David W. Townsend

Causes and consequences of variability in the timing of spring phytoplankton blooms

Abstract Established conceptual models of the initiation and progression of spring phytoplankton blooms are reconsidered in light of recent observations. We use biological simulation modelling as a tool for the analysis of spring plankton blooms in shallow, coastal waters in temperate latitudes of the North Atlantic. The model shows that interannual variability in the timing of bloom initiation arises from year-to-year differences in incident irradiation, as determined by weather (cloudiness). This variability in timing results in some years when the spring bloom occurs in cold water temperatures near 0°C. Model results suggest that due to low temperature inhibition of heterotrophic consumption, more fresh organic material is delivered to the benthos in these cold-water blooms than when the bloom occurs in waters only 3°C warmer. Thus we suggest that variable bloom timing can be important to the trophodynamic fate of bloom products. We suggest that variability in timing of spring phytoplankton blooms in offshore and open ocean waters is also related to weather, through controls on the light field and wind mixing. Our analyses of wind-driven vertical mixing demonstrate such blooms can begin following the winter period of deep convection, and prior to the vernal development of stratification, provided that wind speed is below a certain, predictable threshold, which we estimate. In such cases, there may be several spring bloom pulses, each interrupted by self-shading light limitation or vertical mixing events. Eventually the seasonal thermocline develops and nutrient exhaustion curtails bloom production. This means that the spring phytoplankton bloom in offshore and open ocean areas may be significantly more productive, result in more export production, and be more important to the carbon cycle, than has been previously assumed. Furthermore, these features of temperate marine planktonic ecosystems are not only sensitive to annual variations in weather, but also any trends that might result from greenhouse warming or other factors that affect the climate system.

Offshore blooms of the red tide dinoflagellate, Alexandrium sp., in the Gulf of Maine

Paralytic shellfish poisoning (PSP) occurs nearly every year in the Gulf of Maine. In a study of dynamics of the causative organism, the toxic dinoflagellate Alexandrium sp., we conducted three surveys of the coastal and oshore waters of Gulf of Maine during the summer of 1998, sampling more than 200 stations during each cruise in June, July and August. Hydrographic data were collected and concentrations of phytoplankton chlorophyll, inorganic nutrients and densities of Alexandrium cells were measured in discrete water samples. The distributions of Alexandrium at the surface and in subsurface waters displayed maximum cell densities in the oshore waters of the Gulf on all three cruises. Highest cell densities in surface waters (ca. 5.510 3 cellsl ˇ1 ) were observed in two broad patches: one in the Bay of Fundy and another in shelf and oshore waters of the central and eastern Gulf of Maine in association with the Eastern Maine Coastal Current. Highest subsurface densities of cells appeared to be associated with the frontal edges beyond the cold surface waters associated with the Eastern Maine Coastal Current. As the summer progressed, the highest surface densities of Alexandrium receded toward the eastern portions of the Gulf and the Bay of Fundy. We suggest that the oshore distributions of relatively high densities of Alexandrium are naturally occurring and can be related to inorganic nutrient fluxes, and to the ambient light field as it varies seasonally and vertically. Locations of high cell densities were described and interpreted using a nondimensional light-nutrient parameter, computed as the ratio of the depth of the 10% surface irradiance to the depth of 4mMNO3 concentration. Possible mechanisms responsible for periodic development of PSP outbreaks in nearshore shellfish beds are discussed. # 2001 Elsevier Science Ltd. All rights reserved.

Strontium:calcium ratios in juvenile Atlantic herring Clupea harengus L. otoliths as a function of water temperature☆

The effect of environmental temperature on the atomic concentration ratios of strontium to calcium in fish otoliths was investigated for young-of-the-year juvenile Atlantic herring Clupea harengus L. which were captured from the wild and held in the laboratory for 1 yr. The fish were maintained in large volume, flowing seawater tanks and were subject to the seasonal temperature cycle of Maine coastal waters (≈ 2–18°C). Otolith Sr/Ca ratios were measured for 136 fish and related to temperature by a hyperbolic Michaelis-Menten function. The greatest temperature effect occured at lower temperatures, where Sr/Ca ratios increased with decreasing temperature. There was less of an effect at warmer temperatures where the Sr/Ca ratios approached an asymptote. The results are interpreted in terms of the effect of environmental temperature on the physiological processes that result in a discrimination against the passage of strontium from seawater into the saccular endolymph which baths the otoliths. We suggest that at lower temperatures, where those physiological processes become slowed or impaired, strontium passes more readily into the endolymph and becomes incorporated into the otolith aragonite, thus supporting the use of otolith Sr/Ca ratios in studies of life histories of fishes at environmental extremes.

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.

