Ecology of the Northeast US Continental Shelf
Phytoplankton: The Base of the Food Web
Ultimately, how many organisms that can live and grow in a given area depends on the amount of primary production, or rather the amount of photosynthesis by plants at the base of the food web. Photosynthesis involves the capture of light energy by plant pigments such as chlorophyll a, which is then used to convert water and carbon dioxide into carbohydrates and oxygen. Primary productivity is simply the rate of photosynthesis and uptake of dissolved nutrients such as nitrate, phosphate and silicate to produce more plant matter or biomass. In shallow waters where sunlight reaches the bottom, larger plants, including seaweeds and sea grasses, are important primary producers. In the deeper open continental shelf water, sunlight is insufficient to sustain the seaweeds attached to the seabed, and microscopic plant cells collectively known as phytoplankton are responsible for nearly all of the primary production (Figure 1).
Our estimates of the average daily rate of phytoplankton primary production (PP) for the continental shelf system, and for contrast, the adjacent Slope Sea, Gulf Stream and Sargasso Sea are illustrated in Figure 2. There are large regional differences in primary production on the Northeast U.S. Continental Shelf Large Marine Ecosystem (NES LME). The most obvious pattern is the general onshore-offshore decrease in PP, from the coast to the shelf break.
This pattern, as well as the seasonal patterns derived from the satellite data and model estimates of PP, agrees well with patterns revealed during earlier shipboard surveys which found that the overall levels of primary production in the NES LME place it among the most productive continental shelf systems in the world.
The highest levels of PP are found on Georges Bank and in the immediate near-shore areas (particularly in the Mid-Atlantic Bight) and in the major estuaries where nutrients from land (such as nitrogen and phosphorus) essentially fertilize the sea water. Elevated levels of PP (2-3 gCm-2d-1) are evident in the coastal water adjacent to and generally south of the mouths of the Hudson, Delaware and Chesapeake Bays. Intermediate levels are found on the mid-shelf region of the Mid-Atlantic Bight, and in coastal areas of the Gulf of Maine. Primary production (about 0.7 gCm-2d-1) over the deep basins in the Gulf of Maine are the lowest observed in the NES LME. PP in the deep outer shelf Georges Bank and MAB water is low and similar to the levels in the deep basins of the Gulf of Maine. Note, however, that along the outer shelf the mean PP decreases as one proceeds along the 100m isobath from the southern flank of Georges Bank through the MAB to Cape Hatteras. In contrast to these high primary productivity levels characteristic of continental shelf water, mean PP in the Sargasso Sea is only 0.3-0.4 gCm-2d-1.
There are a number of environmental and oceanographic factors which interact to govern the magnitude of primary production and generate the distributional patterns in Figure 2. The principal factors are the amount of photosynthetically active radiation (PAR - basically visible light) striking the ocean surface, the turbidity and resulting downward penetration of PAR through the water column, the abundance or biomass of the phytoplankton, the availability of major nutrients such as nitrogen, phosphorus and silicon, water temperature which influences phytoplankton respiration and microbial recycling of nutrients, physical processes such as vertical mixing by strong tides and winds, vertical thermal stratification of the water column during summer, and circulation processes such as upwelling and downwelling from strong or persistent winds which may disperse or concentrate the phytoplankton, and the formation of fronts between different water masses.
The abundance or concentration of phytoplankton and the way they are vertically distributed in the upper sun-lit portion of the water column are key determinants of the level of primary production of the water column. A useful index of phytoplankton abundance commonly used is the concentration of the major photosynthetic green pigment chlorophyll a. Ocean color sensors on satellites measure the spectrum (color) of the ocean, that is, the water-leaving radiances at a number of visible and infrared wavelengths, and these data are then used to estimate the concentration of chlorophyll a primarily by comparing blue and green wavelengths.
We see a general pattern of decreasing chlorophyll with increasing depth and distance from the coast (Figure 3). This pattern of phytoplankton biomass distribution lays the foundation, along with light and other factors which will be described below, for the overall primary production pattern illustrated earlier in Figure 2. Georges Bank stands out as an offshore, phytoplankton-rich area, with chlorophyll concentrations in the tidally-mixed central portion exceeding 3 mg m-3. These high levels are comparable to those characteristically found near shore.
The seasonal pattern for satellite-derived surface chlorophyll is illustrated in Figure 3. The Winter-Spring bloom (spring bloom) represents a major pulse in the annual phytoplankton cycle. In the shallow MAB shelf waters the WS-bloom may begin in as early as January, on Georges Bank the WS-bloom initiates in February and intensifies during March-April, while in the deep waters in the Gulf of Maine and over the Slope Sea the peak occurs during April. The southwest-to-northeast progression of the WS-bloom in the GOM during April is illustrated in the accompanying animation. Typically lower levels of phytoplankton prevail during the summer when the water column is vertically stratified. On top of this seasonal pattern there are rather important variations in phytoplankton abundance over relatively shorter periods, from a few days to weeks, usually in response to changes in physical conditions (e.g. winds and associated vertical mixing or upwelling.
