Observations of the Eastern Maine Coastal Current and Its Offshore Extensions in 1994

Cold surface temperatures, reflecting Scotian Shelf origins and local tidal mixing, serve as a tracer of the Eastern Maine Coastal Current and its offshore extensions, which appear episodically as cold plumes erupting from the eastern Maine shelf. A cold water plume emanating from the Eastern Maine Coastal Current in May 1994 was investigated using advanced very high resolution radiometer (AVHRR) imagery, shipboard surveys of physical and biochemical properties, and satellite-tracked drifters. Evidence is presented that suggests that some of the plume waters were entrained within the cyclonic circulation over Jordan Basin, while the major portion participated in an anticyclonic eddy at the distal end of the plume. Calculations of the nitrate transported offshore by the plume show that this feature can episodically export significant quantities of nutrients from the Eastern Maine Coastal Current to offshore regions that are generally nutrient depleted during spring-summer. A series of AVHRR images is used to document the seasonal along-shelf progression of the coastal plume separation point. We speculate on potential causes and consequences of plume separation from the coastal current and suggest that this feature may be an important factor influencing the patterns and overall biological productivity of the eastern Gulf of Maine.

Thermal Conditions at the Bed of the Laurentide Ice Sheet in Maine During Deglaciation: Implications for Esker Formation

The University of Maine Ice Sheet Model was used to study basal conditions during retreat of the Laurentide ice sheet in Maine. Within 150 km of the margin, basal melt rates average similar to 5 mm a(-1) during retreat. They decline over the next 100km, so areas of frozen bed develop in northern Maine during retreat. By integrating the melt rate over the drainage area typically subtended by an esker, we obtained a discharge at the margin of similar to 1.2 m(3) s(-1). While such a discharge could have moved the material in the Katahdin esker, it was likely too low to build the esker in the time available. Additional water from the glacier surface was required. Temperature gradients in the basal ice increase rapidly with distance from the margin. By conducting upward into the ice all of the additional viscous heat produced by any perturbation that increases the depth of flow in a flat conduit in a distributed drainage system, these gradients inhibit the formation of sharply arched conduits in which an esker can form. This may explain why eskers commonly seem to form near the margin and are typically segmented, with later segments overlapping onto earlier ones.

Simulation of the Impact of Dams and Fishing Weirs on Reproductive Potential of Silver-Phase American Eels in the Kennebec River Basin, Maine

I modeled the cumulative impact of hydroelectric projects with and without commercial fishing weirs and water-control dams on the production, survival to the sea, and potential fecundity of migrating female silver-phase American eels, Anguilla rostrata in the Kennebec River basin, Maine, This river basin has 22 hydroelectric projects, 73 water-control dams, and 15 commercial fishing weir sites. The modeled area included an 8,324 km(2) segment of the drainage area between Merrymeeting Bay and the upper limit of American eel distribution in the basin. One set of input,, (assumed or real values) concerned population structure (Le., population density and sex ratio changes throughout the basin, female length-class distribution, and drainage area between dams), Another set concerned factors influencing survival and potential fecundity of migrating American eels (i.e., pathway sequences through projects, survival rate per project by length-class. and length-fecundity relationship). Under baseline conditions about 402,400 simulated silver female American eels would be produced annually reductions in their numbers due to dams and weirs would reduce the realized fecundity (i.e., the number of eggs produced by all females that survived the migration). Without weirs or water-control dams, about 63% of the simulated silverphase American eels survived their freshwater spawning migration run to the sea when the survival rate at each hydroelectric dam was 9017, 40% survived at 80% survival per dam, and 18% survived at 60% survival per dam. Removing the lowermost hydroelectric dam on the Kennebec River increased survival by 6.0-7.6% for the basin. The efficient commercial weirs reduced survival to the sea to 69-76%( of what it would have been without weirs', regardless of survival rates at hydroelectric dams. Water-control dams had little impact on production in this basin because most were located in the upper reaches of tributaries. Sensitivity analysis led to the conclusion that small changes in population density and female length distribution had greater effects on survival and realized fecundity than similar changes in turbine survival rate. The latter became more important as turbine survival rate decreased. Therefore, it might be more fruitful to determine population distribution in basins of interest than to determine mortality rate at each hydroelectric project.

