October 1, 1998
Source Population Dynamics of "Red Tide" Dinoflagellates in the Southwestern Gulf of Maine Coastal Current
Note: GMR-20 was an extension of Project GMR-01
Donald M. Anderson / Biology Dept. / Woods Hole Oceanographic Institution
W.R. Geyer / Dept. of Applied Ocean Physics and Engineering / Woods Hole Oceanographic Institution
Bradford Butman / U.S. Geological Survey
Richard P. Signell / U.S. Geological Survey
Peter J.S. Franks / Biological Oceanography / Scripps Institution of Oceanography
Theodore C. Loder III / Dept. of Earth Sciences / University of New Hampshire
* * * * *
Central to the successful achievement of these objectives was the development of a coupled physical/biological model which will be of use in investigations of Gulf of Maine ecosystems and which may ultimately become a tool for assessing and evaluating resource management strategies.
Field Program. The field effort consisted of shipboard, moored, and drifter measurements in the western Gulf of Maine during the spring run-off periods of 1993 and 1994. (Table 1). Five biweekly large scale shipboard surveys were conducted in 1993. A different temporal approach was attempted in 1994. In early May 1994, just prior to expected outbreaks of shellfish toxicity, two large scale surveys were completed within a single week to access the short term changes in the hydrographic conditions and Alexandrium cell distributions that were not readily apparent in the biweekly 1993 data. Two more "back to back" large scale surveys were mounted during the first week of June, 1994. In addition, shorter surveys were conducted in 1994 along a transect near Cape Porpoise, Maine and in Massachusetts Bay. The hydrographic survey data included vertical profiles of temperature, salinity, fluorescence and beam attenuation. Water samples were collected at surface, 10m and 20m depths to document the abundance of Alexandrium and the associated nutrient conditions. This effort was complemented by river discharge data, meteorological observations from offshore buoys, and satellite observations of sea-surface temperature. The measurements were highly successful, with nearly 100% data return from all components. The data have been analyzed and are now being synthesized in several publications and theses.
|CRUISES||five 3-day (80+ stations) cruises
When: mid-April-June -biweekly
Where: Monhegan Is. to Mass Bay
Data: CTD, Alexandrium cells,
|two extended (5+ days) cruises with
repeat coverage (80+ stations each leg)
When: late April-early May
Where: Penobscot Bay to Mass Bay
Data: CTD, Alexandrium cells,
inorganic nutrients, ADCP
five 1-2 day cruises off Cape Porpoise
two 1-2 day cruises in Mass Bay
|MOORINGS||2 current moorings
(Monhegan, Cape Porpoise)
5m- VMCM, temp, salinity, trans
25m-VACM, temp, salinity
50m-VACM, temp salinity, trans
1 bottom pressure mooring
Duration: mid-March to July
|3 current moorings
(Kennebec River, Cape Porpoise-2)
5m- VMCM, temp, salinity, trans
25m-VACM, temp, salinity
50m-VACM, temp salinity
1 bottom pressure mooring
Duration: late March to September
|DRIFTERS||3 Davis type (2m drogue)
deployed at Kennebec River mouth
|4- Davis type (2m drogue)
deployed at Kennebec River mouth
|SATELLITE IMAGERY||Coastwatch AVHRR SST imagery
(1km) twice daily
processed into hardcopies
|Coastwatch AVHRR SST imagery
(1km) tweice daily
processed into hardcopies
1993 Field Results. The first hydrographic survey during mid-April, 1993 was concurrent with the peak river discharge from the Kennebec-Androscoggin Rivers. It was also concurrent with downwelling winds and moderate down-coast currents. The near-surface salinity distribution indicated a narrow plume between 10 and 20km wide that extended from the Kennebec River mouth to Massachusetts Bay (Fig. 1). As expected, the lowest salinities were found near the mouth of the Kennebec River (18 psu), but low salinities were also observed inshore near Cape Elizabeth (26 psu) 20 km to the southwest. The highest Alexandrium concentrations (>50 cells l-1) were found along the Cape Elizabeth transect coincident with the less-saline waters of the river plume (Fig. 1). However, lower concentrations (20 cells l-1) were also found offshore outside of the plume. These observations suggest that the Casco Bay area, just downstream of the Kennebec River, may act as an initiation site for bloom initiation, since the highest early-season populations of Alexandrium were observed there and shellfish toxicity is historically detected within northeastern Casco Bay before other areas along the Maine coast. The first detection of shellfish toxicity in Casco Bay was about two weeks after the first observation of the cells.
