Jeffrey M. Napp

The contribution of microorganisms to particulate carbon and nitrogen in surface waters of the Sargasso Sea near Bermuda

Abstract Seawater samples were collected from the euphotic zone of the Sargasso Sea near Bermuda in August of 1989 and March–April of 1990. Microbial population abundances, chlorophyll concentration, particulate carbon and particulate nitrogen were measured. Calculations were performed to establish the relative and absolute importance of the various microbial assemblages. The choice of conversion factors (g C and N cell −1 , or g C and N μm −3 ) for the microbial populations dramatically affected the estimation of “living” and “detrital” particulate material in the samples, and the relative importance of the various microbial groups. Averaged over all samples on either of the two cruises, microbial biomass constituted a greater proportion of the total particulate carbon and nitrogen during March–April (55% and 63%, respectively), than during August (≈24% and 30%, respectively) using “constrained” conversion factors that were derived. Accordingly, detrital material constituted the bulk of the particulate material during August, but was similar to the amount of microbial biomass during March–April. The bacterial assemblage constituted the largest single pool of microbial carbon (35%) and nitrogen (45%) in the water, and a significant fraction of the total particulate carbon (≈10–20%) and nitrogen (≈15–30%). Phototrophic nanoplankton (microalgae 2–20 μm in size) were second in overall biomass, and often dominated the microbial biomass in the deep chlorophyll maxima that were present during both cruises. The results temper recent assertions concerning the overwhelming importance of bacterial biomass in the oligotrophic Sargasso Sea but still support a major role for these microorganisms in the open ocean as repositories for carbon and nutrients.

Interannual and decadal variability in zooplankton communities of the southeast Bering Sea shelf

The southeastern Bering Sea shelf ecosystem is an important fishing ground for fin- and shellfish, and is the summer foraging grounds for many planktivorous seabirds and marine mammals. In 1997 and 1998, Northern Hemisphere climate anomalies affected the physical and biological environment of the southeastern Bering Sea shelf. The resulting anomalous conditions provided a valuable opportunity to examine how longer-term climate change might affect this productive ecosystem. We compared historical and recent zooplankton biomass and species composition data for the southeastern Bering Sea shelf to examine whether or not there was a response to the atmosphere–ocean–ice anomalies of 1997 and 1998. Summer zooplankton biomass (1954–1994) over the southeastern shelf did not exhibit a decline as previously reported for oceanic stations. In addition, zooplankton biomass in 1997 and 1998 was not appreciably different from other years in the time series. Spring concentrations of numerically abundant copepods (Acartia spp., Calanus marshallae, and Pseudocalanus spp.), however, were significantly higher during 1994–1998 than 1980–1981; spring concentrations of Metridia pacifica and Neocalanus spp. were not consistently different between the two time periods. Neocalanus spp. was the only taxon to have consistent differences in stage composition between the two time periods—CV copepodites were much more prevalent in May of the 1990s than early 1980s. Since relatively high zooplankton concentrations were observed prior to 1997, we do not attribute the high concentrations observed in the summers of 1997 and 1998 directly to the acute climate anomalies. With the present data it is not possible to distinguish between increased production (control from below) and decreased predation (control from above) to explain the recent increase in concentrations of the species examined.

Spatial and temporal changes in the partitioning of organic carbon in the plankton community of the Sargasso Sea off Bermuda

The vertical distribution of plankton (bacteria, nanozooplankton, microzooplankton, mesozooplankton, macrozooplankton and salps) biomass in the photic zone near the JGOFS time series station off Bermuda was examined during 2–3 week periods in August 1989 and in March/April 1990. The amount of phytoplankton carbon in the photic zone was lower in August as compared to March/April (398 and 912 mg C m−2, respectively). Total heterotrophic biomass in the photic zone was also lower in August as compared to March/April (1106 and 1795 mg C m−2, respectively). Taken together, bacteria and nanozooplankton constituted approximately 70% of the total heterotrophic carbon in the photic zone on both cruises. Considering their high weightspecific carbon demand relative to micro-, meso-, and macrozooplankton, it is clear that most of the carbon in the surface waters of the Sargasso Sea near Bermuda cycles through bacteria and flagellates—the “microbial loop”. However, both seasonal (August vs. March/April) and withincruise variations in the vertical flux of organic material were related to the biomass of macrozooplankton. Macrozooplankton biomass was lower in August than March/April (93 and 267 Mg C m−2, respectively). There was more non-living carbon (detritus) than living carbon in the photic zone during the August cruise (70% of total organic matter) but about equal amounts of detritus and living carbon in March/April.

Mesozooplankton of Shelikof Strait, Alaska : abundance and community composition

Zooplankton was sampled in Shelikof Strait and some of the surrounding shelf waters from March to October 1985, and then in Shelikof Strait during spring 1986–1989, using 150 and 333 μm mesh nets. The median integrated abundance of copepods in spring was 1–2 orders of magnitude greater in the Strait (maximum depth >250 m) than over shallower, adjacent regions of continental shelf. Zooplankton fauna was a mixture of oceanic and continental shelf taxa, and among the copepods was strongly influenced by abundance of two deep-water species,Neocalanus plumchrus andMetridia pacifica Seasonal development of the copepod community in Shelikof Strait followed similar patterns over the 5-year period. Biomass of copepods showed some large interannual differences related mostly to abundance of the oceanic taxa. Large interannual differences were found for some of the other zooplanktonic taxa as well, but there did not appear to be any correspondence between the patterns shown by these groups and those shown by the copepods. Although euphausiids were not quantitatively sampled, the taxonomic composition and relative abundance and seasonality of species within this group are reported.

