Portfolio I: Estuarine Processes

Achievements

The research investments of MDSG are directed at developing important sources of information for understanding and managing Chesapeake Bay. We have consistently demonstrated our commitment to the success of this investment by concentrating on research topics that have real world implications. Issues that we address include academic, commercial, and management communities and often bring partners together who only rarely cross paths without some catalyst to interact. Funds are awarded competitively to investigators and institutions that have the resources, capabilities, and reputations to perform the proposed work successfully.

Historical Trends

An important aspect of understanding changes in the Bay ecosystem is to gauge historical trends and provide a long-term context for contemporary conditions. MDSG has supported some seminal efforts in this area. Research by Brush & Cooper has investigated historical trends in the species composition of phytoplankton in Chesapeake Bay using sediment cores to examine the remains of organisms that are preserved in sediment. By coupling this information with the age of sediment layers, the researchers compiled a stratigraphic history for important components of the regional flora. An important conclusion of this work is that sedimentation, eutrophication, and anoxia have changed significantly since European colonization. There are identifiable impacts on the biota of the estuary, including a change in the species composition of diatoms that has co-occurred with the period of deforestation, increased nutrient loading, and reduced water clarity. This work has earned a high profile with publication in the journal Science. It is the kind of research that provides a historical perspective for comparison with contemporary conditions and relationships. This work is continuing under a current grant awarded to Dr. Brush (R/P-46).

Nutrient Enrichment and Cycling within Chesapeake Bay

It is widely accepted that nitrogen (N) is the principal element that regulates biomass and primary production of phytoplankton in Chesapeake Bay on seasonal to interannual time scales. We know that N processing in the Bay is essential to support maximum rates of primary production that occur in summer. It has been estimated that each atom of N input to the Bay is cycled up to eight times per year as a function of physical, chemical and biological processes that combine to promote retention and resupply of limiting N in a variety of forms other than nitrate, the most abundant form of N in inputs.

MDSG recognized early the need for detailed research on N and the effects on water quality in this estuarine system, and has supported several research efforts in this area. Ongoing research has examined the relation between dissolved organic nitrogen (DON) and primary production (Glibert & del Giorgio R/P-41). The lability of DON on regional and temporal scales was studied by examining the amount and bio-availability of DON from a variety of pristine, agricultural, and urbanized watersheds from Georgia to New York. Seasonal monitoring revealed that the composition of riverine DON changes from low C:N ratios in the summer, to significantly higher C:N ratios in fall and spring. In addition, bacterial production experiments showed that DON from industrial and hardwood-forest river systems (Delaware, Hudson) had higher lability than those of the "blackwater" and agricultural rivers common in the Chesapeake Bay watershed. These findings bear on the seasonally important role of DON in the Bay, and on differences among estuarine systems that drain contrasting watersheds.

The loss of fixed N via denitrification was examined in benthic sediments using newly developed methods and instrumentation (Cornwell & Kana R/P-45). A current effort is extending these measurements to tidal marshes that are thought to be sites of very active denitrification (R/P-49). The methods development by Cornwell and Kana that was funded by MDSG was among the first to apply new technology to this problem.

Oysters were once extremely abundant in Chesapeake Bay and are believed to have been partially responsible for controlling phytoplankton biomass. Researchers Newell & Cornwell (R/P-40) have investigated the effects of oysters on N cycling by examining the role of these filter feeders in organic matter cycling and benthic denitrification. Historically, the Chesapeake Bay oyster population represented a significant mechanism for transferring material from phytoplankton to benthic deposits. Now reduced to <1% of their previous levels, oysters no longer play this role to an appreciable extent. The PIs examined the effects of oysters on Bay water quality using a series of sediment-water flux experiments. Because no dissolved inorganic nitrogen (DIN) was recycled to the water column under oxygenated conditions, they concluded that the rehabilitation of oyster stocks would have the beneficial effect of removing phytoplankton from the water column without stimulating further phytoplankton production.

The results of MDSG-sponsored research on nutrient dynamics have changed the way we view nutrient dynamics in Chesapeake Bay, from marshes to open waters of the estuary. These research projects are also giving us new insights on the cycling of N in that have consequences for other estuaries.

Trophic Dynamics

Key species and communities in the Chesapeake Bay have received a great deal of attention from MDSG because of the pivotal ecological positions they occupy in the Bay's food web, and the commercial and recreational importance of harvestable forms. This focus on trophic dynamics is central to our mission and addresses the interconnections of species within the Bay ecosystem, as shown in relevant examples from the MDSG sponsored research that includes projects spanning bacteria to gelatinous zooplankton to fish.

It is clear that bacteria are instrumental in both nutrient regeneration and oxygen consumption in the Bay ecosystem. Recognition that microbial communities are composed of a diverse array of organism types with a broad range of life histories was the subject of research by Fletcher and Ducklow (R/P-26 and R/DO-20 (part of a national initiative)). The PIs demonstrated how classical models grossly oversimplify the role of bacteria in the Bay as they demonstrated how particle-attached bacteria break down organic matter very differently from free-living bacteria. This study began to differentiate groups of microbes and to provide a more realistic view of how microbial communities function in Chesapeake Bay.