Fig. 1. Surface salinity and Alexandrium distributions during the first cruise of 1993 (Apr 12-14).
Based on the structure of the plume and the trajectories of drifters, it was concluded that patches of Alexandrium cells originating near Casco Bay and generally associated with the plume were transported to the south along the coast with an average velocity of 0.15 m s-1, but during times of downwelling maximum speeds were approximately 0.50 m s-1. While the plume extended down the coast with a relatively uniform distribution during downwelling conditions, the distibution of the cells was non-uniform and quite patchy. One of these patches (>200 cells l-1) was observed offshore of Cape Ann, MA during the second cruise on April 28-30 (data not shown). This patch was strongly associated with the combined plumes of the Merrimack, Kennebec and other upstream rivers. While we believe the patch originated to the north near Casco Bay and not within the Merrimack River itself, the enhanced stratification caused by the Merrimack River inflow over the Kennebec plume may have produced the conditions for re-creation and maintenance of the patch in the plume waters as the cells from the north vertically migrate into the surface waters of the Merrimack outflow. Since the transport speeds calculated for the plume during this time were high, this patch may not be related to the previous observation of patches observed during the first cruise two weeks prior (Fig. 1).
Upwelling favorable conditions occured just prior to the third cruise (May 10-13). The freshwater plume remained generally attached to the coast but widened to >30km and beyond the sampling array. The lowest salinities were observed south of the Kennebec River and further offshore indicating the decrease in river input and the influence of upwelling. The distribution of salinity was also much more irregular than observed under downwelling conditions earlier in April (Fig. 2). Under the upwelling conditons, the Alexandrium cells were generally observed further offshore associated with the widened plume structure (Fig. 2). Shellfish toxicity had increased to detectable levels along the southern Maine coast just prior to the upwelling. However, following the upwelling condition, toxicity declined, most likely due to the movement of the bulk of the population offshore and away from the shellfish.
Fig. 2. Salinity and Alexandrium distribution during the third cruise of 1993 (May 10-13).
The last two cruises in 1993 (May 24-27; Fig. 3 and June 4-6; data not shown) document the accumulation of freshwater and cells in Massachusetts Bay under relatively calm conditions. A nearshore plume had become re-established, perhaps as a result of a weak downwelling wind several days prior to the cruise (Fig. 3). Drifter tracks from earlier releases indicate that there was movement of surface waters inshore near Cape Ann in response to the downwelling conditions. This timing of the wind-driven cross-shelf transport was critical for a sufficient number of Alexandrium cells to be introduced into Massachusetts Bay causing an outbreak of shellfish toxicity along the "south shore" communities of Boston (Fig 3). With the ensuing calm conditions and lack of transport out of Massachusetts Bay, the population remained in the Bay for several weeks and gradually caused shellfish toxicity further south than the Cape Cod Canal, an event that had never before been recorded. Furthermore, a new population re-established itself near Casco Bay suggesting that under the low flow conditions, populations may have been retained in Casco Bay as well. This population may have been responsible for PSP outbreaks along the southern Maine coast in mid-June when no cruises were scheduled. Sea surface temperture (SST) imagery during this time suggested that the plume was re-established during downwelling favorable conditions resulting in the transport of the cells onto the coast causing a second PSP outbreak along portions of the southern Maine coast. Subsequently, upwelling conditions observed in the imagery drove the cells back off the coast, ending the PSP problem for the year.
Fig. 3. Salinity and Alexandrium distributions during the fourth cruise (May 24-27).