One of the more important features of the annual phytoplankton cycle in the Gulf of Maine is that it is somewhat out of phase with the cycle observed for Georges Bank and the MAB: In the Gulf of Maine, the fall bloom is as important as the winter-spring bloom, especially in the northern GOM (see accompanying animation). Under certain conditions phytoplankton blooms can also occur in summer as in the animation showing such an event in the Mid-Atlantic Bight most likely due to upwelling of deepwater nutrients to the surface.
We also see very clear regional differences in the ratio of subsurface to surface chlorophyll during summer. The ratio is low in the tidally energetic, well mixed regions of the coast and very high in the strongly stratified regions where mixing is impeded.
Phytoplanton Vertical Distribution and Size-Composition
The satellite-based map of the chlorophyll distribution shown in Figure 4 provides an index of the average amount of phytoplankton in surface water, but we are equally interested in the species or types of phytoplankton that are present throughout the oceanographically diverse ecosystem. In studies of the flow of organic matter and energy from primary producers through zooplankton grazers to fish, it is particularly important to understand the species-composition and size-distribution of the phytoplankton. While larger phytoplankton or net plankton provide an important food source for larger copepods (which are in turn preyed on by larval fish and other species); the smaller species or nanoplankton are grazed by smaller organisms. The map of netplankton (Figure 5) reveals that the larger netplankton are relatively more abundant in the tidally-mixed shallow water on Georges Bank and in near shore off the coasts of New Jersey and Long Island (particularly during the spring and fall blooms). In the deep water at the shelf break and central Gulf of Maine, the netplankton values are low. Here on average, the smaller nanoplankton prevail.
We also see seasonal patterns in which the predominance of netplankton on Georges Bank and elsewhere is reduced during stratification as nutrients are depleted (Figure 6). During this period much of the primary production comes from recycled rather than new production. This in turn has important implications for the flow of energy in the system. When energy has to flow through additional steps in the food web (as with the smaller organisms grazing on nanoplankton) there is a loss because of inefficiencies in the transfer of energy at successive steps.
Harmful Algal Blooms
While most phytoplankton species are harmless and provide the primary food source for the marine ecosystem, there are a few dozen phytoplankton species that produce toxins or cause ecological damage. These harmful algal bloom (HAB) species can cause fish kills, loss of critical coastal habitats, shellfish harvesting closures, mass mortality of marine animals, unsafe beaches and serious threats to human health from algal toxins. Furthermore, high biomass phytoplankton blooms can cause hypoxia (low dissolved oxygen conditions in the water) in coastal and estuarine regions where human population growth and related activities, such as urban runoff, agriculture, and wastewater treatment, have dramatically increased nutrient inputs over natural levels.
Aureococcus anophagefferens has caused destructive ‘brown tide’ blooms in the Middle Atlantic Bight region of the NES LME. Blooms of Aureococcus substantially decrease light availability in the water column, which can trigger large-scale die-offs of seagrass beds, a critical habitat for scallops, larval fish and other species. Although not considered a toxic species, large ‘brown tide’ blooms can negatively impact populations of clams, scallops and mussels and has caused collapses of the multi-million dollar shellfish industry in the Middle Atlantic Bight.
In the Gulf of Maine region, late spring/early summer blooms of Alexandrium fundyense, a toxic dinoflagellate, cause outbreaks of Paralytic Shellfish Poisoning (PSP). The toxins produced by Alexandrium accumulate in filter-feeding shellfish such as clams, mussels and oysters and can be transferred through the food web where they affect and even kill animals such as zooplankton, shellfish, fish, birds, marine mammals and even humans that feed either directly or indirectly on them. Alexandrium is naturally distributed throughout the Gulf of Maine, but bloom intensity varies from year to year and the distribution is often altered by current and wind patterns.
For more information on PSP in the Gulf of Maine, visit Northeast PSP
Cura, J.J. Jr. 1987. Phytoplankton. Chapter 20. In: (Backus, RH, ed.) Georges Bank. Cambridge, MA: MIT Press; p. 213-218.
O'Reilly, JE; Evans-Zetlin, C; Busch, DA. 1987. Primary Production. Chapter 21. In: (Backus, RH, ed.) Georges Bank. Cambridge, MA: MIT Press; p. 220-233.
O’Reilly, JE; Zetlin, C. 1998. Seasonal, Horizontal, and Vertical Distribution of Phytoplankton Chlorophyll a in the Northeast U.S. Continental Shelf Ecosystem. NOAA Technical Report NMFS 139; 120 p.
For more information, contact Kimberly Hyde