Nitrogen utilization during spring phytoplankton bloom development in the southeast Bering Sea

Thesis (Ph.D.) University of Alaska Fairbanks, 1983Interactions between a high latitude, continental shelf, spring phytoplankton bloom and water column physics and chemistry were studied using measured rates of nitrogen uptake. Peak bloom conditions commenced when the mixed layer shallowed and minimized respirational losses. Integrative light-mixing growth models were accurate during early bloom stages. An advection-diffusion model associated peak bloom nitrate uptake with pycnocline mixing rates of 2.1 m d * in an 18 m mixed layer. The accumulation of surface buoyancy was a reliable index of peak bloom temporal and spatial "patchiness" since mixing rates influenced both respirational losses and nutrient supply. Maximum nitrogen specific uptake rates (h r .- 1 ), unlike those of carbon, coincided with peak bloom conditions. Although species com positions among peak bloom periods were similar, particulate C/N ratios were not. Apparently, both intercellular factors and prevailing mixing conditions influence specific uptake rates and cell composition. A large proportion of new (nitrate) to total productivity was associated with the dominance of the early bloom forming diatoms in the mixed layer. In the absence of these net plankton the residual nanoplankton dominated community exhibited a greater dependence on regenerated nitrogen. Nitrate uptake averaged 700 mg-at m during the spring bloom and 1 g-at m-2 year-1 The yearly f factor was 0.40. Nitrogen uptake based carbon productivity was 188 g C m -2 year -1 A mass balance of the inorganic carbon system indicates that nitrate uptake alone cannot account for all the carbon leaving the surface layer. The correspondence between 1SN0~ uptake measurements and nitrate decreases suggests the diffusion of slope water into the middle shelf is slow. Large scale meteorological patterns may be responsible for the inter annual variability observed in production. Frequent May storm activity prolonged peak bloom periods, while calm conditions promoted extensive Chijl layers. The passage of atmospheric low pressure system s was also associated with the cross shelf "pumping" of water masses

Seasonal variation in light response of polar phytoplankton

The seasonality of light response curves was observed in phytoplankton samples taken in the Eastern Bering Sea (EBS) shortly before and during the spring bloom. Under-ice samples were found to have lower values of both the maximum growth rate (μ0) and the initial slope (α) of the photosynthesis-irradiance (PE) curve. This trend in α was also noted in a literature review of photoacclimation studies that looked at acclimation periods of 30 days or more. A trade-off is proposed between α and maintenance respiration such that below the compensation intensity EC it becomes advantageous to decrease α to mitigate the costs of respiration. An existing NPZD model of the EBS was then extended to reflect this trade-off with a seasonal transition from low to high α, and likewise μ0, at the point where available light is greater than EC. A parameter analysis found that with this seasonal plasticity the model could accurately reproduce the timing and magnitude of the 2009 spring bloom using parameter combinations within realistic ranges. Without this seasonality, no parameter set could be found that reasonably reproduced the observations. This strongly suggests that ecosystem models of phytoplankton should consider the effects of seasonality within parameters, including α which may be lower in over-wintering populations

Spring plankton dynamics in the Eastern Bering Sea, 1971-2050 : mechanisms of interannual variability diagnosed with a numerical model

A new planktonic ecosystem model was constructed for the Eastern Bering Sea based on observations from the 2007-2010 BEST/BSIERP (Bering Ecosystem Study/Bering Sea Integrated Ecosystem Research Program) field program. When run with forcing from a data-assimilative ice-ocean hindcast of 1971-2012, the model performs well against observations of spring bloom time evolution (phytoplankton and microzooplankton biomass, growth and grazing rates, and ratios among new, regenerated, and export production). On the southern middle shelf (57°N, station M2), the model replicates the generally inverse relationship between ice-retreat timing and spring bloom timing known from observations, and the simpler direct relationship between the two that has been observed on the northern middle shelf (62°N, station M8). The relationship between simulated mean primary production and mean temperature in spring (15 February to 15 July) is generally positive, although this was found to be an indirect relationship which does not continue to apply across a future projection of temperature and ice cover in the 2040s. At M2, the leading direct controls on total spring primary production are found to be advective and turbulent nutrient supply, suggesting that mesoscale, wind-driven processes - A dvective transport and storminess - may be crucial to long-term trends in spring primary production in the southeastern Bering Sea, with temperature and ice cover playing only indirect roles. Sensitivity experiments suggest that direct dependence of planktonic growth and metabolic rates on temperature is less significant overall than the other drivers correlated with temperature described above

