James A. Yoder

Effect of upwelling on phytoplankton productivity of the outer southeastern United States continental shelf

Abstract Gulf Stream frontal disturbances cause nutrient-rich waters to frequently upwell and intrude onto the southeastern United States continental shelf between Cape Canaveral, Florida and Cape Hatteras, North Carolina. Phytoplankton response in upwelled waters was determined with three interdisciplinary studies conducted during April 1979 and 1980, and in summer 1978. The results show that when shelf waters are not stratified, upwelling causes productive phytoplankton (diatom) blooms on the outer shelf. Phytoplankton production averages about 2 g C m −2 d −1 during upwelling events, and ‘new’ production is 50% or more of the total. When shelf waters are stratified, upwelled waters penetrate well onto the shelf as a subsurface intrusion in which phytoplankton production averages about fives times higher than the nutrient-depleted overlying mixed layer. Phytoplankton within the intrusion deplete upwelled NO 3 in about 7 to 10 days, at which point no further net increase in phytoplankton biomass occurs. Current meter records show that upwelling occurs roughly 50% of the time on the outer shelf during November to April (shelf not stratified), and we estimate that seasonal primary production in upwelled waters is 175 g C m −2 6 months −1 of which at least 50% is ‘new’ production. More than 90% of outer shelf primary and ‘new’ production occurs during upwelling and thus upwelling is the dominant process affecting primary productivity of the outer shelf. Our seasonal estimates of outer shelf primary and ‘new’ production are, respectively, three and ten times higher than previous estimates that did not account for upwelling.

Phytoplankton dynamics within Gulf Stream intrusions on the southeastern United States continental shelf during summer 1981

Abstract During July and August 1981 subsurface intrusion of upwelled nutrient-rich Gulf Stream water was the dominant process affecting temporal and spatial changes in phytoplankton biomass and productivity of the southeastern United States continental shelf between 29 and 32°N latitude. Intruded waters in the study area covered as much as 10 1 km including virtually all of the middle and outer shelf and approximately 50% of the inner shelf area. Within 2 weeks following a large intrusion event in late July, middle shelf primary production and Chl a reached 3 to 4 gC m − d −1 and 75 mg m − , respectively. At the peak of the bloom 80% of the water column primary production occurred below the surface mixed-layer, and new primary production (i.e., NO 3 -supported) exceeded 90% of the total. Chl a-normalized photosynthetic rates were very high as evidenced by high mean assimilation number (15.5 mg C mg Chl a −1 h −1 ), high mean α (14 mg C mg Chl a −1 Ein −1 m), and no photoinhibition. As a result of the high photosynthetic rates, mean light-utilization index (Ψ) was 2 to 3 times higher than reported for temperature sub-arctic and arctic waters. The results imply a seasonal (June to August) middle shelf production of 150 g C m −1 , about 15% higher than previous estimates of annual production on the middle shelf. Intrusions of the scale we observed in 1981 may not occur every summer. However, when such events do occur, they are by far the most important processes controlling summer phytoplankton dynamics of the middle and outer shelf and of the inner shelf in the southern half of the study area.

Phytoplankton and nutrient variability along a cross-shelf transect off Savannah, Georgia, U.S.A.

Monthly cruises were conducted for one year to examine temporal and spatial variability in nutrients, chlorophyll, and the relative rate of photosynthesis of the −1 ) close to shore with little cruise-to-cruise variability. At offshore stations (>20–30 km) nutrients and chlorophyll concentrations were generally low (nitrate m , chlorophyll −1 ). The outer shelf ( c . 100 km offshore), however, exhibited considerable variability. When we found upwelling at the shelf break, chorophyll a concentrations at the surface were as high as 6.0 μg l −1 . At other times surface chlorophyll at the same locations averaged 0.1–0.2 μg l −1 with nitrate concentrations m . Phytoplankton >10 μm dominated the near shore stations. The Skeletonema costatum and Asterionella japonica were found in high abundance. Our study shows that phytoplankton abundance exhibits little seasonal variability in the Georgia Bight, but that upwelling events induce high frequency (days to weeks) variability on the outer shelf.

Statistical analysis of the distribution of fish eggs and larvae on the southeastern U.S. continental shelf with comments on oceanographic processes that may affect larval survival

Abstract Data collected in 1953 and 1954 by the U.S. Fish and Wildlife Service were statistically analyzed to ascertain where and when fish eggs and larvae are most abundant on the southeastern U.S. continental shelf. The results are related to some oceanographic processes that might affect the survival of larval fish. Along-shelf differences in numbers of eggs and larvae are minimal compared with differences that occur across the shelf. Highest numbers of fish larvae are found on the outer shelf during fall, winter and spring, but larvae are evenly distributed across the shelf in summer. Upwelling strongly influences the dynamics of plankton production on the outer shelf, and thus during most seasons of the year upwelling may be the most important process controlling the amount of food available to larval fish. During winter and spring, mean winds do not favor shoreward transport of larval fish from the outer shelf if the larvae are located in near-surface waters. Thus, during these seasons variability of winds on the ‘event’ time scale may be more important to onshore and offshore transport of larval fish than the mean strength and direction of monthly or seasonally averaged winds.

