Frederic H. Nichols

The Modification of an Estuary

The San Francisco Bay estuary has been rapidly modified by human activity. Diking and filling of most of its wetlands have eliminated habitats for fish and waterfowl; the introduction of exotic species has transformed the composition of its aquatic communities; reduction of freshwater inflow by more than half has changed the dynamics of its plant and animal communities; and wastes have contaminated its sediments and organisms. Continued disposal of toxic wastes, the probable further reduction in freshwater inflow, and the possible synergy between the two provide the potential for further alteration of the estuarys water quality and biotic communities.

Increased benthic grazing: An alternative explanation for low phytoplankton biomass in northern San Francisco Bay during the 1976-1977 drought

Abstract Among the consequences of extremely low river flow into northern San Francisco Bay during a two-year drought were (1) a gradual increase in salinity, (2) an unusual decline in chlorophyll a concentration, and (3) the upstream migration of estuarine benthic invertebrates to the normally brackish area of the bay. Total abundance in the benthos at a shallow monitoring site increased from a normal 2000 to greater than 20 000 individuals m −2 during the summer of 1977, presumably in response to the increased salinity. Estimated filtration rates derived from equations in the literature for one of the species, the suspended-feeding bivalve Mya arenaria ranged from 1 to 4 m 3 m −2 day −1 during 1977 depending on abundance and mean size on sampling dates. Because water depth at this site is less than 2 m, Mya could have filtered all of the particles (including diatoms) from the water column on the order of once per day. Several other immigrant species undoubtedly contributed to the removal of particles from the near-bottom water as well. Increased benthic grazing, therefore, could have accounted for the anomalously low phytoplankton biomass observed during the drought. These results suggest that during periods of prolonged low river flow and increased salinity benthic food webs could become more important than planktonic food webs in the upper part of the estuary.

Time scales and mechanisms of estuarine variability, a synthesis from studies of San Francisco Bay

This review of the preceding papers suggests that temporal variability in San Francisco Bay can be characterized by four time scales (hours, days-weeks, months, years) and associated with at least four mechanisms (variations in freshwater inflow, tides, wind, and exchange with coastal waters). The best understood component of temporal variability is the annual cycle, which is most obviously influenced by seasonal variations in freshwater inflow. The winter season of high river discharge is characterized by: large-scale redistribution of the salinity field (e.g. the upper estuary becomes a riverine system); enhanced density stratification and gravitational circulation with shortened residence times in the bay; decreased tissue concentrations of some contaminants (e.g. copper) in resident bivalves; increased estuarine inputs of river-borne materials such as dissolved inorganic nutrients (N, P, Si), suspended sediments, and humic materials; radical redistributions of pelagic organisms such as copepods and fish; low phutoplankton biomass and primary productivity in the upper estuary; and elimination of freshwater-intolerant species of macroalgae and benthic infauna from the upper estuary. Other mechanisms modulate this river-driven annual cycle: (1) wind speed is highly seasonal (strongest in summer) and causes seasonal variations in atmosphere-water column exchange of dissolved gases, resuspension, and the texture of surficial sediments; (2) seasonal variations in the coastal ocean (e.g. the spring-summer upwelling season) influence species composition of plankton and nutrient concentrations that are advected into the bay; and (3) the annual temperature cycle influences a few selected features (e.g. production and hatching of copepod resting eggs). Much of the interannual variability in San Francisco Bay is also correlated with freshwater inflow: wet years with persistently high river discharge are characterized by persistent winter-type conditions.Mechanisms of short-term variability are not as well understood, although some responses to storm events (pulses in residual currents from wind forcing, erosion of surficial sediments by wind waves, redistribution of fish populations) and the neap-spring tidal cycle (enhanced salinity stratification, gravitational circulation, and phytoplankton biomass during neap tides) have been quantified. In addition to these somewhat predictable features of variability are (1) largely unexplained episodic events (e.g. anomalous blooms of drift macroalgae), and (2) long-term trends directly attributable to human activities (e.g. introduction of exotic species that become permanent members of the biota).

