OS32B-01 INVITED
Physical and Biological Controls on the Invasion Speed of a Marine Invader with Planktonic Larvae, Illustrated with Examples From Carcinus maenas in the Gulf of Maine
Existing estimates of the downstream invasion speed of newly introduced species or novel genotypes with planktonic larvae have usually found that they invade much less rapidly downstream than would be expected from the mean alongshore currents in their habitat. For example, a new, genetically-distinct population of Carcinus maenas was introduced at the northern-edge of its range in Cape Breton, Nova Scotia (Roman 2006). Recently, we have found that this newly introduced genotype is being transported south- westward along the coast by the prevailing currents, displacing established populations of C. maenas. However, the rate of southward spread of the northern genotype is much less than what we would expect from the mean currents along the Scotian Shelf and Gulf of Maine. We examine four reasons for errors in the naive estimates of the rate of spread of an introduced species or genotype from mean currents. 1) Cross-shelf shear in the alongshore currents can interact with cross-shelf dispersion of larvae to bias larval settlement towards larvae which have spent more time in the weaker nearshore currents. 2) Weakly retentive embayments and other coastal features, even if unable to retain larvae for more than a small fraction of their planktonic duration, will tend to reduce the alongshelf transports of those larvae and increase the likelihood that they remain close enough to the coast to successfully recruit. This will reduce the mean alongshore transport of the larvae that recruit. 3) Alongshore gradients in the population density of competing species/genotypes will, if the population density increases in the direction of the mean currents, tend to retard the downstream spread of an introduced species/genotype. 4) Greater relative fitness or competitiveness in the downstream genotype/species will retard the spread of the upstream genotype/species. Examples of and evidence for and against these possible explanations will be provide for C. maenas in the Gulf of Maine
OS32B-02 INVITED
Biological and Physical Forcing of Picophytoplankton on the New England Shelf: New Insights From Submersible Flow Cytometry at the Martha's Vineyard Coastal Observatory
Phytoplankton communities of temperate continental shelf systems typically exhibit dramatic seasonal variations. We are taking advantage of the Martha's Vineyard Coastal Observatory (MVCO), a cabled facility on the New England inner shelf, to better understand the physical and biological processes that interact to produce this variability. Our approach depends on high resolution (~hourly) multi-year time series of taxonomically resolved phytoplankton acquired with FlowCytobot and Imaging FlowCytobot, custom-built automated submersible flow cytometers optimized for measurement of picophytoplankton and microphytoplankton, respectively. These observational capabilities enable new approaches to understanding classical problems in plankton ecology. Our time series document a seasonal shift from microphytoplankton (especially large, often chain-forming diatoms) in fall and winter to pico- and small nanophytoplankton in spring and summer. The shift is associated not only with a decline in large cell abundance but also with a dramatic increase (100- to 1000- fold) in small cells. This seasonal pattern reflects a regional-scale phenomenon that emerges despite affects of advection and spatial patchiness. To begin quantifying the factors that produce these seasonal patterns, we have focused first on the cyanobacterial genus, Synechococcus, the numerically dominant picoplankter in this coastal system. In contrast to prevailing views in picoplankton ecology, the annual Synechococcus bloom in this system suggests a persistent decoupling from grazer control in spring. Synechococcus population growth rates, determined from diel changes in cell size distributions, show strong seasonal variability associated with physiological consequences of environmental conditions. Low growth rates are linked to temperature limitation in winter/early spring and a combination of temperature and light limitation in fall associated with decreasing solar angles, seasonal cooling, and breakdown in stratification. Despite low macronutrient concentrations during stratified summer conditions, observed growth rates are high, suggesting little or no role for nutrient limitation in contributing to picoplankton seasonality. In on-going work, we are focusing attention on the frequency and amplitude of diatom bloom events, which appear to be more dependent on processes that supply nutrients to the inner shelf.
OS32B-03
Investigation of the Gulf of Maine Circulation and Alexandrium fundyense Bloom in summer 2006: In Situ Observations and Numerical Modeling
In situ observations and a coupled bio-physical model are used to study the Gulf of Maine (GOM) coastal ocean hydrodynamics, and the initiation, and development of the Alexandrium fundyense bloom in 2006. Hydrographic measurements indicate that 2006 is a year with normal coastal water temperature, salinity, current and river runoff conditions in comparison with the GOM climatology. A. fundyense cyst observations preceding the 2006 bloom show lower concentrations compared to other recent survey data. A coupled bio- physical model is used to hindcast coastal hydrodynamics and A. fundyense cell concentrations. Field data including water temperature, salinity, velocity time series and surface cell concentration maps are used to gauge the model's fidelity, indicating this coupled model is able to reproduce coastal circulation and the temporal and spatial distributions of A. fundyense cell concentration reasonably well. Model hindcast solutions are further used to diagnose physical and biological factors controlling the bloom dynamics. The initial cyst distribution is found to be the key factor controlling the severity of the bloom. Surface wind fields play an important role in modulating bloom's horizontal and vertical distribution. During the bloom's initiation phase, seasonal warming is a major factor determining the net growth rate. Later in the season, nutrient availability and cell mortality collectively control three-dimensional bloom distribution.
OS32B-04
Toward the Characterization of Upwelling and Productivity Potential Along the California Coast.
