Phytoplankton

Northeast ecosystem component

Overview

Marine ecosystem productivity depends on the amount of primary production by phytoplankton, unicellular photosynthetic algae and cyanobacteria suspended in the water column that form the base of the marine food web. Measurements of the primary photosynthetic pigment, chlorophyll a (CHL), are commonly used as a proxy for phytoplankton biomass. Near-surface CHL can be measured remotely by ocean color sensors on satellites and then incorporated into integrated models to estimate primary production (PP). Phytoplankton composition and changes in the timing (phenology) and magnitude of blooms (peaks in biomass), can provide sensitive indicators of ecosystem responses to major external disturbances1. The Northeast Large Marine Ecosystem (NE-LME) may be particularly vulnerable to phenological shifts that are in response to climate change because recruitment success of higher trophic levels is highly reliant on synchronization with pulses and composition of phytoplankton production2,3. Blooms of certain toxin producing phytoplankton can also result in harmful algal blooms (HABs) that can negatively impact the health of other organisms, including humans.

Annual chlorophyll anomalies in the NE-LME
Chlorophyll α anomaly ratios in the Northeast Large Marine Ecosystem between 1998-2017. 

Gulf of Maine

There is an onshore to offshore gradient of decreasing PP in the Gulf of Maine, with the lowest PP in the NE- LME occurring over the deep basins. Mean seasonal patterns include low concentrations during winter, except along coastal margins and shallow banks, a region-wide spring bloom followed by a mid-summer decrease and often a fall bloom4. Variations within the GOM are often related to patterns of vertical stability and nutrient sources.

Georges Bank

Primary production rates on Georges Bank are among the highest of any continental shelf seas5. Relatively high phytoplankton concentrations are consistently found within the shallow (< 60 m), tidally well-mixed regions of Georges Bank with mean concentrations equivalent to those observed in nearshore areas6. Strong currents generated by diurnal and semidiurnal tides interacting with the shallow bottom topography keep the shallow portions of Georges Bank mixed throughout the year7 and help support an extensive food web including high levels of fish production that have sustained commercial fisheries since the 16th century8.

Mid-Atlantic Bight

The Mid-Atlantic Bight is a relatively productive continental shelf system supporting a wide variety of fauna and flora, including commercially important shellfish and groundfish. Primary productivity is highest in the nearshore and estuarine regions with coastal phytoplankton blooms initiating in the fall and winter, although the timing and spatial extent of blooms varies interannually. Offshore, strong stratification during the summer limits nutrient inputs to the surface layer, often resulting in subsurface chlorophyll maxima that are not detectable by satellite remote sensors. Near the shelf-break, a thermohaline front develops during summer upwelling deep, nutrient rich waters to the surface and supporting localized enhanced phytoplankton biomass that propagates up the food web generating local patches of zooplankton, fish and top predators9.


Next component

    1. 1. Song H, Ji R, Stock C, Wang Z (2010) Phenology of phytoplankton blooms in the Nova Scotian Shelf - Gulf of Maine region: remote sensing and modeling analysis. Journal of Plankton Research 32:1485-1499

    2. 2. Cushing DH, Blaxter JHS, Southward AJ (1990) Plankton Production and Year-class Strength in Fish Populations: an Update of the Match/Mismatch Hypothesis. Advances in Marine Biology, Book Volume 26. Academic Press

    3. 3. Edwards M, Richardson AJ (2004) Impact of climate change on marine pelagic phenology and trophic mismatch. Nature 430:881-884
    4. 4. Thomas AC, Townsend DW, Weatherbee R (2003) Satellite-measured phytoplankton variability in the Gulf of Maine. Continental Shelf Research 23:971-989

    5. 5. O'Reilly JE, Evans-Zetlin C, Busch DA (1987) Primary Production. In: Backus RH (ed) Georges Bank. MIT Press, Cambridge, MA

    6. 6. NEFSC (2018) State of the Ecosystem - Mid-Atlantic Bight. Woods Hole, MA

    7. 7. O'Reilly JE, Zetlin C (1998) Seasonal, horizontal, and vertical distribution of phytoplankton chlorophyll a in the northeast U.S. continental shelf ecosystem. U. S. Department of Commerce, Seattle, Washington

    8. 8. Fogarty Michael J, Murawski Steven A (1998) Large-scale disturbance and the structure of marine systems: fishery impacts on Georges Bank. Ecological Applications 8:S6-S22

