Pelagic Zone
IV. Zooplankton

Zooplankton are the major trophic link to pelagic vertebrates, and their abundant populations draw many birds, fish and whales to the area. Whereas phytoplankton production is limited to upper, sunlit waters, zooplankton growth occurs at all depths, including the epipelagic (0-200 m), mesopelagic (200-100 m) and bathypelagic (>1000 m) zones. In the MBNMS, zooplankton are usually most abundant in neritic and inshore regions (Colebrook 1977), as compared with waters of the offshore California Current. Biomass patterns appear strongly linked to mesoscale circulation features. For instance, the eddy/meander off Monterey Bay may be a focus for zooplankton (Huntley et al. 1995), and such mesoscale features may show predictable gradients in zooplankton species (Mackas et al. 1991). A strong seasonal pattern is evident in zooplankton biomass, with highest amounts inshore and maxima in spring and summer (U.S. GLOBEC 1994). The seasonal input of larvae, many from shallow benthic invertebrates, is important regionally; it is likely that spawning is triggered by phytoplankton blooms (Starr et al. 1990). (Local return and settlement of larvae, however, may be strongly dependent on mesoscale current features [Farrell et al. 1991, Graham 1994]). In additional to the onshore-offshore, mesoscale, and seasonal variations, there is also the well known decline in zooplankton abundance from the epipelagic to the mesopelagic zone and below (Vinogradov and Tseitlin 1983). However, in the epi- and mesopelagic zones these are complicated by daily vertical migrations of many species. Vertical distributions of local plankton also are known to be influenced by the intensity of turbulence (Haury et al. 1992)

A. Shallow Zooplankton

The taxonomic composition of many zooplankton groups has been documented regionally. Dominant epipelagic groups inshore are crustacean larvae, copepods, euphausiids, ctenophores, hydrozoan medusae and siphonophores, and the chaetognaths (Bigelow and Leslie 1930, Colebrook 1977). In most cases, the numerical and biomass dominants are often copepods. These include small, neritic surface-dwelling Acartia and Paracalanus, large, more oceanic Calanus pacificus, vertically migrating Metridia pacifica and Pleuromamma and deeper, nonmigrating Oithona (Haury et al. 1990, Huntly et al. 1995, Steinberg unpublished data). Copepods appear to be separated to some extent into niches that are onshore-offshore, depth and migration related, but patterns have not been well described locally, though they likely resemble those described to the north (Peterson et al. 1979).

Euphausiids (krill) are particularly important in food webs of the MBNMS and are major prey for marine vertebrates, including anchovy, squid, salmon, hake, blue sharks and many other vertebrates (Morejohn et al. 1978). Krill swarms, especially of the more oceanic Euphausia pacifica and the more neritic Thysanoessa spinifera draw a variety of predators. Dense aggregations can exist at depth, for example along the walls of Monterey submarine canyon, where they attract diving blue whales (Schoenherr 1991). Though krill are normally found at depth during the day, surface swarms occasionally occur that may be related to reproduction (Smith and Adams 1988). Surface patches, sometimes colored, are particularly important to seabirds and cetaceans (also see Seabird & Shorebird and Marine Mammal sections.) Distributions of krill and other zooplankton recently have been linked to mesoscale features (Huntley et al. 1995).

"Jelly" species are conspicuous in MBNMS zooplankton communities, especially since SCUBA divers, manned submersibles (Barham 1956) and MBARI's remotely operated vehicle (ROV: Robison 1994)) have provided visual access to the Bay's fauna. Surface water communities include a host of seasonally-produced (mostly spring-summer) hydrozoan cnidarians, which are millimeter- to centimeter-sized jellyfish that prey heavily on small zooplankton and are often budded from benthic polyps (Smith 1977, Morris et al. 1980, Wrobel 1990). Other larger scyphozoan medusae, with bells sometimes exceeding 10 or 20 cm in diameter, occasionally swarm in the area. Abundant species, with tentacles and mouth lobes trailing sometimes meters below the swim bell, include the purple striped jelly Pelagia, the sea nettle Chrysaora, and the abundant moon jelly, Aurelia, all important predators on copepods, fish larvae and other zooplankton (Morris et al. 1980; Wrobel 1990). The first two of these occasionally wash up on local beaches. When abundant in summer, these scyphozoans may draw sea turtles to this region (Starbird et al. 1993). The large jellies often are colonized by juvenile crustaceans, including the megalopae of Cancer crabs. They are sometimes held offshore during upwelling by fronts (Wrobel 1990; Graham 1994).

Other jelly predators include the siphonophores. These are colonial hydrozoan jellyfish that range from the surface to the deepest waters of the MBNMS. Important surface forms include Muggiaea and other species (Bigelow and Leslie 1930), but the group is poorly known from conventional plankton net samples because the delicate colonies often are damaged beyond recognition during capture. Surface species often have small gas-filled floats and prey on zooplankton.