A Model Study of the Seasonal Circulation in the Gulf of Maine

The Princeton Ocean Model is used to study the circulation in the Gulf of Maine and its seasonal transition in response to wind, surface heat flux, river discharge, and the M-2 tide. The model has an orthogonal-curvature linear grid in the horizontal with variable spacing from 3 km nearshore to 7 km offshore and 19 levels in the vertical. It is initialized and forced at the open boundary with model results from the East Coast Forecast System. The first experiment is forced by monthly climatological wind and heat flux from the Comprehensive Ocean Atmosphere Data Set; discharges from the Saint John, Penobscot, Kennebec, and Merrimack Rivers are added in the second experiment; the semidiurnal lunar tide (M-2) is included as part of the open boundary forcing in the third experiment. It is found that the surface heat flux plays an important role in regulating the annual cycle of the circulation in the Gulf of Maine. The spinup of the cyclonic circulation between April and June is likely caused by the differential heating between the interior gulf and the exterior shelf/slope region. From June to December the cyclonic circulation continues to strengthen, but gradually shrinks in size. When winter cooling erodes the stratification, the cyclonic circulation penetrates deeper into the water column. The circulation quickly spins down from December to February as most of the energy is consumed by bottom friction. While inclusion of river discharge changes details of the circulation pattern, the annual evolution of the circulation is largely unaffected. On the other hand, inclusion of the tide results in not only the anticyclonic circulation on Georges Bank but also modifications to the seasonal circulation.

Sexually Different Growth Histories of the American Eel in Four Rivers in Maine

Growth histories of yellow-phase American eels Anguilla rostrata collected in four rivers in Maine, were back-calculated from sagittal otolith increments. Our objectives were to first determine whether sexually dimorphic growth rates exist and then compare the growth histories of American eels from four rivers within a geographic region. For female eels, the maximum growth rate was 31.9 +/- 1.7 mm/year at age 8, decreasing to 25.1 +/- 2.9 mm/year at age 14. Males attained a maximum of 29.8 +/- 1.6 min/year at age 3, decreasing to a minimum of 17.9 +/- 1.3 mm/year at age 11. Females grew faster than males after age 4 and had a slower reduction in growth rate with age. These faster growth rates among females were similar in all four rivers. The observed growth rates are not consistent with current life history hypotheses and may indicate an alternative life history strategy. Because female eels benefit from a larger size (i.e., size refuge, increased fecundity, and greater niche breadth), they would benefit from a higher-risk growth strategy that increases growth rate during their earlier years and reduces the amount of time spent in an unfavorable size-class. The tradeoffs (i.e., mortality, developmental rate, pathogen resistance, and longevity) associated with this faster growth rate may not favor the males' life history requirements. Male eels do not achieve the size of females and therefore are not subject to the advantages associated with being larger. Therefore, they may use a risk-averse strategy that maintains submaximum growth rates to obtain the minimum size necessary to mature and complete the spawning migration while reducing the adverse affects of faster growth rates. We postulate that, in eels, intrinsic growth rates should be considered a life history trait that has evolved to meet the life history requirements of each sex.