Time scales of change in the San Francisco Bay benthos

Results from multi-year investigations in the San Francisco Bay estuary show that large abundance fluctuations within benthic macroinvertebrate populations reflect both (1) within-year periodicity of reproduction, recruitment, and mortality that is not necessarily coincident with seasonal changes of the environment (e.g., the annual temperature cycle), and (2) aperiodic density changes (often larger than within-year fluctuations) following random perturbations of the environment.Density peaks of the small, short-lived estuarine invertebrates that comprise the vast majority of individuals in the bays relatively homogeneous benthic community normally occur between spring and autumn depending on the species, in large part a reflection of reproductive periodicity. However, because mild winters permit reproductive activity in some of the common species throughout much of the year, other factors are important to within-year density fluctuations in the community. Seasonally predictable changes in freshwater inflow, wind and tidal mixing, microalgal biomass, and sediment erosion/deposition patterns all contribute to observed seasonal changes in abundance. For example, the commonly observed decline in abundance during winter reflects both short-lived species that die after reproducing and the stress of winter conditions (e.g., inundation by less saline, sediment-laden water and the decline in both planktonic and benthic algal biomass — a direct source of food for the shallow-water benthos). On the other hand, data from several studies suggest that observed ‘recruitment’ and ‘mortality’ may in fact be the migration of juveniles and adults to and from study sites. For example, the common amphipod Ampelisca abdita apparently moves from shallow to deep water, or from up-estuary to down-estuary locations, coincident with periods of high river runoff in winter. Growth of individuals within the few studied species populations is also highly seasonal, and appears to be coincident with seasonal increases in the abundance of planktonic and/or benthic microalgae.Two multi-year studies have shown that, in addition to within-year periodicity, major restructuring of the benthic community can occur as a result of anomalous (usually climate-related) perturbations of the benthic habitat. For example, during wet years freshwater-intolerant species disappear from the upper part of the estuary and from shallow areas of the bay. During a two-year drought these same species colonized the extreme upper end of the estuary in large numbers. Other aperiodic perturbations include localized instances of sediment erosion or deposition and algal mat accumulations that greatly depress abundance. Additionally, there is evidence (observations that the clam Macoma balthica establishes large populations only when the amphipod A. abdita is not abundant) that species interactions can contribute greatly to interannual variations. Thus, while community composition may change little over the long term, year-to-year predictability of species abundances is low.

Seasonal Growth in the Bivalve Macoma balthica near the Southern Limit of its Range

Shell-length growth inMacoma balthica from San Francisco Bay, California, as measured on living animalsin situ, is highly seasonal despite a mild Mediterranean climate: a long period of near non-growth from May to the following February is followed by a short period of rapid growth between March and May. The rapid-growth period follows the spawning period during January/February and ends as water temperature rises above about 15°C. Despite the shortness of the growth period,M. balthica grows larger at a given age in San Francisco Bay than is recorded elsewhere in the world. Application of a model, developed elsewhere from these same field measurements, shows that (1) measurable growth occurs during the summer/autumn/early winter “nongrowth” period, (2) there is an autumn recruitment, and (3) both spring and autumn recruits combine to form a single “one-year-old” size grouping. None of these features is detectable through growth-ring analysis of field samples, apparently because of indistinct climatic seasons, or through size-frequency histogram analysis because of the combined effects of slow growth and intermittent recruitment.

Temporal dynamics of an estuary : San Francisco Bay

1. Environmental setting of San Francisco Bay.- 2. Time scales of circulation and mixing processes of San Francisco Bay waters.- 3. Interannual variability in dissolved inorganic nutrients in Northern San Francisco Bay Estuary.- 4. Gas exchange in San Francisco Bay.- 5. Benthic fluxes in San Francisco Bay.- 6. Temporal fluctuations in grain size, organic materials and iron concentrations in intertidal surface sediment of San Francisco Bay.- 7. Temporal fluctuations of silver, copper and zinc in the bivalve Macoma balthica at five stations in South San Francisco Bay.- 8. Time scales of change in the San Francisco Bay benthos.- 9. The distribution and temporal dynamics of the estuarine macroalgal community of San Francisco Bay.- 10. Temporal dynamics of estuarine phytoplankton: A case study of San Francisco Bay.- 11. Seasonal cycles of zooplankton from San Francisco Bay.- 12. Seasonal and interannual variation in distribution and population abundance of the shrimp Crangon franciscorum in San Francisco Bay.- 13. Distribution and abundance of fishes in the San Francisco Bay estuary between 1980 and 1982.- 14. Time scales and mechanisms of estuarine variability, a synthesis from studies of San Francisco Bay.

Abundance fluctuations among benthic invertebrates in two pacific estuaries

Long-term studies were used to examine (1) contrasting time scales and mechanisms of structural variations within two benthic communities and (2) the usefulness of long data sets for evaluating human impact. A 10-year study of a San Francisco Bay mudflat, the details of which are reported elsewhere, has revealed large short-term (on the order of months) variations in species abundances within a community composed predominantly of opportunistic species. The study site, located in a highly urbanized estuary, is subject to the influence of a nearby sewage-treatment facility. However, rapid changes in population size of the common species, in part due to periodic natural habitat disturbance, impedes the detection of anthropogenic influences on community composition. Only a very long-term data set may provide evidence of progressive change. Data collected for a 20-year period on the benthic community at 200 m depth in the main basin of Puget Sound, an environment subject to little apparent habitat disturbance show that numerical abundance of the common species can also change markedly. Here, however, numerical dominance shifts from one species to another at irregular, multiyear intervals. Recent increases in two heretofore rare species, and a significant increase in total numbers of individuals suggest that long-term changes may be occurring in this community. These two long-term data sets demonstrate the importance of measuring both the amplitude and the periodicity of fluctuations in population size of aquatic species as well as long-term fluctuations and patterns in environmental factors before attempting to demonstrate the effect of anthropogenic influences on aquatic communities. The results of these studies also demonstrate the usefulness of long-term data sets for revealing the potential importance of interactions among species in determining abundance patterns in the soft-bottom benthos.