1994 Field Results. The 1994 field approach sought to examine the short term changes of plume structure and cell distribution by conducting repeated large scale surveys with a separation of only several days. The most notable features of these observations were the rapid changes in near-surface salinity structure and cell distribution between the closely-spaced cruises (Fig. 4 and 5) Whereas the 29 psu contour was confined to the vicinity of the Kennebec River mouth in the first survey (Fig. 4), it extended more than 50km down the coast less than a week later during the next survey (Fig. 5). Similarly, the Alexandrium cells were generally confined to small patches offshore of Casco Bay during the first cruise, but several days later the patch was much larger in the alongshore direction and had been transported down the coast with maximum concentrations approximately double those observed several days earlier. It appears that as the gradients of the plume structure became tighter, the Alexandrium patch became more defined. Thus, physical accumulation at the frontal boundary designated by the 29-31 psu isohalines and growth of the cells may be responsible for the development of the patch. The repeated observations during the two "back to back" surveys were comparable from one cruise to the next because the Alexandrium populations were not transported out of the study site before the next survey. The 1993 surveys were separated too much in time to provide this resolution.
Fig. 4. Salinity and Alexandrium distributions during the first cruise (leg I) of 1994 (Apr 28-May1)
Fig. 5. Salinity and Alexandrium distributions during the first cruise (Leg II) of 1994 (May 2-4).
The 1994 observations indicate similar conditions as 1993 during April and early May,
but subsequent data show a reduction in the freshwater content of the coastal waters (Fig. 6) and reversal of the coastal current during sustained periods (from drifter tracks). This difference was not attributed to a difference in run-off; in fact the run-off characteristics were similar between the two years. The variations in wind forcing between the two years is a more likely explanation for the difference. Upwelling-favorable winds were more frequent and sustained in 1994, and there were fewer downwelling events during the period of maximum run-off. These hydrographic conditions were strongly reflected in the biology. The sustained upwelling-favorable winds in 1994 transported initial Alexandrium populations offshore (Fig. 6), preventing the development of a bloom large enough to cause significant shellfish toxicity along the coast. In 1994, virtually no toxicity was detected in western Maine or in Massachusetts Bay, suggesting that the persistent upwelling kept the low density populations offshore, prevented them from establishing themselves and seeding the southern waters via advection.
Fig. 6. Salinity and Alexandrium distributions during the second large scale survey (Leg I) of 1994 (May 31-June 3).
Analysis of the observations from the spring freshets of 1993 and 1994 emphasize that the local winds were primarily responsible for plume variability over short time scales. Meteorological records show variability in both wind speed and direction over time scales of a few days. Winds fluctuated between upwelling, downwelling, and neutral conditions; the longer time-scale (monthly) averaged wind stress was nearly zero. One important implication is that the timing of forcing conditions is critical in determining the transport and fate of the freshwater and associated Alexandrium populations. In 1993, there was a sufficient interval of downwelling winds at the time when the Alexandrium populations were becoming established to allow the cells to be transported alongshore in numbers that can cause toxicity. In contrast, in 1994 this process was interpreted frequently by upwelling winds, which moved the plume and its cells away from shore and dispersed them.
Nutrients. During the 1993 and 1994 field seasons, nearly 4000 water samples were collected for inorganic nutrient analyses including nitrate, nitrite, ammonium, silicate and phosphate. The analyses of the samples have been completed and are summarized in the thesis of UNH student Caroline Martorano (1997). Data analysis suggested that the major sources of nutrients to the surface waters of the western coastal Gulf of Maine were: the Kennebec/Androscoggin freshwater plume, the Eastern Maine Coastal Current, wind-induced upwelling; tidally-driven wave-sill interaction on Stellwagen Bank, local river inputs (such as the Merrimack), and sewage effluent from Boston Harbor.