    9. 9. Genin A (2004) Bio-physical coupling in the formation of zooplankton and fish aggregations over abrupt topographies. Journal of Marine Systems 50:3-20
Simplified food web tracing primary productivity through the ecosystem (Credit: Kimberly Hyde/NOAA)
Food web tracing primary productivity through a generalized marine ecosystem (Credit: Kimberly Hyde/NOAA)

Ecological Interactions

The NES LME is a highly productive continental shelf ecosystem, which supports several important commercial fisheries1,2. Winter convective mixing, vertical tidal mixing and estuarine outflows supply nutrients into surface waters supporting phytoplankton productionand influencing trophic food-web dynamics. Strong correlations between primary production and marine fish production4,5 and their fundamental role in the global carbon cycle make primary producers significant components of the marine ecosystem. Thus it is critical to consider phytoplankton in ecosystem based management plans and to understand how changes in the phytoplankton community and its productivity can alter the structure and function of marine ecosystems.

 

    1. 1. O'Reilly JE, Evans-Zetlin C, Busch DA (1987) Primary Production. In: Backus RH (ed) Georges Bank. MIT Press, Cambridge, MA

    2. 2. Link J, Overholtz W, O'Reilly J, Green J, Dow D, Palka D, Legault C, Vitaliano J, Guida V, Fogarty M, Brodziak J, Methratta L, Stockhausen W, Col L, Griswold C (2008) The Northeast U.S. continental shelf Energy Modeling and Analysis exercise (EMAX): Ecological network model development and basic ecosystem metrics. Journal of Marine Systems 74:453-474

    3. 3. Townsend DW, Thomas AC, Mayer LM, Thomas MA, Quinian JA (2006) Oceanography of the Northwest Atlantic Continental Shelf. In: Robinson AR, Brink K (eds) The Global Coastal Ocean: Interdisciplinary Regional Studies and Syntheses, The Sea, Book 14A. Harvard University Press, Cambridge, MA

    4. 4. Ryther JH (1969) Photosynthesis and fish production in the sea. Science 166:72-76

    5. 5. Iverson RL (1990) Control of marine fish production. Limnology and Oceanography 35:1593-1604

Environmental Drivers

Marine phytoplankton biomass, production and composition are driven by changes in the environmental forcings (bottom-up control) and consumers (top-down control). Changes in day length, localized weather events, regional currents, water column stratification and mixing, and basin-scale climate patterns alter the physical environment, subsequently affecting overall phytoplankton production and seasonal variability. The seasonal cycle of phytoplankton growth, composition, and biomass is strongly correlated with dissolved nutrient concentrations in the euphotic zone. During winter, nutrients accumulate in the surface waters when lower light levels limit phytoplankton growth. As growing conditions improve in spring, uptake by phytoplankton reduces nutrient concentrations and summer stratification limits mixing of deep water nutrients to the surface. During summer, productivity is at its highest, but is reliant on recycled nutrients within the water column by smaller phytoplankton species (i.e. the microbial loop). Despite high primary productivity during the summer, phytoplankton biomass is actually low; however localized upwelling events or storms can periodically mix deep water nutrients to the surface creating episodic phytoplankton blooms.

Monthly animation of chlorophyll a in the NE-LME (2017)
Monthly average chlorophyll α in the NE-LME during 2017.

Gulf of Maine

In the GOM, the principal supply of nutrients supporting new primary production are from the cold, fresh Labrador Sea Slope Water and the warmer and saltier Warm Slope Water originating from the Gulf Stream. The nitrate load in the Warm Slope Waters is approximately 50% higher than that of the Labrador Slope Waters, which can alter the structure and function of the planktonic ecosystem, including the cell densities of the toxic dinoflagellate Alexandrium fundyense1,2.

Georges Bank

On the shoals of GB, vigorous tidal mixing injects deep nutrient-rich water onto the Bank, which is then dispersed and advected to drive phytoplankton growth throughout the year. In the shallow central portion of GB, light is not a limiting factor of photosynthesis, thus winter concentrations of phytoplankton are often higher than the surrounding areas. Following the winter-spring bloom, cross-frontal nutrient fluxes on the northwest flank to higher phytoplankton concentrations that are advected around the Bank to the southern flank, fueling secondary production and leading to higher stocks of zooplankton in that region3.

Mid-Atlantic Bight

The MAB is characterized by well-mixed, light-limited, nutrient replete conditions in the winter, and stratified, nutrient-limited, conditions in the summer. Near-shore upwelling, river plumes, and shelf break processes create seasonally varying production gradients through different physical mechanisms4. At the shelf break, retention of upwelled offshore nutrients enhances summer phytoplankton production, aggregating and attracting lower and mid-trophic level species, and thus creating persistent and predictable foraging areas for marine predators5.


