James P. Manning

Suppression of the 2010 Alexandrium fundyense bloom by changes in physical, biological, and chemical properties of the Gulf of Maine

For the period 2005-2009, the abundance of resting cysts in bottom sediments from the preceding fall was a first-order predictor of the overall severity of spring-summer blooms of Alexandrium fundyense in the western Gulf of Maine and southern New England. Cyst abundance off mid-coast Maine was significantly higher in fall 2009 than it was preceding a major regional bloom in 2005. A seasonal ensemble forecast was computed using a range of forcing conditions for the period 2004-2009, suggesting that a large bloom was likely in the western Gulf of Maine in 2010. This did not materialize, perhaps because environmental conditions in spring-summer 2010 were not favorable for growth of A.fundyense. Water mass anomalies indicate a regional-scale change in circulation with direct influence on A. fundyenses niche. Specifically, near-surface waters were warmer, fresher, more stratified, and had lower nutrients than during the period of observations used to construct the ensemble forecast. Moreover, a weaker-than-normal coastal current lessened A. fundyense transport into the western Gulf of Maine and Massachusetts Bay. Satellite ocean color observations indicate the 2010 spring phytoplankton bloom was more intense than usual. Early-season nutrient depletion may have caused a temporal mismatch with A. fundyenses endogenous clock that regulates the timing of cyst germination. These findings highlight the difficulties of ecological forecasting in a changing oceanographic environment, and underscore the need for a sustained observational network to drive such forecasts.

Historical and recent evidence of Scotian Shelf Water on southern Georges Bank

Abstract Historical data from 1912–1987 indicate that low salinity (less than 32.0 psu) near-surface water occurs occasionally on southern Georges Bank during May, while none occurs during April. Composite monthly plots of historical near-surface salinity and estimated advection rates show that the southwestern Scotian Shelf is the immediate upstream source of this low salinity water. Optimally interpolated satellite-derived SST and hydrographic data reveal that very cold (less than 2.0°C), low salinity (less than 32.0 psu) Scotian Shelf Water (SSW), initially located south of the 200-m isobath off southern Georges Bank in early March 1992, moved north onto southern Georges Bank during April 1992. SSW was not as extensive during the same period in 1993, as evidence by significantly higher temperatures and salinities. These differences show large interannual variability in the transport and/or properties of SSW flowing onto Georges Bank. Lower (higher) salinities measured during spring 1992 (1993) on southern Georges Bank are consistent with higher (lower) St Lawrence River discharge noted during spring 1991 (1992) and the ∼ nine month lag between annual discharge maxima from rivers located upstream and minimum salinity at Cape Sable in February. However, comparisons between historical occurrences of low salinity (less than 32.0 psu) SSW on southern Georges Bank noted for May 1966, 1971, and 1978, and cumulative St Lawrence River discharge from the spring prior to each occurrence, show no relationship. This suggests that the occurrence of low salinity water on southern Georges Bank is not directly related to variations in upstream river discharge. Although an idealized model of wind forcing on a bank-trapped density front shows that near-surface flow is related to superposition of an Ekman layer and the along-bank gyral circulation, evidence from late winter-spring 1992 and 1993 on Georges Bank shows that except for on-bank movement of the shelf-slope front during late spring 1992, simple Ekman theory does not explain on-bank movement of the SSW plume during 1992 nor the immobile nature of the shelf-slope front during late winter 1992 and late winter-spring 1993. Surface heat flux and satellite-derived SST from Georges Bank and the Scotian Shelf together with a slab mixed-layer model allows estimation of the advective heat flux and transport of SSW flowing onto Georges Bank from the Scotian Shelf during late winter-spring 1992. Mean advective heat flux onto Georges Bank during March–May 1992 is −14.5 mW cm−2 with an estimated error of ∼3.5 mW cm−2, demonstrating that advection of cold SSW onto Georges Bank is required. Mean velocity and transport of SSW flowing onto Georges Bank during March–May 1992 are 13.4 cm s−1 and 0.21 Sv, assuming a SSW depth (width) of 40 m (40 km), demonstrating that flow of SSW onto Georges Bank was robust during the period. At this time, the dynamics that control the flow of SSW across Northeast Channel are unknown.

