David G. Mountain

Effect of small-scale turbulence on feeding rates of larval cod and haddock in stratified water on Georges Bank

Altatract-A set of vertically stratified MOCNESS tows made on the southern flank of Georges Bank in spring 1981 and 1983 was analyzed to examine the relationship between larval cod and haddock feeding success and turbulent dissipation in a stratified water column. Observed feeding ratios (mean no. prey larval gut-‘) for three size classes of larvae were compared with estimated ingestion rates using the Rothschild and Osbom (Journal of Plankton Research, 10, 1988,465-474) predator-prey encounter rate model. Simulation of contact rates requires parameter estimates of larval 6sh and their prey cruising speeds, density of prey, and turbulent velocity of the water column. Turbulent dissipation was estimated from a formulation by James (Estuarine and Coastal Marine Science, 5, 1977,339-353) incorporating both a wind and tidal component. Larval ingestion rates were based on swallowing probabilities derived from calm-water laboratory observations. Model-predicted turbulence profiles generally showed that dissipation rates were low to moderate (10-“-10-7 W kg-‘). Turbulence was minimal at or below the pycnocline (a25 m) with higher values (l-2 orders of magnitude) near the surface due to wind mixing and at depth due to shear in the tidal current near bottom. In a stratified water column during the day, first-feeding larvae (5-6 mm) were located mostly within or above the pycnocline coincident with their copepod prey (nauplii and copepodites). The 7-8 mm larvae were most abundant within the pycnocline, whereas the 9-10 mm larvae were found within and below the pycnocline. Feeding ratios were relatively low in early morning following darkness when the wind speed was low, but increased by a factor of 2-13 by noon and evening when the wind speed doubled. Comparison of depth-specific feeding ratios with estimated ingestion rates, derived from turbulence-affected contact rates, generally were reasonable after allowing for an average gut evacuation time (4 h), and in many cases the observed and estimated values had similar profiles. However, differences in vertical profiles may be attributed to differential digestion time, pursuit behavior tie&d by high turbulence, vertical migration of the larger larvae, an optimum light level for feeding, smaller-scale prey patchiness, and the gross estimates of turbulence. Response-surface estimation of averaged feeding ratios as a function of averaged prey density (O50 m) with a minimum water-column turbulence value predicted that 5-6mm larvae have a maximum feeding response at the highest prey densities (> 30 prey 1-i) and lower turbulence estimates ( 10 prey I- *) as turbulence increases to intermediate levels, clearly showing an interaction effect. In general, maximum feeding ratios occur at low to intermediate levels of turbulence where average prey density is greater than 10-20 prey 1-l. Copyright 0 1996 Elsevier Science Ltd

Acoustical study of the spatial distribution of plankton on Georges Bank and the relationship between volume backscattering strength and the taxonomic composition of the plankton

Abstract High frequency (420 kHz) sound was used to study the volume backscattering from plankton and micronekton over Georges Bank as part of a study designed to determine the correlation length scales of plankton spatial patterns in relation to physical structure and to intercompare different kinds of sampling and remote-sensing instrumentation. Two physically distinct areas were studied: a well-mixed area in a shallow portion of the Bank and a stratified area on the deeper southern flank of the Bank. A submersible echo sounder with a down-looking transducer was mounted in a towed V-fin. Volume backscattering data were collected from near the sea surface to the bottom (40–80 m). Vertical and horizontal volume backscattering structure in the stratified region differed from that in the well-mixed area in both mean and variance, providing evidence that physical forcing of the pattern varied significantly between the two areas. Internal waves appeared to modulate the depth of dense mid-depth volume scattering layers in the stratified sites. In the mixed area, there was little horizontal layering or coarse-scale horizontal structure. However, fine-scale vertical lineations were evident with horizontal length scales on the order of the depth of the water column. One hypothesis to explain these vertical lineations in the well-mixed areas involves the development of secondary vertical circulation cells associated with the tidal flows over a rough bottom. Although volume backscattering at the stratified sites was 4–7 times higher than at the mixed site, there was no significant difference in MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) collected biovolumes between these locations. The difference in volume backscattering was due to differences in both the acoustic scattering properties of zooplankton taxa and the taxonomic composition of the plankton between the sites. Correlations between taxon abundance and volume scattering were positive and significant only for pteropods and euphausiid larvae. The abundances of copepods, chaetognaths, fish larvae, and amphipods were not significantly correlated with volume scattering. When taxon-specific model predictions of acoustic backscattering cross-section, developed by Stanton et al . ( ICES Journal of Marine Science , 51 (1994) 505–512), were used with field collected individual size and abundance data to predict measured volume backscattering data, good agreement was found between observed and predicted volume backscattering strengths.