Analysis also indicated that the occurrence and persistence of red tide blooms in the southwestern Gulf of Maine was not a result of inorganic nutrient enrichment. Although the nutrient levels were generally higher inside the Kennebec plume than outside, the data suggest that the surface nutrients decreased as the plume advected down the coast.The toxic Alexandrium blooms typically occurred during the late spring after nutrients in the coastal current were well depleted by early spring phytoplankton blooms mainly consisting of diatoms. In particular, the highest Alexandrium cell concentrations observed during the 1993 and 1994 sampling seasons were measured in waters which contained DIN levels of 0.30 and 0.73 ÁM, respectively. These low nitrogen levels indicate that the nitrogen was already "stripped" from the surface waters by the earlier diatom bloom. It is unlikely that Alexandrium itself, as only a minor component of the phytoplankton community, could have a major impact on the nutrient concentrations. Alexandrium could be relying on its ability to persist at very low levels (0-1 uM) of DIN concentrations, and/or by an ability to obtain nitrogen from the pycnocline region of the water column through vertical migration. We note, however, that results from another study indicate that Casco Bay strains may lack the ability to vertically migrate or to take up nitrate at night. Thus, Alexandrium growth might have been nutrient-limited in both 1993 and 1994.
We suspect that once established at low levels, Alexandrium could grow rapidly if it were transported into areas rich in nutrients caused by upwelling, vertical mixing, or anthropogenic inputs. The initiation of the blooms in the Casco Bay region near the Kennebec plume (Fig. 1 and Fig. 4) may be a response not only to the greater stability of the water column and accumulation of cells at frontal boundaries, but also to increased nutrients found there before rapid consumption of the nutrients by diatoms. Similarly, the Alexandrium patch observed off Cape Ann near the frontal boundary of the Merrimack plume during the second cruise on April 28-30, 1993 (see above), may have responded to new nutrient inputs. Also, the higher populations observed in Massachusetts Bay along the "south shore" of Boston in 1993 were associated with higher nutrients presumably originating from the Boston Harbor sewage outfall. Thus, there is evidence to suggest that localized elevated nutrient conditions encountered by the cells may potentially stimulate growth.
However, high nutrients are not common in the nearshore surface waters of the WMCC, so Alexandrium populations must rely on recycled nitrogen. We are now exploring the hypothesis that low nutrient availability in Casco Bay or coastal current water, combined with rapid transport to the south, results in low population cell densities of less than a thousand cells per liter, as observed in both 1993 and 1994. Lower Alexandrium concentrations are a characteristic feature of the Casco Bay area and the WMCC, and are in major contrast to the high concentrations of Alexandrium in areas such as the Bay of Fundy, which often exceed 50,000 cells per liter. These insights into the importance of the nutrient environment, combined with the demonstrated linkage between plume behavior and local small-scale physical forcings are extremely important findings of this study. These topics will be explored in great detail in the ECOHAB-Gulf of Maine project. Thus, both the chemical and physical environments were dominant factors regulating the distribution of Alexandrium in the southwestern Maine coastal current during 1993 and 1994.
Modeling. The factors influencing the variations of the coastal current structure and the Alexandrium sp. distribution within the coastal current were investigated in a numerical modeling study. This effort involves both simple, idealized models to explore the underlying mechanisms, and a full, three-dimensional and time-dependent model of the western Gulf of Maine that provides detailed comparison with the observations.
An idealized model was used to explore the sensitivity of the structure of the coastal current to variability of wind forcing. The model results indicate that the time-averaged response of the coastal current system depends on the event-scale variability -- particularly the duration of upwelling. As a generalization of the wind conditions present in the western Gulf of Maine, initial experiments were conducted by forcing the model with sinusoidally varying (in time) wind stresses of different amplitudes. These experiments demonstrated that even with a zero temporal mean wind stress, wind fluctuations result in significant mixing of the plume. In particular, strong sustained upwelling fluctuations appear capable of thoroughly mixing the plume such that its freshwater signal is largely removed.