Strontium:calcium concentration ratios in otoliths of herring larvae as indicators of environmental histories

SynopsisElemental analyses, using wave-length dispersive electron microprobe techniques on otoliths from reared Atlantic herring larvae, Clupea harengus, showed trace quantities of strontium relative to that of calcium, and an inverse relationship between Sr/Ca concentration ratios and rearing temperature. These data are consistent with those for coral aragonite, in that there appears to be an inverse temperature effect on physiological incorporation of strontium in the otolith aragonite. Our determinations of Sr/Ca concentration ratios of lab-reared herring larvae showed that the deposition of strontium relative to calcium and the rearing temperature were related, where: T (° C) = −2.955 [Sr/Ca] × 1000 ± 19.172. This principle thus makes it possible to use Sr/Ca concentration ratios in fish otoliths to delineate past temperatures experienced by an individual. Further, combining electron microprobe analyses with scanning electron microscope (SEM) examinations of daily increments in the same otolith makes it possible to reconstruct the temperature history for an individual fish on a time scale of days. An example of the application of the technique to an approximately six-month-old field-caught herring larva is given, and the limitations of the technique are discussed.

Blooms of the coccolithophore Emiliania huxleyi with respect to hydrography in the Gulf of Maine

Abstract We present results of oceanographic surveys of visually turbid blooms of the coccolithophore Emiliania huxleyi in the Gulf of Maine during the summers of 1988, 1989 and 1990. In each year, hydrographic stations within the blooms could be distinguished from non-bloom stations on a temperature-salinity diagram. In 1988 and 1989 the blooms were confined to the surface waters of the central western Gulf of Maine; T-S analyses showed they occurred in higher salinity surface waters at stations characterized by a well-defined upper mixed layer overriding a sharp pycnocline. Nutrients (not measured in 1988) were near depletion in the surface waters of both bloom and non-bloom stations in 1989, with surface phosphate being lower in the bloom waters (0.02–0.16 μM in the top 15 m) than in non-bloom waters (0.21–0.49 μM). Phosphate was not as low in the surface waters of the 1990 bloom. The bloom that year was much smaller in areal extent than in 1988 or 1989, and was limited to the northern part of the Great South Channel and western Georges Bank area of the Gulf of Maine. T-S analyses indicated significant mixing of different water masses in the area of the bloom in 1990, with the bloom being confined to those stations having less dense surface waters, of lower salinity, than the non-bloom stations. There also was evidence of a subsurface salinity minimum beneath the bloom waters in 1990. Blooms of E. huxleyi with surface expressions of visually turbid waters do not occur every year in the Gulf of Maine, and we discuss possible causative factors, specifically as related to the age or maturity of surface waters and macro- and micro-nutrient levels, that could facilitate bloom formation and which could vary between years.

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.

Sources and cycling of nitrogen in the Gulf of Maine

Abstract An analysis of water mass flows and nitrogen fluxes in the Gulf of Maine region shows that deep Slope Water that enters the Gulf through the Northeast Channel, and Scotian Shelf Water that enters at the surface, dominate the flux of nitrogen into the Gulf. A box model is developed that examines internal vertical nitrogen fluxes, and reveals that the flux of nitrogen into surface waters is sufficient to explain only about 59 gC m −2 yr −1 of new primary production, which is 20% of the total estimated Gulf of Maine primary production of 290 gC m −2 yr −1 . This means that the Gulf-wide f ratio (of “new” NO 3 -based production to the total production based on both new NO 3 and recycled NH 4 ) is 0.20, which is more typical of oligotrophic oceans than a productive continental shelf sea like the Gulf of Maine. The expected f ratio is nearer to 0.4, which would require an additional flux of new NO 3 into the Gulf equal to about 40% of the total flux already accounted for by all sources: Slope Water, Scotian Shelf Water, rivers and atmospheric deposition. This additional supply of “new” nitrogen is argued to be the result of water column nitrification. The box model also shows, surprisingly, that nutrients delivered to surface waters of the Gulf by Scotian Shelf Water are roughly equal to that of Slope Water. It is concluded that better estimates are needed of water flows into and out of the Gulf, along with more measurements of their nutrient loads, and that measurements should be made of water column nitrification rates. An overall conclusion is that the energetics of vertical mixing processes that deliver nutrients to the productive surface waters set the upper limit to biological production in the Gulf of Maine, and that construction of carbon and nitrogen budgets that consider only fluxes into and out of the Gulf, and not internal recycling, will be in error.

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.

Early spring phytoplankton blooms in the Gulf of Maine

Abstract Our observations of the distribution of early spring phytoplankton blooms (27 March to 5 April 1984) in the Gulf of Maine support previous contentions that bloom propagation begins when the depth averaged in situ solar irradiance within the upper mixed layer is about 40 ly d −1 . We found that prior to vernal warming and thermal stratification, the formation of an upper mixed layer in these waters appears to be determined in three ways: (1) by density stratification resulting from freshwater runoff immediately adjacent to the coast, (2) by intrusions of dense slope water and its subsequent doming to form a pycnocline in Jordan Basin, and (3) by local bathymetry.