Central and northern California coastal temperature time series were combined with temperature-nitrate relationships, obtained from the literature and other available data sets, to generate a set of "proxy nitrate" time series from shallow (often pier-based) locations along the coast. These time series allow us to assess along-coast variability in the primary production potential of upwelling over the region. When fluorescence data are also available we determine which bloom events appear to respond directly to nutrient forcing and which do not. A significant strength of the observing network is its vast scale: the importance of oceanographic/offshore forcing versus local forcing for shallow bloom events can be examined concurrently for the entire coast. Such work will improve development of future primary productivity indices, but also points to the need for coastal observing systems to make more direct measurements of nutrients and Chlorophyll.
OS32B-05
Quantifying Connectivity in the Coastal Ocean
The quantification of coastal connectivity is important for a wide range of real-world applications ranging from marine pollution to nearshore fisheries management. For these purposes, coastal connectivity is best defined as the probability that water parcels from one nearshore location are advected to another site over a given time interval. Here, we demonstrate how to quantify coastal connectivity using Lagrangian probability- density function (PDF) methods, a classic modeling approach for many turbulent applications, and numerical solutions of coastal circulation for the Southern California Bight. Mean dispersal patterns from a single release site (or Lagrangian PDFs) show a strong dependency to the particle-release location and seasonal variability, reflecting circulation patterns in the Southern California Bight. Strong interannual variations, responding to El Nino and La Nina transitions are also observed. Mean connectivity patterns, deduced from Lagrangian PDFs, is spatially heterogeneous for the advection time of around 30 days or less, resulting from distinctive circulation patterns, and becomes more homogeneous for a longer advection time. A given realization of connectivity is stochastic because of eddy-driven transport and synoptic wind forcing changes. In general, mainland sites are good sources while both Northern and Southern Channel Islands are poor source sites, although they receive substantial fluxes of water parcels from the mainland. The predicted connectivity gives useful information to ecological and other applications for the Southern California Bight (e.g., designing marine protected areas, understanding gene structures, and predicting the impact of a pollution event) and provide a path for assessing connectivity for other regions of the coastal ocean.
OS32B-06
Role of Wind Forcing in Estuarine-Shelf Exchange and Hypoxia in a Partially-Mixed Estuary
Episodic hypoxia in Narragansett Bay, Rhode Island disrupts ecological processes and raises concerns for environmental managers. To better understand the drivers of hypoxia in this estuarine system, the Narragansett Bay Coastal Hypoxia Research Program (NB-CHRP) is developing a hybrid physical-ecological model. In this presentation, we will describe physical processes that are relevant to the larger NB-CHRP project goals, using model results and field observations. We adapted the Regional Ocean Modeling System, a three-dimensional hydrodynamic model, to the Narragansett Bay and neighboring shelf system. In addition to modeling, we collected Acoustic Doppler Current Profiler time series from the head of the estuary to the inner shelf during summer months. Both the model and observational data suggest that the hypoxic events are strongly linked to physical circulation patterns that control flushing and exchange rates. A key aspect of flushing is the intrusion of deep, oxygenated waters from the continental shelf into the estuary. We examined the environmental conditions that promote the intrusion of shelf water and the fate of that water mass in the estuary. New field data and preliminary model results emphasize the importance of wind forcing on the retention of oxygen-depleted water within, and the resupply of source water to regions of the estuary that experience hypoxia. Wind conditions strongly influence whether the sources of resupplied water come from other impacted regions of the estuary or from the less impacted water of the shelf.
OS32B-07
Numerical simulations of buoyancy driven flow on the Texas-Louisiana continental shelf
A realistic hydrodynamic numerical model of the Mississippi/Atchafalaya river plume system is used to examine the effects of seasonal hypoxia on the Texas-Louisiana continental shelf in the context of physical processes. Circulation over the Texas-Louisiana continental shelf is dominated by the Mississippi/Atchafalaya River plume system. The Mississippi/Atchafalaya River plume system is unique in that there are two large fresh water sources, with the Mississippi River plume forming a surface trapped plume near the shelf edge, whereas the Atchafalaya empties into a broad, shallow shelf and forms a bottom trapped plume. Results show that simple parameterizations of respiration can reproduce the spatial and temporal structure of hypoxia on the shelf. Also, the model suggests that different kinds of respiration are important over different shelf regions: bottom respiration is important in controlling hypoxia over the broad, shallow region south of Atchafalaya Bay, water column respiration is important in creating hypoxia in the Louisiana Bight, west of the Mississippi River Delta. Thus, physical processes are critical in the creation, maintenance, and destruction of seasonal hypoxia on the Texas-Louisiana shelf.
OS32B-08
Spatial and Temporal Variability Analyses of the Chesapeake Bay Outflow Plume with Satellite Ocean Color Data
Estuarine outflow plume plays an important role in mixing processes between estuaries and the continental shelf. The Chesapeake Bay outflow plume has been subjected to intensive research, particularly in field investigation and numeral modeling. This study identifies the outflow plume from the Chesapeake Bay by utilizing satellite-based ocean color data (SeaWiFS and MODIS). By analyzing 11 years of SeaWiFS and 6 years of MODIS satellite observation data, the spatial and temporal characteristics of the Chesapeake Bay outflow plume are described. The general spatial pattern of the Chesapeake Bay outflow plume is presented, and the seasonal variability is determined. The time-averaged plume dynamic behaviors in the inner continental shelf are discussed by examining other related oceanographic data. In this study, the long-term various spatial and temporal analyses of the ocean color data are performed by the Giovanni, which is a robust, multifunctional, and easy-to-use web application used to visualize, analyze, and access the Earth science data.