 

    1. 1. Townsend DW, Rebuck ND, Thomas MA, Karp-Boss L, Gettings RM (2010) A changing nutrient regime in the Gulf of Maine. Continental Shelf Research 30:820-832

    2. 2. Townsend DW, McGillicuddy Jr DJ, Thomas MA, Rebuck ND (2014) Nutrients and water masses in the Gulf of Maine–Georges Bank region: Variability and importance to blooms of the toxic dinoflagellate Alexandrium fundyense. Deep Sea Research Part II: Topical Studies in Oceanography 103:238-263

    3. 3. Townsend DW, Thomas AC, Mayer LM, Thomas MA, Quinian JA (2006) Oceanography of the Northwest Atlantic Continental Shelf. In: Robinson AR, Brink K (eds) The Global Coastal Ocean: Interdisciplinary Regional Studies and Syntheses, The Sea, Book 14A. Harvard University Press, Cambridge, MA

    4. 4. Mouw CB, Yoder JA (2005) Primary Production Calculations in the Mid-Atlantic Bight, including Effects of Phytoplankton Community Size Structure. Limnology and Oceanography 50:1232-1243

    5. 5. Thorne LH, Foley HJ, Baird RW, Webster DL, Swaim ZT, Read AJ (2017) Movement and foraging behavior of short-finned pilot whales in the Mid-Atlantic Bight: importance of bathymetric features and implications for management. Marine Ecology Progress Series 584:245-257

Human Activities

While high abundances of phytoplankton typically provide increased food for secondary production, some phytoplankton species can negatively impact human activities and other higher trophic levels. Collectively these occurrences are called Harmful Algal Blooms (HABs) and can cause significant impacts to fisheries and human health.

 

 

Pseudo-Nitzchia

Alexandrium spp.

Toxic dinoflagellate blooms of Alexandrium occur annually and produce saxitoxin and related neurotoxins. Filter feeding bivalves can accumulate the toxins, causing Paralytic Shellfish Poisoning (PSP) when consumed by humans. All states in the NE have rigorous monitoring programs to insure that commercially available shellfish are safe to eat. Blooms in coastal waters of the Gulf of Maine are typically found first off the coast near Portland, Maine and southward along the coast to Cape Ann in late May and early June. Cell densities are highest in eastern Maine later in summer, in July and August, and blooms in the Bay of Fundy can occur as late as September and October. Both inshore blooms and offshore blooms, which remain largely undetected, can cause mortalities of fish marine mammals, turtles and birds1.

Pseudo-nitzschia spp.

Domoic acid, produced by the some diatoms of the genus Pseudo-nitzschia, accumulates in filter-feeding bivalves and fish and causes amnesic shellfish poisoning (ASP) in humans and can lead to fatalities of marine mammals and seabirds2,3. Blooms of Pseudo-nitzchia have become more common in the past decade causing closures of shellfish beds in Maine and Rhode Island in 2016 and 20174.

Other HAB species

In the lower Chesapeake Bay, annual blooms of the dinoflagellate Cochlodinium polykrikoides have been observed for several decades and more recently, blooms of Alexandrium monilatum, a toxin-producing dinoflagellate common to the Gulf of Mexico, have invaded the region. Both dinoflagellate species have been associated with fish kills either directly or indirectly, and may also negatively impact shellfish aquaculture. Blooms of C. polykrikoides and Aureococcus anophagefferens (responsible for “brown tides”) occur regularly in the Long Island Sound region, with the most extensive and longest-lasting brown tide event ever recorded occurring in 2017.


 

    1. 1. Starr M, Lair S, Michaud S, Scarratt M, Quilliam M, et al. (2017) Multispecies mass mortality of marine fauna linked to a toxic dinoflagellate bloom. PLOS ONE 12(5): e0176299.

    2. 2. Lefebvre KA, Bargu S, Kieckhefer T, Silver MW (2002) From sanddabs to blue whales: the pervasiveness of domoic acid. Toxicon 40:971-977

    3. 3. Fernandes LF, Hubbard KA, Richlen ML, Smith J, Bates SS, Ehrman J, Léger C, Mafra Jr LL, Kulis D, Quilliam M, Libera K, McCauley L, Anderson DM (2014) Diversity and toxicity of the diatom Pseudo-nitzschia Peragallo in the Gulf of Maine, Northwestern Atlantic Ocean. Deep Sea Research Part II: Topical Studies in Oceanography 103:139-162.

    4. 4. NEFSC (2018) State of the Ecosystem - Mid-Atlantic Bight. Woods Hole, MA