High-frequency acoustic volume backscattering in the Georges Bank coastal region and its interpretation using scattering models

High-frequency (120 and 420 kHz) sound was used to survey sound scatterers in the water over Georges Bank. In addition to the biological sound scatterers (the plankton and micronekton), scattering associated with internal waves and suspended sediment was observed. Volume backscattering was more homogeneous in the vertical dimension (with occasional patches) in the shallow central portion of the Bank where there is significant mixing. In the deeper outer portion of the Bank where the water is stratified, volume backscattering was layered and internal waves modulated the vertical position of the layers in the pycnocline. The internal waves typically had amplitudes of 5-20 m, but sometimes much higher. Species composition and size data from samples of the animals and suspended sediment used in conjunction with acoustic scattering models revealed that throughout the region the animals generally dominate the scattering, but there are times and places where sand particles (suspended as high as up to the sea surface) can dominate. The source of the scattering in the internal waves is probably due to a combination of both animals and sound-speed microstructure. Determination of their relative contributions requires further study.

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.

Influence of ocean freshening on shelf phytoplankton dynamics

[1] Climate change-induced freshening of the ocean can enhance vertical stratification and alter circulation patterns in ways that influence phytoplankton dynamics. We examined the timing of spring phytoplankton blooms and the magnitude of net primary productivity in the Nova Scotian Shelf (NSS) - Gulf of Maine (GoM) region with respect to seasonal and interannual changes in surface water freshening from 1998 to 2006. The general pattern of temporal westward progression of the phytoplankton bloom corresponds with the gradient of increasing sea surface salinity from the NSS in the east to the western GoM. Increased freshening enhances the spatial gradients in bloom timing by stimulating earlier blooms upstream (NSS), but it has less impact downstream (the western GoM). Strong spatial gradients (increasing westward) of mean chlorophyll concentration and net primary productivity during post-bloom months (May-June) indicate that lower sea surface salinity upstream can likely impede nutrient fluxes from deep water and therefore affect overall productivity.

The volume of Shelf Water in the Middle Atlantic Bight: seasonal and interannual variability, 1977-1987

The volume of Shelf Water (salinity <34psu) is determined for three regions in the Middle Atlantic Bight (MAB) and for Georges Bank from a combination of hydrographic data and shelf/slope front positions derived from satellite imagery. The hydrographic data were obtained on 49 surveys of the shelf between 1977 and 1987. In each region the shelf water volume exhibits a significant annual cycle with a range of 50–80% of the mean volume. The annual volume cycle appears to be advected southwestward from Georges Bank through the MAB with the mean circulation. Large interannual changes in volume are observed which have a range comparable to the mean volume of Shelf Water in the MAB. Both the seasonal and interannual changes in volume appear to be related to changes in the inflow to the Gulf of Maine through Northeast Channel and from the Scotian Shelf.

Does the fall phytoplankton bloom control recruitment of Georges Bank haddock, Melanogrammus aeglefinus, through parental condition?

In 2003, the Georges Bank stock of haddock (Melanogrammus aeglefinus) experienced the largest recruitment event recorded during its assessed history. Several hypotheses have been advanced to explain recruitment variability in this much-scrutinized stock, including variability in the retention of eggs and larvae on Georges Bank, the timing of haddock spawning, and variability in the spring bloom, which influences larval growth and survival. Although these processes may contribute to the formation of successful year classes, none of the factors associated with these previous hypotheses provides an adequate explanation of the 2003 recruitment event. We analyzed data on the dynamics of the fall phytoplankton bloom the year prior to spawning and show it to be highly correlated with subsequent recruitment. We suggest that the fall bloom affects recruitment through enhanced condition of adults and by increasing the quantity and quality of their reproductive output, which in turn leads to a higher probability of ...

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.

Surface heat flux in the Gulf of Maine

Abstract The surface heat flux in the Gulf of Maine is estimated on a daily basis for the period 1979–1987 by combining air temperature, water temperature and wind speed data measured at a NOAA buoy in the central Gulf with insolation data from coastal stations and climatological parameters. The heat flux is compared with water column temperatures measured during the same period. The mean annual cycle of heat flux is similar to that reported by Bunker (1976) and to the rate of change in the heat content of the upper 100 m of the water column across the Gulf. The interannual variability in heat flux is dominated by the latent and sensible heat flux terms during winter and by insolation variability during summer. In the western Gulf the interannual variability in water temperature is significantly correlated with the heat flux variations. In the eastern Gulf, no relationship is found and the temperature variability is believed dominated by advective changes.

Spatial coherence of interannual variability in water properties on the U.S. northeast shelf

Interannual variability in the surface and bottom temperature and salinity on the U.S. northeast shelf is described by using hydrographic data from the Northeast Fisheries Science Center (NEFSC) MARMAP program. This 10 year data set provides the spatial resolution to describe spatial patterns in the variability of the shelf water properties. An empirical orthogonal function analysis is used to determine the primary modes of variability. All parameters investigated have significant first modes, which contain 40–50% of the variance in the winter and 25–35% in the summer period. All parameters, except the summer surface temperature, exhibit coherent variability across the entire shelf region from near Cape Hatteras to the central Gulf of Maine. The summer surface temperature appears to have two regions of coherent variability: in the central Middle Atlantic Bight and in the western Gulf of Maine. This regionalization is believed due to the effect of local sources of coastal runoff.