Seasonally, Chesapeake Bay ciliates reach maximum water-column abundance and biomass during the spring and early summer (May-June), with minima in mid- to late summer. Minor peaks also occur periodically in fall and winter. The relative contribution of tintinnids and oligotrichs is greatest during the warmer months, while non-choreotrich taxa forming a major portion of the population during seasonal maxima. The phototrophic ciliate Myrionecta rubra is a minor part of the community throughout the year, but is usually most prevalent in spring and fall.

Water-column abundance, biomass, and composition of Chesapeake Bay ciliates.

    The decline in ciliate populations from spring through summer is closely linked to seasonal stratification of the water column and development of bottom water anoxia. Ciliates are almost evenly distributed relative to depth during spring when stratification is moderate, phytoplankton stocks are vertically homogeneous, and oxygen concentrations in bottom waters are relatively high. With intensified stratification, reduced Chl a concentrations and development of anoxic conditions in early summer, ciliate populations become vertically heterogeneous and often exhibit mid-water maxima coincident with the oxycline-pycnocline.

Vertical distributions in ciliate abundance. phytoplankton stock, and hydrographic parameters at mesohaline stations during early (A), mid- (B), and late summer (C).

    Ciliate populations also exhibit interannual variations that reflect major changes in the timing and magnitude of freshwater flow from the Susquehanna River and associated fluctuations in phytoplankton stocks. For example, 1988-1989 were years of highly contrasting hydrographic conditions that resulted in correspondingly divergent phytoplankton and microzooplankton populations. Annual flow from the Susquehanna was near or slightly below average in both years; however, the spring freshet of 1989 was much later and more prolonged than usual. As a result, surface salinities in the upper Bay were reduced to 8-12 ppt for a relatively short period in late May and June of 1988, while low salinities (6-12 ppt) persisted throughout the summer of 1989. In addition, the winter-spring phytoplankton bloom that typically occurs in the mid- to lower Bay was clearly present in 1988, but failed to develop in 1989.

Surface salinity iospleths (A) and integrated water-column contours for Chl a (B), abundance (C), and ciliate biomass (D) along the main stem of Chesapeake Bay.

    During early May of 1988, peak ciliate concentrations were associated with a declining diatom bloom at lower Bay stations, whereas maximum values in late May were located in the upper Bay where phytoplankton populations were dominated by nanoflagellates and dinoflagellates. Ciliate populations also reached high levels during the spring of 1989, but peak concentrations were lower and less widely distributed than in 1988.

    The association of dense ciliate populations with the declining diatom bloom in 1988 is difficult to justify by a direct phytoplankton-microzooplankton link, as diatom species that bloom in the lower Bay during late winter and spring are well outside the size-range of particles ingested by most ciliates. Ciliate maxima in the upper estuary in late May 1988 may be directly related to high Chl a concentrations resulting from the mixed flagellate bloom in that area. However, the occurrence of ciliate peaks in the lower Bay in 1988 and upper Bay in 1989 indicate either that low biomass and production of nanoplankton are sufficient to promote spring ciliate maxima or that ciliate populations are utilizing an alternate food source such as bacteria or detritus.

Distribution of ciliate trophic guilds.

    Data on the distribution of microphagous (~bacterivorous) and macrophagous (algivorous and omnivorous) ciliates during 1988 and 1989 show clear temporal and spatial separations of trophic guilds relative to seasonal phytoplankton blooms. Microphagous species dominated ciliate populations that were associated with the declining diatom bloom in early May 1988 and were also the major contributors to the spring maxima in the upper Bay during 1989, when a winter-spring phytoplankton bloom failed to develop. Macrophagous taxa formed peak concentrations associated with the mixed flagellate bloom of the upper estuary in late May 1988 and were locally abundant in the summers of both years. Thus, the spring ciliate maximum of Bay does not constitute a single event that is broadly represented in time and space. Rather, it consists of discrete ciliate assemblages that exploit primary production through different pathways. Phytoflagellate stocks may be directly utilized, while winter-spring diatom production of the lower Bay is probably channeled through the "microbial loop." The spring peak of microphagous species may also reflect elevated bacterial concentrations associated with high freshwater flow and suspended particulates.