The comb jellies, members of the phylum Ctenophora, include important local species, mostly poorly known because of their fragility (Wrobel 1990). The sturdiest of these is Pleurobrachia. An occasional dominant in the spring and summer, it consumes a variety of small prey, especially copepods (Bigelow and Leslie 1930). Pleurobrachia can occasionally be found in strand lines along the area beaches, looking like centimeter-sized, colorless marbles. Others comb jellies, like Leucothea, are extraordinarily fragile (Wrobel 1990) and reported mostly by divers.

A particularly abundant jelly in the region is the By-the-wind-sailor, Velella velella, a sail-bearing cnidarian that blows ashore in windy periods in spring and summer (Francis 1985, Wrobel 1990). The lovely, small blue floats possess dinoflagellate (algal) symbionts, but also consume small zooplankton. The bleached floats can persist in drifts for months (also see Sandy Beach section).

Locally, larvaceans (gelatinous invertebrate chordates) can be important in the epipelagic zone (Bigelow and Leslie 1930). These organisms are generally small (millimeter-sized) and tadpole-like in appearance; they inhabit transparent feeding devices, called "houses" which are food-concentrating structures (Alldredge 1976). Several species can be abundant locally, and one - Oikopleura longicauda - occasionally occurs in long, red, surface windrows (Alldredge 1982, M. Silver unpublished data). Larvaceans discard their millimeter- to centimeter-sized houses every few hours (Alldredge 1972), contributing importantly to "marine snow", the visible, non-living particles found abundantly in the MBNMS (Silver et al. 1978). Along with phytoplankton, marine snow reduces visibility for recreational divers during much of the year, especially during the upwelling season.

Other species of gelatinous plankton visit surface waters of the MBNMS, often in association with inshore movements of more oceanic, California Current waters. These visitors include salps and doliolids, pteropods (sea butterflies) including the larger mucus web feeders (e.g. Corolla) as well as the smaller carnivores (e.g. Clione) (regional offshore distributions in CalCOFI Atlases, condensed in Colebrook 1977, unpublished inshore obs. by authors).

Changes in the local currents, especially those associated with ENSO (El Niño-Southern Oscillation) events, can dramatically alter both the abundance and species composition of the zooplankton in upper waters. Thus, the presence of the swimming red crab Pleuroncodes is a sure sign of southern water, as this species normally lives off Baja California (McGowan 1984). Likewise, the decline of temperate species including local krill, a change that occurs with warming events like ENSO, may negatively effect marine birds and mammals. Decadal scale changes also have been reported in zooplankton abundance in the in southern California (Roemmich and McGowan 1995), but such changes are not as so clear in the MBNMS area (Chelton et al. 1982). Decadal scale changes have been linked less with alterations in upwelling intensity than with variations in the water flow rates of the southward-moving California Current (Chelton et al. 1982, Roemmich and McGowan 1995).

B. Deep Zooplankton

At depth, the zooplankton fauna is much less well known. Observations from ROVs (Remotely Operated Vehicles) and submersibles have been particularly important in the MBNMS in discovering new species, learning more about previously named species, and studying behavior. One of the more common deepwater hydrozoans (a narcomedusa) is Solmissus , a predator on other gelatinous zooplankton (Mills and Goy 1988). The siphonophores are particularly prominent in deep waters and are important predators. Some, such as Apolemia, may reach 40 meters in length in the Monterey Submarine Canyon (Barham 1956, Robison 1995).The fragile, deep water ctenophore fauna was virtually unknown until the studies using MBARI's ROV. Matsumoto and Robison (1992) recently described Kiyohime usagi, the large (up to 28 cm) , "rabbit-eared" ctenophore. It is a large, passive predator that ambushes euphausiids. Other poorly known ctenophores of the MBNMS's mesopelagic and bathypelagic waters have also been recorded recently by Matsumoto (1990).

A number of species are now being studied extensively using the MBARI ROV. One example is the yellow mesopelagic worm Poeobius, a mesopelagic detrital feeder that uses mucus threads to capture sinking detritus (Utall and Buck 1996) Another example is the mesopelagic "giant larvacean" Bathochordaeus that builds houses up to 1 m across (Hamner and Robison 1972); these houses support a unique fauna of associated zooplankton (Steinberg et al. 1994). Bathochordaeus occurs in the high salinity California Undercurrent and is known from more southerly waters (Barham 1979). After being discarded, the large houses sink rapidly, carrying shallower materials to the seafloor below (Hamner and Robison 1992).

Below 500 m, most plankton are colored deep reds, browns, purples and black: these color changes are generally associated with the lack of light, especially red light, at depth. (Illuminated by the faint blue-greenish light that penetrates deepest, red and dark colored objects are non-reflective and invisible.) This is the zone whose fauna is poorly known, because of fragility of individuals (resulting in unrecognizable specimens from net tows) and low abundance. Forms include the archaic, football-sized "vampire" squid Vampyroteuthis infernalis, and the beautiful red/purple medusa Atolla (Robison 1995). Vampyrocrossota childressi, the only known black medusa, occurs in the Monterey Canyon at depths from 600-1500 m (Thuesen 1993). Many of the mesopelagic and bathypelagic species are bioluminescent, using body lights and extruded glowing clouds for various purposes (Robison 1995).