Leaf respiratory CO2 is C-13-enriched relative to leaf organic components in five species of C-3 plants

Here, we compared the carbon isotope ratios of leaf respiratory CO2 (delta(13)C(R)) and leaf organic components (soluble sugar, water soluble fraction, starch, protein and bulk organic matter) in five C-3 plants grown in a glasshouse and inside Biosphere 2. One species, Populus deltoides, was grown under three different CO2 concentrations. The Keeling plot approach was applied to the leaf scale to measure leaf delta(13)C(R) and these results were compared with the delta(13)C of leaf organic components. In all cases, leaf respiratory CO2 was more C-13-enriched than leaf organic components. The amount of C-13 enrichment displayed a significant species-specific pattern, but the effect of CO2 treatment was not significant on P. deltoides. In C-3 plant leaves, C-13-enriched respiratory CO2 appears widespread. Among currently hypothesized mechanisms contributing to this phenomenon, non-statistical carbon isotope distribution within the sugar substrates seems most likely. However, caution should be taken when attempting to predict the delta(13)C of leaf respiratory CO2 at the ecosystem scale by upscaling the relationship between leaf delta(13)C(R) and delta(13)C of leaf organic components

Fluorescence, pigment and microscopic characterization of Bering Sea phytoplankton community structure and photosynthetic competency in the presence of a Cold Pool during summer

Spectral fluorescence measurements of phytoplankton chlorophyll a (Chl a), phytoplankton phycobilipigments and variable fluorescence (Fv/Fm), are utilized with High Performance Liquid Chromatography (HPLC) estimates of phytoplankton pigments and microscopic cells counts to construct a comprehensive picture of summer-time phytoplankton communities and their photosynthetic competency in the eastern Bering Sea shelf. Although the Bering Sea was ice-free during our study, the exceptionally cold winter that preceded the summer of 2008 when our cruise took place, facilitated the formation of a “Cold Pool” (<2 °C) and its entrapment at depth in the northern middle shelf. The presence of a strong pycnocline over the entire middle and outer shelves restricted inorganic nutrient fluxes into the surface waters resulting in phytoplankton populations that were photo-physiologically stressed due to nutrient limitation. Elevated Chl a concentrations recorded in the Green Belt along the shelf edge of the Bering Sea, were due to Phaeocystis pouchetii and nano-sized cryptophytes. Although inorganic nutrients were not limiting in the Green Belt, Fv/Fm values were low in all probability due to iron limitation. Phytoplankton communities in the low biomass surface waters of the middle shelf were comprised of prasinophytes, haptophytes, cryptophytes and diatoms. In the northern part of the middle shelf, a sinking bloom made up of the centric diatoms Chaeotoceros socialis, Thalassiosira nordenskioeldii and Porosira glacialis was located above the Cold Pool. The high biomass associated with this senescent bloom and its accretion above the pycnocline, suggests that the Cold Pool acts as a barrier, preventing sinking phytoplankton from reaching the bottom where they can become available to benthic organisms. We further posit that if summer-time storms are not energetic enough and the Cold Pool is not eroded, its presence facilitates the transfer of the large spring phytoplankton bloom to the pelagic ecosystem

Nutrient and Phytoplankton Dynamics on the Inner Shelf of the Eastern Bering Sea

In the Bering Sea, the nitrogen cycle near Nunivak Island is complicated due to limited nutrient replenishment across this broad shelf, and substantial nitrogen loss through sedimentary processes. While diffusion at the inner front may periodically support new production, the inner shelf in this region is generally described as a regenerative system. This study combines hydrographic surveys with measurements of nitrogen assimilation and benthic fluxes to examine nitrogen cycling on the inner shelf, and connectivity between the middle and inner shelves of the southern and central Bering Sea. Results establish the inner shelf as primarily a regenerative system even in spring, although new production can occur at the inner front. Results also identify key processes that influence nutrient supply to the inner shelf and reveal coupling between the middle shelf nutrient pool and production on the inner shelf

Influence of (sub)mesoscale eddies on the soft-tissue carbon pump.