Phytoplankton and Hydrography of the Kennebec Estuary, Maine, USA

The biomass, abundance and species composition of phytoplankton in the Kennebec estuary, Maine, USA, were investigated in relation to hydrography and Light regime during 7 seasonal survey cruises. The salinity distribution ranged from 32 at the mouth to between 0 and 5 at the head, depending on the magnitude of freshwater discharge at the time of each survey. Maximum Vertical salinity and temperature gradients were observed at the mouth. while local tidal mixing, combined with the freshwater flow, produced a well-mixed water column at the head of the estuary. The middle portion of the estuary was stratified on flooding and ebbing tides, but was vertically well mixed at high and low tides. Phytoplankton biomass was lowest in winter (chlorophyll a approximate to 1 mu g l(-1)) and highest in summer (up to 10 mu g l(-1)) The phytoplankton species assemblages at the seaward and the riverine ends of the estuary were made up of taxa with corresponding salinity preferences. Both cell numbers and biomass (chlorophyll a) exhibited a bimodal distribution along the length of the estuary in the warmer months, with the middle portions of the estuary having depressed phytoplankton standing stocks compared with the seaward and landward ends. This bimodal distribution was related to Light limitation and nutrient regeneration in the middle portion of the estuary and to the production of and advective contributions of phytoplankton from both the freshwater and seaward ends.

Effects of Surface Forcing on Interannual Variability of the Fall Phytoplankton Bloom in the Gulf of Maine Revealed Using a Process-Oriented Model

SeaWiFS (Sea-viewing Wide Field-of-view Sensor) chlorophyll data revealed strong interannual variability in fall phytoplankton dynamics in the Gulf of Maine, with 3 general features in any one year: (1) rapid chlorophyll increases in response to storm events in fall; (2) gradual chlorophyll increases in response to seasonal wind-and cooling-induced mixing that gradually deepens the mixed layer; and (3) the absence of any observable fall bloom. We applied a mixed-layer box model and a 1-dimensional physical-biological numerical model to examine the influence of physical forcing (surface wind, heat flux, and freshening) on the mixed-layer dynamics and its impact on the entrainment of deep-water nutrients and thus on the appearance of fall bloom. The model results suggest that during early fall, the surface mixed-layer depth is controlled by both wind-and cooling-induced mixing. Strong interannual variability in mixed-layer depth has a direct impact on short-and long-term vertical nutrient fluxes and thus the fall bloom. Phytoplankton concentrations over time are sensitive to initial pre-bloom profiles of nutrients. The strength of the initial stratification can affect the modeled phytoplankton concentration, while the timing of intermittent freshening events is related to the significant interannual variability of fall blooms.

Recruitment Habitats and Nursery Grounds of the American Lobster Homarus Americanus: A Demographic Bottleneck?

We have identified benthic recruitment habitats and nursery grounds of the American lobster Homarus americanus Milne Edwards in the coastal Gulf of Maine, USA, by systematically censusing subtidal sediment, cobble, and ledge substrata. We distinguish lobsters between settlement size (5 mm carapace length (CL) to ca 40 mm CL as the 'early benthic phase' (EBP) because they are ecologically and behaviorally distinct from larger lobsters. EBP lobsters are cryptic and apparently restricted to shelter-providing habitats (primarily cobble substratum) in coastal Gulf of Maine. In these habitats we found average population densities of EBP lobsters as high as 6.9 m-2. EBP lobsters were virtually absent from ledge and sedimentary substrata devoid of vegetation although larger lobsters are commonly found there. It is possible that the requirement for shelter-providing substrata by this life phase creates a natural demographic 'bottleneck' to benthic recruitment for the species. Prime cobble recruitment habitat is relatively rare and comprises ca 11 % of the 60.2 km of shoreline at our study area in midcoast Maine. If this low availability of cobble exists throughout the Gulf of Maine, as other studies indicate, it could limit lobster production potential. We verified the geographic extent of recruitment to cobble habitats censused in 3 of 4 regions spanning ca 300 km of the coastal Gulf of Maine (from Nahant, Massachusetts to Swans Island, Maine). Early benthic phase lobsters were absent from cobble censused in the northeastern extreme of our survey (Swans Island). This pattern is consistent with earlier speculation that relatively cool water temperatures may limit larval settlement in this region.