Drifter observations of the Gulf of Maine Coastal Current

Two-hundred and twenty seven satellite-tracked drifters were deployed in the Gulf of Maine (GoM) from 1988 to 2007, primarily during spring and summer. The archive of tracks includes over 100,000 kilometers logged thus far. Statistics such as transit times, mean velocities, response to wind events, and preferred pathways are compiled for various areas of the coastal GoM. We compare Lagrangian flow with Eulerian estimates from near-by moorings and evaluate drifter trajectories using Ekman theory and 3-D ocean circulation models. Results indicate that the Gulf of Maine Coastal Current is a strong and persistent feature centered on the 94 ± 23 meter isobath, but that particles: a) deviate from the seasonal-mean core fairly regularly, and are often re-entrained; b) follow a slower (9 cm/s), less-constrained path in the western portion off the coast of Maine relative to the eastern (16 cm/s) section; and c) can be affected by wind events and small scale baroclinic structures. Residence times calculated for each ½ degree grid cell throughout the GoM depict some regions (Eastern Maine and Western Nova Scotia) as being relatively steady, flow-through systems, while others (Penobscot, Great South Channel) have more variable, branching pathways. Travel times for drifters that are retained within the coastal current along the entire western side of the Gulf of Maine are typically less than two months (55 days).

Seasonal and interannual variability in the properties of the surface waters of the Gulf of Maine

Abstract The MARMAP hydrographic data set (1977–1987) is used to determine the mean annual cycle of temperature, salinity, and density structure of surface waters throughout the Gulf of Maine. The temperatures follow the expected seasonal warming and cooling pattern. In the eastern Gulf the salinity cycle is dominated by influx of low salinity Scotian Shelf water which enters near Cape Sable in the winter, and in the western Gulf by the local spring runoff. Phasing of temperature and salinity cycles in different parts of the Gulf results in the western Gulf of Maine being more strongly stratified in the summer and more vertically uniform in the winter than is the eastern Gulf. The interannual variability, derived by subtracting the annual cycles from the original data, reveals relatively little temperature variability (1–2°C) during the period 1977–1987, compared to observed changes of 4–6°C in previous decades. Large interannual changes in salinity (0.5 psu), however, are evident in the data. The salinity variability is shown to be due primarily to changes in local fresh water sources—precipitation and runoff. Comparison of salinity changes in the Gulf of Maine with data from Georges Bank and the Middle Atlantic Bight shows that the salinity variability is coherent over the northeast continental shelf region from the western Gulf (Wilkinson Basin) to Cape Hatteras.

Middle Atlantic Bight salinity: interannual variability

Abstract Twenty-eight hydrographic surveys of the Middle Atlantic Bight (MAB) were conducted during 1977–1987 as part of the National Marine Fisheries Services Marine Resources, Monitoring, Assessment, and Prediction program. The average temperature and salinity for Shelf Water (

Data assimilative hindcast of the Gulf of Maine coastal circulation

[1] A data assimilative model hindcast of the Gulf of Maine (GOM) coastal circulation during an 11 day field survey in early summer 2003 is presented. In situ observations include surface winds, coastal sea levels, and shelf hydrography as well as moored and shipboard acoustic Doppler D current profiler (ADCP) currents. The hindcast system consists of both forward and inverse models. The forward model is a three-dimensional, nonlinear finite element ocean circulation model, and the inverse models are its linearized frequency domain and time domain counterparts. The model hindcast assimilates both coastal sea levels and ADCP current measurements via the inversion for the unknown sea level open boundary conditions. Model skill is evaluated by the divergence of the observed and modeled drifter trajectories. A mean drifter divergence rate (1.78 km d � 1 ) is found, demonstrating the utility of the inverse data assimilation modeling system in the coastal ocean setting. Model hindcast also reveals complicated hydrodynamic structures and synoptic variability in the GOM coastal circulation and their influences on coastal water material property transport. The complex bottom bathymetric setting offshore of Penobscot and Casco bays is shown to be able to generate local upwelling and downwelling that may be important in local plankton dynamics.

