Ra is lost from the sediments from a depth of ~ 10 cm upward. Modelling the distribution of excess 228 Th and 234 Th in the surficial layers of the deep basin sediments results in a mean sediment mixing coefficient of 0.2 cm 2 /yr and a sedimentation rate close to that based on 210 Pb. There is no evidence of changing sedimentation rate in the central basin during the past century. Fluxes of excess 210 Pb, 230 Th and 231 Pa to the central Santa Monica Basin sediments are much higher than what can be predicted from local supply. Advective input of open ocean waters coupled with enhanced scavenging of these reactive nuclides at the ocean margin is considered to be the primary cause.