In an idealized situation of a baroclinically unstable single eddy, we study the impact of eddy-induced mixing on the soft-tissue carbon pump. The new element here is the coupling of a three-dimensional nonhydrostatic ocean model with a physiological plankton model that is able to represent a variable plankton C:N ratio. During the development and breakup of the eddy, a complicated vertical velocity field appears. The processes of transport and plankton growth, as well as the effect of the flow on the C:N ratio, are studied in detail. The physical processes associated with eddy breakup have a strong impact on the local environment in which the plankton grows. The changes in the local environment lead to a decrease of the C:N ratio (about 30% throughout the upper 150 m of the domain) and hence a weakening of the soft-tissue carbon pump. According to a sensitivity analysis, the decrease of the C:N ratio as a consequence of the flow field appears robust; it does not depend on specific parameter values in the model. Copyright 2007 by the American Geophysical Union

A history of vegetation, sediment and nutrient dynamics at Tivoli North Bay, Hudson Estuary, New York

We conduct a stratigraphic paleoecological investigation at a Hudson River National Estuarine Research Reserve (HRNERR) site, Tivoli Bays, spanning the past 1100 years. Marsh sediment cores were analyzed for ecosystem changes using multiple proxies, including pollen, spores, macrofossils, charcoal, sediment bulk chemistry, and stable carbon and nitrogen isotopes. The results reveal climatic shifts such as the warm and dry Medieval Warm Period (MWP) followed by the cooler Little Ice Age (LIA), along with significant anthropogenic influence on the watershed ecosystem. A five-fold expansion of invasive species, including Typha angustifolia and Phragmites australis, is documented along with marked changes in sediment composition and nutrient input. During the last century, a ten-fold sedimentation rate increase due to land-use changes is observed. The large magnitude of shifts in vegetation, sedimentation, and nutrients during the last few centuries suggest that human activities have made the greatest impact to the marshes of the Hudson Estuary during the last millennium. Climate variability and ecosystem changes similar to those observed at other marshes in northeastern and mid-Atlantic estuaries, attest to the widespread regional signature recorded at Tivoli Bays

Nutrients, oxygen and inferred new production in the Northeast Water Polynya 1992

Nutrient and oxygen distributions were measured during a hydrographic survey of the Northeast Water Polynya off the northeast coast of Greenland (77–81°N, 6–17°W) during July–August 1992 and were interpreted in the context of satellite imagery of the region. Satellite imagery revealed a convoluted plume of cold water flowing along isobaths from underneath fast ice in the southwestern portion of the polynya toward the northeast. This plume carried relatively high nutrient and low oxygen inventories. Nitrate to phosphate ratios were low in the polar water, consistent with an ultimate source of this water mass in the Pacific sector of the Arctic Ocean. It is hypothesized that the low N:P Arctic outflow might be the cause of nitrate limitation along the east coast of America as far as Cape Hatteras. Gradients of both nutrients and oxygen inventories in the euphotic zone were observed along and across the axis of mean flow within the polynya and are shown to be due to net production of organic matter. On the basis of these spatial gradients of nitrate and oxygen, an assumed along-axis current velocity of 10 cm s−1, and the observed relationships of biologically removed inorganic carbon with nitrate and oxygen, the net organic matter production was estimated to be 40–60 mmol(C) m−2 d−1. This represents the organic carbon available for export from the polynya euphotic zone. Nutrient-deficient and oxygen-rich waters were observed merging with the southward flowing East Greenland Current, suggestive of possible export, however, the ultimate fate of organic carbon produced within the polynya requires further study