More detailed and quantitative analyses were then conducted to investigate the role of individual upwelling and downwelling favorable wind events. Strong upwelling and downwelling winds were both capable of initiating significant mixing between the plume and ambient coastal waters. Downwelling winds compress the plume against the coast which results in a reduction in overall stratification; the reduced stratification, coupled with enhanced shears in the alongshore velocity field promotes enhanced mixing through shear instability processes. Upwelling winds, in contrast, spread and advect the plume offshore consistent with Ekman drift theory (see Fong et al., 1996); the associated shears with the cross-shore Ekman transport work against the stratification and enhance mixing.
A Blumberg and Mellor 3-dimensional circulation model was developed for the entire study area between Massachusetts Bay and Penobscot Bay. The objectives of that modeling effort were: 1) to simulate the transport and water properties of the WMCC during 1993-94 as functions of surface, boundary and topographic forcing; and 2) to test hypotheses concerning the mechanisms for initiation, growth, accumulation and transport of Alexandrium cells.
Simulations show many of the observed elements of the WMCC behavior, including trapping along the coast during downwelling and offshore spreading during upwelling. It is evident that wind forcing is an important and possibly dominant source of variability in the WMCC structure. A simple model for Alexandrium growth in response to temperature and salinity was then coupled to the physical transport model. The 1993-94 field seasons were then numerically hindcast using realistic bathymetry, measured wind, river and heat flux data, and boundary conditions derived from our in situ observations. One hypothesis tested was that differences in wind forcing and river discharge caused the large differences in toxicity that were observed in the WMCC between 1993 and 1994. In both years, however, the simulations show low-salinity lenses containing Alexandrium cells traveling along the coast and into Mass Bay, whereas this only happened in 1993. (Movies of model output: http://crusty.er.usgs.gov/ecohab/). Toxicity in the WMCC thus cannot be attributed solely to physical variability of wind and river discharge - it is also heavily dependent on biological variability.
One cause of biological variability is in the process by which Alexandrium cysts inoculate overlying waters. Several hypotheses were thus tested, including the location of the germination input (all along the coast versus discrete seedbeds), but only the scenario in which Alexandrium cells were seeded near the Kennebec river mouth generated patterns of cells that resembled field data. The simulations provide yet another independent indication that the Casco Bay area is a source region.
In the simulations, cells were continuously introduced into the source region at fixed concentrations, giving realistic results for some situations, but not for others. The source function was clearly over-simplified and the modeling team concluded that without a realistic, time-varying input (based on estimates of the germination rates and the duration of germination events), further progress was unlikely in modeling PSP in the WMCC region. This impasse will be addressed in a follow-up project called ECOHAB-GOM (see below).
A manuscript describing the results of the modeling work are in preparation for a special issue of Estuarine and Coastal Shelf Science that will be distributed on CD-ROM, allowing the submissions to utilize animation.
This project generally followed the proposed work plan. The only significant change was in the cruise schedule. In 1994, instead of running cruises every several weeks, as was the case in 1993, an effort was made to document the short-term (i.e. about one week) variability in physical, chemical and biological fields. This was necessary because the velocity of the coastal current and the high frequency nature of the wind changes made it difficult to extrapolate between cruises spaced 2-3 weeks apart. In 1994, therefore, cruises consisted of two legs - one which traversed the study domain from south to north, and a second one which retraced the same stations in reverse order. This provided a very useful picture of the short-term development of the coastal current and its Alexandrium population.
On re-examination of the Alexandrium data during the extension period, a greater importance has been placed on the significance of low level populations (about 20 cells l-1) found offshore and upstream of the Kennebec River. These populations were observed in both 1993 and 1994 and were originally thought to be only "background" levels as they were just above the detection limits of our sampling methods. These populations may in fact be the source populations germinating from offshore cyst beds. During southward transport within the general counterclockwise Gulf of Maine circulation, these populations may interact with the Kennebec River Plume and become enhanced and entrained within the plume due to favorable physical/chemical/biological environments found there. Alternatively, one of the short cruises in 1994, documented an Alexandrium bloom within the New Meadows River of Casco Bay following a localized outbreak of PSP. The source of these cells could be either within Casco Bay itself or transported from offshore into the New Meadows River via the plume during downwelling conditions. The ECOHAB program which is now focused on the initiation of the blooms in the Casco Bay/Kennebec River region may eventually resolve these issues.