Interannual and regional variability in distribution and ecology of juvenile pollock and their prey in frontal structures of the Bering Sea

Abstract The distribution, size, length-specific weight, growth, and feeding of age-0 walleye pollock ( Theragra chalcogramma ) were examined along with their prey distribution patterns in two contrasting transects over a 4-year period (1994–1997) in relation to biophysical properties of frontal regions around the Pribilof Islands, Bering Sea. There were significant interannual differences in catch of age-0 pollock, but transect and habitat differences (inshore vs. front vs. offshore) were not significant for either catch or size of pollock. There were significant variations in length-specific weight and growth of pollock, but the trends were inconsistent. Copepods dominated the zooplankton biomass in all habitats and years; there were no consistent differences in the densities of the dominant zooplankton taxa among the habitats. There were, however, strong habitat and transect differences in juvenile pollock diet, particularly for the larger and presumably rarer prey taxa (euphausiids, chaetognaths, fish). We did not find any evidence that occupying a particular habitat was beneficial to young pollock, although other factors (e.g. bioenergetic advantage and predation refuge) that we did not examine here could have been more variable and critical to pollock survival. In a physically dynamic system such as the Pribilof Islands, age-0 pollock may need to continuously search for optimal conditions of high prey availability and low predation pressure.

How Does Climate Change Affect the Bering Sea Ecosystem

The Bering Sea is one of the most productive marine ecosystems in the world, sustaining nearly half of U.S. annual commercial fish catches and providing food and cultural value to thousands of coastal and island residents. Fish and crab are abundant in the Bering Sea; whales, seals, and seabirds migrate there every year. In winter, the topography, latitude, atmosphere, and ocean circulation combine to produce a sea ice advance in the Bering Sea unmatched elsewhere in the Northern Hemisphere, and in spring the retreating ice; longer daylight hours; and nutrient-rich, deep-ocean waters forced up onto the broad continental shelf result in intense marine productivity (Figure 1). This seasonal ice cover is a major driver of Bering Sea ecology, making this ecosystem particularly sensitive to changes in climate. Predicted changes in ice cover in the coming decades have intensified concern about the future of this economically and culturally important region. In response, the North Pacific Research Board (NPRB) and the U.S. National Science Foundation (NSF) entered into a partnership in 2007 to support the Bering Sea Project, a comprehensive

Intra‐ and interannual measurements of zooplankton biomass in the Gulf of Alaska using bioacoustical sensors

52 million investigation to understand how climate change is affecting the Bering Sea ecosystem, ranging from lower trophic levels (e.g., plankton) to fish, seabirds, marine mammals, and, ultimately, humans. The project integrates two research programs, the NSF Bering Ecosystem Study (BEST) and the NPRB Bering Sea Integrated Ecosystem Research Program (BSIERP), with substantial in-kind contributions from the U.S. National Oceanic and Atmospheric Administration (NOAA) and the U.S. Fish and Wildlife Service.

Comparison and evaluation of methods for distinguishing among several functional classes of zooplankton and fish using acoustic backscatter data at four frequencies

Sensors that monitor temperature and salinity in the sea are commonly deployed for long periods on moorings. Meteorological measurements and surface conditions such as wave height are also routinely available from surface buoys. A few moorings include a variety of optical sensors, e.g., fluorometers and spectral backscattering sensors. Ocean currents, shear, and parameters that describe mixing are sensed over long periods by autonomous acoustic Doppler current profilers (ADCP). Bioacoustical devices, used to detect and study marine zooplankton and micronekton communities, are relatively new additions to the growing suite of ‘‘moorable’’ sensors. In each of 2002, 2003, and 2004, several months of continuous bioacoustical measurements of zooplankton were made concurrently with measurements of several physical parameters and meteorological variables at a single mooring site located south of Seward, Alaska in the coastal Gulf of Alaska. Volume scattering strength spectra were collected at 20‐min intervals wit...

Climate impacts on eastern Bering Sea foodwebs: a synthesis of new data and an assessment of the Oscillating Control Hypothesis

Zooplankton net samples data were collected at 30 locations near the Pribilof Islands, Alaska in September 1999 using a MOCNESS. Simultaneous volume backscatter data were collected with an HTI echosounder system at: 43, 120, 200, and 420 kHz. Three algorithms were compared for their ability to classify acoustic data into the dominant zooplankton functional/size groups (euphausiids, copepods, and ostra cods) and separating them from fish. The algorithms tested include: (1) combining morphological image processing and image differences to identify patches in different size ranges, and using the forward problem calibrated to plankton found in MOCNESS hauls to ascribe size/functional groups to the different patches; (2) application of canonical correlation between acoustics (backscatter volume and target strengths) and plankton biomass captured by the MOCNESS; and (3) application of inverse techniques to estimate the number of individuals in set size classes using multiple theoretical models of backscatter vo...