DYNAMICS AND PRODUCTION OF Pectinaria koreni (Malmgren) IN KIEL BAY, WEST GERMANY *

ABSTRACT Samples collected periodically for 2 years (November 1973 to November 1975) along a 2.5-km transect crossing the channel system of southwestern Kiel Bay are used to describe the dynamics and production of a numerically dominant macroinvertebrate, the polychaete Pectinaria koreni. This species has, in recent years, increased in abundance and become an important food for demersal fish. Recruitment of young normally occurred in early summer, with an additional recruitment in late summer of some years. Population size varied widely from year to year, from less than 100 adult specimens/m 2 in autumn of 1973 and 1975 to greater than 1000 specimens/m 2 in 1974. Biomass and production reached highest levels during late summer of 1974 (from 0.7 to 2.5 g C/m 2 and from 0.5 to 1.0 g C/m 2 /month respectively), with highest values recorded at deeper stations. The previous and following years showed very low levels of both, due to poor recruitment or high mortality. The periodic development of an oxygen deficiency in bottom waters appears to be the major factor determining year to year differences in the abundance and biomass of this and other dominant species in the community. In late summer of those years having calm and warm summer weather conditions (in this case, 1973 and 1975), bottom waters below 20 m became anoxic causing nearly total destruction of the benthic community. In the autumn of 1973 the result was a greatly reduced standing stock of Pectinaria throughout the following winter and spring. During summer of 1974 there was much rain and wind, and similar hydrographic stability leading to oxygen deficiency in the bottom water did not develop. The recruitment in 1974 of all species, but especially that of Pectinaria , was heavy, and biomass and numbers of specimens were large. Although oxygen deficiency again developed during the hot summer of 1975, an autumn recruitment of Pectinaria allowed for reestablishment of its dominance during the same year.

Emergence of burrowing urchins from California continental shelf sediments—A response to alongshore current reversals?

Abstract Two sequences of bottom photographs taken every two or four hours for two months during the Coastal Ocean Dynamics Experiment (CODE) off the Russian River, California, reveal the dynamic nature of interations between the water column, the sediments, and benthic organisms in the mid-shelf silt deposit. Time-lapse photographs taken between late spring and early summer in 1981 and 1982 show that the subsurface-dwelling urchin Brisaster latifrons (one of the largest invertebrates found in shelf-depth fine sediment off the U.S. Pacific coast) occasionally emerged from the sediment, plowed the sediment surface during the course of a few hours to several days, then buried themselves again. Frame-by-frame study of the film sequences shows that the urchins typically emerged following relaxation of coastal upwelling, periods characterized by current direction reversals and increases in bottom water turbidity. Among the possible causes of the emergence of urchins and the consequent bioturbation of the upper few cm of sediment, a response to an enhanced food supply seems most plausible. Circumstantial evidence suggests the possibility that phytoplankton sedimentation during periods of upwelling relaxation could provide a new source of food at the sediment surface.

Interdecadal change in the deep Puget Sound benthos

Data from quantitative samples of the benthos at a 200-m site in central Puget Sound, collected twice yearly in most years between 1963 and 1992, were evaluated to determine the extent to which species composition in a continental-shelf depth community exhibits long-term persistence. Study results showed that the most abundant species were consistently present over the 30-year period. However, measures of species composition (e.g., similarity, diversity) reveal a subtle, gradual change in the community over time. Among the changes are (1) multi-year periods of greatly increased abundance of the common species; (2) an overall increase in the total abundance of the benthic community beginning in the mid-1970s; (3) periods of increased abundance, during the late 1970s and early 1980s, of two species that are tolerant of organic enrichment; and (4) the steady decline in abundance of the large burrowing echinoderm, Brisaster latifrons as a consequence of the lack of recruitment to the site since 1970. Despite the conspicuousness of these changes, there are no observed environmental factors that readily explain them. Circumstantial evidence suggests that climate-related change in Puget Sound circulation beginning in the mid-1970s, organic enrichment associated with a nearby large source of primary-treated sewage, and the influence of changes in the abundance of the large echinoderms on the smaller species are potential agents of change. The principle reasons for our inability to identify causes of long-term change in the Puget Sound benthos are (a) inconsistent long-term monitoring of environmental variables, (b) the lack of quantitative information about long-term changes in plankton and fish populations, (c) lack of knowledge of specific predator/prey and competitive interactions in soft bottom benthos, (d) unknown influence of moderate levels of contamination on biota; and (e) lack of understanding of possible linkages between climate regime shifts and fluctuations in local biological populations.