Although the mean flows of all of the moorings were generally directed along the coast to the southwest, there was significant offshore veering in most of the near-surface observations. The most extreme veering occurred at the outer mooring, where the mean flow was oriented more than 45 degrees offshore in both 1993 and 1994 (Geyer et al., submitted ms.). These observations could not be explained by wind-induced Ekman transport due to interfacial drag, or by non-linearity in the wind-induced Ekman transport. However, the freshwater transport in the plume was comparable to the freshwater discharge of the major rivers feeding the western Gulf of Maine. There was a significant, along-coast reduction in the transport, which may indicate "leakage" of roughly a third of the low-salinity water in the seaward direction. The mean, offshore flow at the moorings may be related to this leakage. Analysis of project data is continuing in an effort to resolve this important issue.
One of the major modeling problems identified was that both the 1993 and 1994 simulations produced red tide blooms in Massachusetts Bay, while only the bloom in 1993 actually occurred. One of the reasons implicated was poor knowledge of source conditions for Alexandrium as stated above. Another possible reason was inadequate representation of river plume water transport down the coast from the Gulf of Maine rivers to Massachusetts Bay. This was suggested by the fact that in 1993, the model failed to reproduce the low salinity water found in Massachusetts Bay during June, the period when the blooms occurred. This was examined carefully during the extension period of the project, and it was found that the periodic upwelling and downwelling winds that carry the plume offshore and onshore were causing too much mixing of the plume in the model. The result was that the plume lost its dynamic structure too quickly and the transport of fresh water and cells downstream was then underrepresented. One of the problems implicated was that the Blumberg and Mellor model used in this study requires that each model layer be a fixed percentage of the water depth. Along the western Gulf of Maine, the steep topography and the large offshore excursions of the plume dictate that the number of model layers representing the plume change dramatically as the plume moves on and offshore, and therefore numerical mixing occurs along model layers that act to smear out the plume. Increasing the number of vertical layers cannot fix this problem. This suggests that using a model with a more flexible configuration for model layer thickness (such as the S-coordinate Rutgers University Model) may be an important factor in successful modeling of river plumes and Alexandrium transport.
This project was able to document the distribution and abundance of toxic Alexandrium cells in a major hydrographic system at a level of detail never before possible. The Alexandrium bloom dynamics in the western Gulf of Maine are now better understood than any other toxic or harmful algal species anywhere in the world. The factors that regulate bloom development in the coastal current were elucidated, and the importance of upwelling and downwelling winds documented. The linkage between the location of patches of cells and wind events was shown to be a critical determinant of the pattern of toxicity along the coast, especially in southern Maine and Massachusetts. Furthermore, the importance of the Casco Bay region as a "source" or initiation zone was clearly demonstrated in both 1993 and 1994, indicating that the toxicity that develops along several hundred miles of coastline has its origin in a relatively small, restricted zone. The significance of "upstream" low level populations is still unknown. Further work is clearly needed to characterize the biology and physics of that initiation zone.
The study has important implications in the understanding of the structure of the coastal current in the Gulf of Maine, particularly with respect to wind-forcing. We obtained a unique physical and chemical dataset describing the western Maine coastal current. No such data exist with similar resolution or detail. Thus, the work represents a major step forward in our understanding of the dynamics of river plumes in general and the Gulf of Maine regime in particular. Likewise, a three-dimensional model was formulated which recreated the coastal current behavior in the study area. The addition of biology to the model was successful in large part, but highlighted the need for better paramaterization of the source function for Alexandrium cells germinating from benthic cysts. Another important development is that a team of investigators was formed during this project has been successful in competing nationally for funding of a 5-year Gulf of Maine field research program within the ECOHAB program. The skill, knowledge and data obtained during this RMRP project were instrumental in that award.
