In open waters of the MBNMS, the food webs are supported almost entirely
by phytoplankton. Seasonally, the production and biomass of phytoplankton
change markedly due to variations in the supply of nutrients need for growth.
Some nutrients are entrained in water entering the area and some arrives
from local coastal upwelling. As waters move past two promontories in the
MBNMS, Point Año Nuevo and Point Sur, upwelling may occur, resulting
in a downstream plume of cold water visible on satellite images (see
Physical Oceanography Section). Simultaneously, surveys of ocean color
usually show a corresponding color change, reflecting the growth of phytoplankton
(Abbott and Zion 1985, Traganza et al. 1987). These upwelling events are
wind-driven and episodic and occur mostly during spring and summer months
during the upwelling season. In general, highest phytoplankton biomass occurs
in a relatively narrow band inshore (Barale and Fay 1986) with additional
high pigment rings and filaments offshore (Abbott and Barksdale 1991) and
when water is coldest (Palaez and McGowan 1986).
Phytoplankton populations, which show strong seasonal cycles, have been
studied in Monterey Bay since the 1950s (Bolin and Abbott 1963, Abbott and
Albee 1967). Since the late 1980s, an intensive and continuous sampling
program at the Monterey Bay Aquarium Research Institute (MBARI) has been
studying phytoplankton (and other oceanographic phenomena) using a station
grid in Monterey Bay and 2 permanent moorings in the Monterey Bay region.
The moorings measure optical properties, temperature, phytoplankton biomass
and currents (Chavez et al. 1994). The production cycle is shown in Figure 1, with the broad seasonal maximum during the spring/summer upwelling
season. Individual blooms may last several weeks, and phytoplankton growth
usually occurs from the surface down to 10-40 m (Garrison 1976). After the
upwelling winds have slacked, a relaxation phase may occur, with warmer
offshore waters flooding the surface of Monterey Bay with their more oceanic
species in clear, low productivity water (Bolin and Abbott 1963). Thus the
upwelling season is characterized by high average productivities, but highly
variable conditions on shorter time scales. After the upwelling favorable
winds slack, in late summer or fall, there is usually lower phytoplankton
productivity and the species composition of the plankton changes. At any
time, however, the onset of upwelling-favorable winds can alter this pattern.
The larger MBNMS phytoplankton, i.e. "netplankton" captured in
nets with meshes usually larger than 20 microns, are typical of those from
colder coastal waters throughout California. During the upwelling period,
colony forming diatoms dominate (Bolin and Abbott 1963, Garrison 1979),
particularly species of Chaetoceros, Rhizosolenia, Skeletonema,
and a group of species formerly called Nitzschia (now mostly renamed Pseudo-nitzschia;
see also Cupp 1943). The large colonies, which may reach > 1 mm, sustain
many filter feeders and account for the wealth of marine life in the area.
It is thought that waters like the MBNMS, with abundant colonial diatoms,
support the world's most prolific fisheries because of their short food
chains: large colonies often can be grazed directly by small schooling fish
like anchovies and sardines (Ryther 1969).
In the fall and occasionally during interludes in the upwelling season,
netplankton may be dominated by dinoflagellates, especially during warming
intervals. These sometimes produce red tides from aggregations of species
of Prorocentrum, Ceratium, and Gonyaulax (Bolin
and Abbott 1963, Smith 1991). Waters of the MBNMS may glow blue at night
during this time, from the bioluminescence (light production) of dinoflagellates,
a phenomenon which helped illuminate schools of sardines to the fishing
fleet in earlier days. The fairly frequent red tides in the MBNMS are mostly
benign. However, there have been occasional episodes of paralytic shellfish
poisonings (a group of dinoflagellate toxins) in the region, with over 100
poisonings and a dozen deaths in San Mateo and Santa Cruz counties between
1929-89 that were caused by humans consuming shellfish contaminated with
the toxins (Nishitani and Chew 1988, Price and Kizer 1990). The California
Department of Public Health, the oldest program in the U.S. to monitor dinoflagellate
toxins, regularly samples to prevent poisonings (Price and Kizer 1990).
Recently, outbreaks of phytoplankton causing "Amnesic Shellfish Poisoning"
have been discovered in the MBNMS. In 1991, seabird deaths in Monterey Bay
were traced to anchovies that consumed a diatom now renamed Pseudo-nitzschia
australis (formerly know as Nitzschia) (Work et al. 1993,
McGinness et al. 1995). This common, colony-forming diatom was shown to
produce domoic acid (Garrison et al. 1992), a toxin that causes neurological
and digestive disturbances and occasionally death in vertebrates, including
humans. Members of this genus are common in the MBNMS area (Buck et al.
1992; Walz et al. 1994). Previous episodes may have resulted in the occasional
bird kills reported in the area, including an event that likely led to Alfred
Hitchcock's movie, "The Birds" (Haywood 1995). As in previous
years, waters of the MBNMS continue to be regularly monitored for blooms
of these and other phytoplankton toxins, which are difficult to predict.
The most studied phytoplankton of the MBNMS are the netplankton, which are
responsible for seasonal blooms. The phytoplankton smaller than 20 microns
(the 2-20 micron nanoplankton and the under-2 micron picoplankton) are relatively
poorly known locally, but are always present in modest abundance. In the
MBNMS, these smaller phytoplankton can contribute the largest proportion of the phytoplankton biomass
and production during non-upwelling periods (Malone 1971, Garrison 1976).
As phytoplankton largely support the offshore pelagic life of most of the
MBNMS species, changes in their production have important consequences for
higher trophic levels. During El Niño events, water flow from the
south increases, upwelling decreases, and phytoplankton production decreases
(McGowan 1984). Comparatively little is known about the ENSO effects on
phytoplankton productivity in the MBNMS. However, recent data on the 1992-93
ENSO showed reductions of about 20-30% in productivity in Monterey Bay (Figure 1) and
a delay in the onset of the normal upwelling season (Lenarz et al. 1995).
For the 1982-83 ENSO, the strongest in a century, dramatic changes in the
phytoplankton composition of Monterey Bay were documented, as were proportional
increases in dinoflagellate and tropical species contributions and decreases
in diatom contributions, significant increases in species diversity of the
netplankton overall, and an overall decline in phytoplankton abundance (Smith
Next - Section IV. Zooplankton
Pelagic Zone Table of Contents