The negative impact of jellyfish, or sea nettles, of the genus Chrysaora on recreational activities is well known as these summer occupants of Bay waters make swimming uncomfortable or impossible for several months. Beyond the obnoxious role of jellyfish, Houde, Breitburg and Purcell (R/P-29 and R/P-50) have shown that hypoxia actually benefits the sea nettle populations. Larval fish tend to enter areas of low dissolved oxygen to escape predation from larger fish. Gelatinous zooplankton, however, are not affected by hypoxia to the same extent as other predators, and can prey effectively on the juvenile fish weakened by low dissolved oxygen concentrations.

Following on these findings, Purcell and Roman (R/P-28 and R/P-35) explored the effect of sea nettle abundance on herbivorous copepods, an important trophic link between primary production and fisheries. Results showed that copepod abundance was not constrained by predation from gelatinous zooplankton or by food supply. The potential top-down role of gelatinous zooplankton on copepods is complicated by the occurrence of large populations of ctenophores of the genus Mnemiopsis, an important prey item of Chrysaora that at the same time is a voracious consumer of herbivorous copepods. Regional meteorological conditions also influence the trophic dynamics of the gelatinous zooplankton as years of strong freshwater flow suppress or delay Chrysaora, while permitting an expansion of the Mnemiopsis population. These interactions accentuate the complex trophic interactions in the plankton of this ecosystem.

The ecological roles of two abundant forage fish in Chesapeake Bay, the bay anchovy and menhaden, were investigated by Houde and Brandt (R/F-65) using sophisticated acoustic methods, traditional net surveys, and bioenergetics models. The PIs tested the hypothesis that these species are major consumers of plankton in the Bay by estimating seasonal biomasses and abundances, and defining their trophic relationships, growth statistics and production in relation to the physical structure of the environment. Bay anchovy was shown to reach peak abundance during the August-October period when numerous fast-growing, young-of-the-year anchovy entered the population and contributed most of the annual production. Based on an empirical analysis, annual production (wet wt) was extremely high at approximately 10 g m^-3 yr^-1, and mean biomass in late summer-early fall peaked at >3 g m^-3. Population consumption by the bay anchovy population exceeded 3,000 kg km^-2 d^-1 during the August-October period. Consistent with the hypothesis, a significant fraction (~33%) of Chesapeake Bay copepod production in August-October was consumed by the anchovy population at peak abundance. Outputs of bioenergetics models indicated that for one month (June 1990), the anchovy population had the potential to consume most of the zooplankton production. Empirical results confirmed the model outputs, showing that bay anchovy switched their diet to include benthic invertebrates at that time. The important ecological role of the bay anchovy also affects N movement from the Bay as approximately 1% of the N input to the Chesapeake Bay may be translocated during the winter emigration by bay anchovy.

Ecosystem Scale Analyses

Study of the Chesapeake Bay on an ecosystem scale complements extensive, focused studies on particular organisms and processes, and provides a view of the Bay as a whole that lends itself to prediction. For some years, MDSG has invested in aircraft and satellite remote sensing of ocean color and temperature to provide synoptic, whole system data and information on phytoplankton distributions and temperature structure. The advantage of remote sensing for measuring water quality properties is the spatial and temporal resolution that can be attained, and the rapidity of data and information turnaround. MDSG saw the opportunity to invest in the development of an aggressive campaign to enhance and develop large-scale observations of Bay water quality as quantified by phytoplankton biomass through the use of aircraft-mounted radiometric instruments.

Early in the 1990s, Harding's work produced Bay-wide estimates of phytoplankton abundance from weekly to twice-weekly flights focused on the spring bloom. Now teamed with Hood, the most recent grant supports research to couple highly resolved data from ocean color measurements with circulation models. Harding & Hood (R/P-44) are continuing to develop a long-term data stream, use the products in collaboration with partners in management at the Chesapeake Bay Program to assess model outputs for key ecosystem properties (chlorophyll, primary productivity), and produce a strong outreach effort to disseminate information to a broader user community. As the result of the program, we now have a much improved view of how seasonal and interannual variability of freshwater flow and nutrient loading influences plankton dynamics in the receiving waters of the Bay. Current efforts are combining data from aircraft and satellite instruments with data from shipboard measurements to produce spatially explicit "maps" of primary productivity for the Bay at high resolution in time and space. From these data will emerge an improved understanding of annual integrated production (AIP) in Chesapeake Bay, an important ecological indicator of ecosystem health and an important variable for estimating fish production. By investing early and sharing the cost with NASA, MDSG ensured the establishment and success of this research program.

Primary productivity is the autotrophic "engine" that drives material and energy transfers in the Bay. For almost two decades, Kemp and Boynton have explored links of pelagic production and benthic-pelagic consumption and have made major contributions to our understanding of the Bay's energetics. In recent studies supported by MDSG, Kemp and Boynton (R/P-32 and R/DO-22) applied the concept of "net community metabolism" to Chesapeake Bay as an index that expresses the trophic state of the plankton community. We now recognize that the Bay has both strongly heterotrophic and autotrophic regions and is slightly net-autotrophic on a Bay-wide scale. The upper Bay is light-limited and heterotrophy dominates, while the mid- to lower Bay receives ample nutrients and light to support net autotrophy. Seasonal and interannual shifts in net community metabolism accompany changes in freshwater flow and nutrient loading, such that in years of high flow, autotrophy is enhanced. This work has ramifications for the functioning of the Bay ecosystem as a whole, and represents some of the most important research of its kind in U.S. estuaries.