Jia Wong (Postdoctoral Investigator, Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution).
Derek Fong (PhD, Woods Hole Oceanographic Institution; advisor-Geyer)
Dynamics of freshwater plumes: observations and numerical modeling of
the wind-forced response and alongshore freshwater transport.
Completed May, 1998.
Caroline Martorano (MS, Earth Sciences, University of New Hampshire; advisor-Loder)
Nutrient dynamics during blooms of Alexandrium spp. in the southwestern Gulf of Maine.
Andrew Juhl (Graduate student, Scripps Insittute of Oceanography, advisor-Franks)
Turbulence in dinoflagellate blooms
Joanna Suidek (WHOI summer student fellow)
Influence of hydrography on the distribution of Alexandrium sp.
Anderson, D.M. 1997. Bloom dynamics of toxic Alexandrium species in the northeastern U.S. Limnology and Oceanography 42:1009-1022.
Anderson, D. M. Physiology and bloom dynamics of toxic Alexandrium species, with emphasis on life cycle transitions. In: The Physiological Ecology of Harmful Algal Blooms. D. M. Anderson, A.D. Cembella, and G. M. Hallegraeff (eds.), Springer-Verlag, Heidelberg. In press.
Fong D.A., Geyer W.R., and R.P. Signell, The wind-forced response of a buoyant coastal current: Observations of the western Gulf of Maine plume, Journal of Marine Systems, 12, 69-81.
Franks, P.J.S. 1997. Models of harmful algal blooms. Limnology and Oceanography 42:1273-1282.
Franks, P.J.S. 1997. Coupled physical-biological models for the study of harmful algal blooms. Ocean Research 19:153-160
Franks, P.J.S. 1998. Spatial patterns in dense algal blooms. Limnology and Oceanography 42:1297-3105
Geyer, W.R., Signell R.P., Fong D.A., Wang J., Anderson, D.M., and Keafer B.A. (submitted) The Kennebec River plume and the coastal current in the western Gulf of Maine. Continental Shelf Research.
Anderson, D.M., B.A. Keafer, W.R. Geyer, and R.P. Signell. Bloom Dynamics of Alexandrium in the Gulf of Maine: The plume advection hypothesis revisited.
Fong and Geyer. "The response of a river plume during an upwelling wind event".
Fong and Geyer. "The alongshore transport of fresh water in a surface-trapped river plume".
Richard P. Signell and Peter J.S. Franks. "Modeling Red Tide in the Western Gulf of Maine".
For submission to special issue of Estuarine and Coastal Shelf Science.
Anderson, D.M., B.A. Keafer, W.R. Geyer, and R.P. Signell. Bloom Dynamics of Alexandrium in the Gulf of Maine: Alonshore Transport in a Buoyant Coastal Current. Invited talk at the Seventh International Conference on Toxic Phytoplankton. July, 1995.
Geyer, W.R., R.P. Signell, D.M. Anderson, and D.A.Fong. Modeling the Transport of Fresh Water and Toxic Dinoflagellates in the Coastal Current of the Western Gulf of Maine. Presented as a poster at the AGU/ASLO Ocean Sciences meeting, 1996.
Franks, P.J.S. Spatial patterns in dense algal blooms. Invited talk to the AGU/ASLO Ocean Sciences meeting, 1996.
Anderson, D.M., B.A. Keafer, W.R. Geyer, and R.P. Signell. Bloom Dynamics of Alexandrium in the Gulf of Maine: Alongshore Transport in a Buoyant Coastal Current. Invited talk presented at the AGU/ASLO Ocean Sciences meeting, 1996.
Fong, D.A. and W.R. Geyer. Mixing of a river plume by a sustained upwelling favorable wind event. Gordon Conference on Coastal Ocean Circulation, New London, NH., 1997.
Fong, D.A. and W.R. Geyer. Alongshore transport of a surface-trapped river plume. American Geophysical Union Ocean Sciences Meeting, San Diego, CA., 1998.