Model simulations of the Bay of Fundy Gyre: 2. Hindcasts for 2005–2007 reveal interannual variability in retentiveness

A persistent gyre at the mouth of the Bay of Fundy results from a combination of tidal rectification and buoyancy forcing (Aretxabaleta et al., J. Geophys. Res., vol. 113, 2008). Here we assess interannual variability in the strength of the gyre using data assimilative model simulations. Realistic hindcast representations of the Gyre are considered over the course of cruise surveys in 2005, 2006 and 2007. Assimilation of shipboard and moored ADCP velocities are used to improve the skill of the simulations, as quantified by comparison with non-assimilated drifter trajectories. Our hindcast suggest a weakening of the Gyre system during May 2005. Retention of simulated passive particles in the Gyre during that period was highly reduced. A recovery of the dense water pool in the deep part of the basin by June 2006 resulted in a return to particle retention characteristics similar to climatology. Retention estimates reached a maximum during May 2007 (sub-surface) and June-July 2007 (near-surface). Interannual variability in the strength of the gyre was primarily modulated by the stratification of the dense water pool inside the Grand Manan Basin. These changes in stratification may be attributed to mixing conditions the preceding fall/winter and/or advectively-driven modification of water mass properties.

Evidence for vertical circulation cells in the well-mixed area of Georges Bank and their biological implications

Two surveys were conducted in the well-mixed region of Georges Bank to look for secondary vertical circulation cells, the first in 1996 and the second in 1997. Each survey collected high-frequency acoustic, temperature, and fluorescence data along a 1-n.mile square grid. Concurrent ADCP measurements also were made in the second year. MOCNESS and pump samples from both years caught large amounts of sand and organisms typical of this regions such as copepods and hydroids. However, forward problem calculations suggest that the acoustic scattering was dominated by post-larval bivalves. Sand and copepods also accounted for significant amounts of the estimated backscatter. The acoustic data from both surveys contained near-surface vertical bands of high-volume backscatter. The frequency and intensity of these bands was strongly correlated with the magnitude of the current velocity. Significant upwelling and downwelling were observed in the ADCP records, and the acoustic bands often co-occurred in the downwelling zones. Simulations of particle distributions within idealized circulation cells, consistent with the acoustic and ADCP data, suggest that the acoustic bands are caused by aggregations of positively buoyant or upward-swimming scatterers. The circulation cells proposed could have an important effect on the ecology of the well-mixed region by aggregating upward-swimming fish and zooplankton in near-surface patches.

Assessment of Georges Bank recirculation from Eulerian current observations in the Great South Channel

Abstract Twenty Vector Averaging Current Meters (VACMs) were deployed at various depths in the vicinity of the Great South Channel during several experiments conducted between 1976 and 1986 (average record length ∼ 5 months). Since little more than mean current vectors from some records has been reported previously, a basic description of the flow field is presented here. With at least one current record occurring in every month of the year, the hypothesis of increased subtidal flow around Georges Bank during the stratified season is supported. Anemometer data obtained at nearby NOAA environmental buoys and the Nantucket Lightship are used to estimate wind stress, the primary forcing in the well-mixed season. Empirical modes of current structure demonstrate the spatial structure and the seasonal variability of wind and density-driven dynamics in this region. When averaged over a few months, the northward transport through the Great South Channel (fall mean ∼ 0.095 Sv) is small compared to the westward transport along the southern flank of Georges Bank (fall mean ∼ 0.421 Sv), but the subtidal variability of the Great South Channel transport is highly energetic, with a standard deviation (∼ 0.11 Sv) greater than the mean, indicating that large reversals in transport can occur over a few days.

Particles in the coastal ocean : theory and applications

Part I. Background: 1. The coastal ocean 2. Drifters and their numerical simulation 3. Probability and statistics - a primer 4. Dispersion by random walk 5. BCs, boundary layers, sources 6. Turbulence closure Part II. Elements: 7. Meshes: interpolation, navigation, and fields 8. Particles and fields Part III. Applications: 9. Noncohesive sediment - dense particles 10. Oil - chemically active particles 11. Individual-based models - biotic particles Part IV. Appendixes.