Phytoplankton advection off the southeastern United States continental shelf

Abstract We observed an anomalous water mass below 100 m over the continental slope just seaward of the southeastern U.S. continental shelf. The water mass was colder than expected from T-S relations and contained relatively high concentrations of phytoplankton compared to surrounding waters. Hydrographic and biological characteristics indicated that the water mass was formed from waters that upwelled into the euphotic zone on the outer shelf. Based on our analyses, we conclude that the atmosphere extracted heat from the upwelled water thereby increasing its density and causing it to cascade (advect) off the shelf. Our preliminary calculations indicate that this process could represent a significant off-shelf flux of phytoplankton carbon during winter.

Nutrient limitation of phytoplankton growth in Georgia nearshore waters

Nutrient enrichment experiments were conducted to investigate the utilization of dissolved organic and inorganic nitrogen by marine phytoplankton in Georgia coastal waters. Natural populations of marine phytoplankton, enriched with different concentrations of ammonium chloride and other plant nutrients, were grown under controlled temperature and irradiance conditions until the populations reached “stationary phase.” Results showed that (1) phytoplankton are limited by DIN up to ca. 20μM, when another nutrient (phosphate or silicate) becomes limiting, (2) very little naturally-occurring DON is directly utilized for growth, (3) very little DON is indirectly made available for growth over time periods of days to ca. 1 week, and (4) trace metals and vitamins do not significantly limit phytoplankton growth.

Climatological and hydrographic influences on nearshore food webs off the southeastern United States: bacterioplankton dynamics

Abstract Bacterioplankton productivity, numbers, and cell specific activity were studied in nearshore waters of the southeastern U.S. continental shelf during seasons of maximum freshwater discharge. In April 1984, coastal waters were stratified from normal spring discharge and typical northeastward wind stress. In April 1985, shelf waters were vertically homogeneous due to below normal runoff and southwestward wind stress. In 1984, nearshore bacterial productivity ranged from 7.0 to 14.7 × 10 6 cells l −1 h −1 and midshelf rates were 40–50% less. In 1985, nearshore productivity ranged from 0.9 to 2.4 × 10 6 cells 1 −1 h −1 , and productivity was extremely patchy over the entire shelf. The cell-specific activity (thymidine incorporation per cell) suggests that although productivity was high in 1984, only a fraction of the bacterioplankton was actively growing or incorporating thymidine (0.9–2.9 × 10 −21 mol cell −1 h −1 ). In 1985, a higher percentage of cells appeared to be active and incorporating thymidine (5–13 × 10 −21 mol cell −1 h −1 ) even though productivity was low. Hydrographic conditions along the southeastern coastline may have had a significant impact on the overall community structure and carbon flow through the microbial food web. When coastal waters were stratified in 1984, bacterial biomass was a significant percentage (35–320%) of the phytoplankton biomass. During vertically homogeneous conditions of 1985, bacterial production and biomass were a small percentage (2–13%) of the phytoplankton production and biomass across the shelf. The interannual variation in the microbial food web was attributed to the interannual variability of the southeastern U.S. hydrology due to changes in freshwater discharge and wind direction and intensity. The ecological implications of these results extend to the potential impact of seasonal microbial food webs on nearshore allochothonous and autochothonous organics before removal from the southeastern U.S. coastline.

Interactions of phytoplankton, zooplankton and microorganisms

Abstract We present evidence that there are significant interactions between heterotrophic microorganisms, doliolids and Fritillaria within intrusions of nutrient-rich Gulf Stream water stranding on the continental shelf. During the summer of 1981 cold, nutrient-rich water from below the surface of the Gulf Stream was repeatedly intruded and stranded on the continental shelf off northeastern Florida. On August 6 old, stranded Gulf Stream water depleted of nitrate occupied the lower layer on the outer shelf. The upper water was continental shelf water, older but of undefined age. On August 6 free-living bacteria were >106ml−1 everywhere at all depths, an order of magnitude greater than normal bacterial numbers on the northeastern Florida continental shelf. Over 10 days the numbers of free bacteria doubled while bacteria attached to particles increased by a factor of four. The adenylate/chlorophyll ratio showed that phytoplankton dominated the lower layers of intruded water, while the surface water became increasingly dominated by heterotrophic microorganisms (bacteria and protozoa) over 10 days. There were significant, negative correlations between bacteria and doliolids and between bacteria and Fritillaria. Regions of maximum bacterial numbers did not coincide with locations of salp swarms. The increased numbers of bacteria at all depths in a highly stratified system in which most phytoplankton are in the lower layer suggests a diverse source of bacterial growth substrates, some of which involve zooplankton as intermediaries. Production of autotrophs is more than twice that of microheterotrophs on average, but because of their differential distribution, microheterotrophs are the dominant biomass in much of the surface water and may be significant in energy flux to metazoan consumers as well as competitors for mutually useable sources of nutrition.