Richard L. Wetzel

Relative effects of nutrient enrichment and grazing on epiphyte-macrophyte (Zostera marina L.) dynamics

The independent and interactive effects of nutrient concentration and epiphyte grazers on epiphyte biomass and macrophyte growth and production were examined in Zostera marina L. (eelgrass) microcosms. Experiments were conducted during early summer, late summer, fall, and spring in a greenhouse on the York River estuary of Chesapeake Bay. Nutrient treatments consisted of ambient or enriched (3× ambient) concentrations of inorganic nitrogen (ammonium nitrate) and phosphate. Grazer treatments consisted of the presence or absence of field densities of isopods, amphipods, and gastropods. epiphyte biomass increased with both grazer removal and nutrient enrichment during summer and spring experiments. The effect of grazers was stronger than that of nutrients. There was little epiphyte response to treatment during the fall, a result possibly of high ambient nutrient concentrations and low grazing pressure. Under low grazer densities of early summer, macrophyte production (g m−2 d−1) was reduced by grazer removal and nutrient enrichment independently. Under high grazer densities of late summer, macrophyte production was reduced by enrichment only with grazers absent. During spring and fall there were no macrophyte responses to treatment. The relative influence of epiphytes on macrophyte production may have been related to seasonally changing water temperature and macrophyte requirements for light and inorganic carbon.

Seasonal pulses of turbidity and their relations to eelgrass (Zostera marina L.) survival in an estuary

The light environment of one Chesapeake Bay tributary where seagrasses have decreased in abundance was described using both continuous and discrete measures of irradiance and related to the growth and survival of transplanted eelgrass (Zostera marina L.). After 8 months of continuous growth at an upriver site, a decline and eventual complete loss of eelgrass transplants began during a month long (May–June) period of increased turbidity (Kd>3.0). Transplant loss continued even after light conditions improved (Kd<2.0). At a downriver site where there has been some natural seagrass regrowth, the pulse of high turbidity was not as evident and transplants survived. Other than this spring period of high turbidity at the upriver site, the light environments of the two areas were similar with minimum turbidity in January and maximum in the spring and summer. Annual median daily attenuation coefficients (Kd) at the upriver and downriver sites were 1.77 and 1.96, respectively, and were not significantly different (P=0.49). Total downwelling quantum flux at transplant depths of 0.8 m below mean sea level were 2618 and 2556 mol·m−2·yr−1, or approximately 24.9 and 24.3% of annual solar PAR. The high spring turbidity pulse corresponded to an increase in non-chlorophyll particulate matter. Chlorophyll specific attenuation (Kc) accounted for 6.7–9.0% of Kd in June. Differences in attenuation were greatest in the 400–500 nm spectral region. Therefore, measures of total PAR attenuation can overestimate the usable irradiance available to the macrophytes. Scalar quantum fluxes during the period of elevated turbidity were 2.7 and 13.4 mols·m−2·day−1 at the upriver and downriver sites. The duration and intensity of total PAR measured upriver during this period were insufficient to support eelgrass growth and survival, and below literature estimates for eelgrass community light compensation at in situ temperatures (20–25°C). Therefore late spring, month-long pulses in turbidity, such as measured here can account for the loss of transplanted vegetation and, potentially, explain lack of successful recruitment into formerly vegetated upriver sites.

Epiphyte-grazer relationships in seagrass meadows: consequences for seagrass growth and production

Studies of seagrass meadows have shown that the production of algal epiphytes attached to seagrass blades approaches 20% of the seagrass production and that epiphytes are more important as food for associated fauna than are the more refractory seagrass blades. Since epiphytes may compete with seagrasses for light and water column nutrients, excessive epiphytic fouling could have serious consequences for seagrass growth. We summarize much of the literature on epiphytegrazer relationships in seagrass meadows within the context of seagrass growth and production. We also provide insights from mathematical modeling simulations of these relationships for a Chesapeake BayZostera marina meadow. Finally we focus on future research needs for more completely understanding the influences that epiphyte grazers have on seagrass production.

Seasonal variations in eelgrass (Zostera marina L.) responses to nutrient enrichment and reduced light availability in experimental ecosystems

Abstract The single and interactive effects of altered water column nutrient concentrations and light availability on the growth of the seagrass Zostera marina L. (eelgrass) and its attached epiphytes were investigated in 110 liter microcosms. Experiments lasting 4 to 6 weeks were conducted seasonally during spring, summer and fall in a greenhouse equipped with flow-through seawater from the adjacent York River estuary of the Chesapeake Bay. Nutrient treatments consisted of inflow seawater with ambient or enriched (2× to 3×) concentrations of dissolved inorganic nitrogen and phosphorus and with rapid turnover (16 d −1 ). Enrichment levels were chosen to evaluate conditions found in regions of the Chesapeake Bay where Z. marina has declined. Light reductions were accomplished by shading individual microcosms with neutral density screening so that mean scalar irradiance was 42, 28, or 9% of solar PAR. These levels were chosen to simulate light reductions observed along gradients of turbidity which characterize present and former Z. marina habitats in the region. Epiphytic grazers consisted of gastropods ( Bittium varium and Mitrella lunata ) which were applied at consistent densities (5200 m −2 ) for all experiments. Growth of both the seagrasses and their associated epiphytes decreased with increased shading. There was little additional response to nutrient enrichment except at highest light levels during the spring when macroepiphytes increased to over 10× the seagrass mass and seagrass growth decreased. The results suggest that it is principally light availability which governs seagrass growth in moderately nutrient enriched regions of the bay. In systems such as the York River, given adequate grazer densities, observed levels of nutrient enrichment are unlikely to cause excessive epiphyte loads and subsequent seagrass declines. Although Z. marina tissue levels of nitrogen and phosphorus increased significantly with enrichment and with shading no direct effects of nitrate toxicity were observed.

Spatial and temporal characteristics of nutrient and phytoplankton dynamics in the York River Estuary, Virginia: Analyses of long-term data

Ten years (1985–1994) of data were analyzed to investigate general patterns of phytoplankton and nutrient dynamics, and to identify major factors controlling those dynamics in the York River Estuary, Virginia. Algal blooms were observed during winter-spring followed by smaller summer blooms. Peak phytoplankton biomass during the winter-spring blooms occurred in the mid reach of the mesohaline zone whereas peak phytoplankton biomass during the summer bloom occurred in the tidal fresh-mesohaline transition zone. River discharge appears to be the major factor controlling the location and timing of the winter-spring blooms and the relative degree of potential N and P limitation. Phytoplankton biomass in tidal fresh water regions was limited by high flushing rates. Water residence time was less than cell doubling time during high flow seasons. Positive correlations between PAR at 1 m depth and chlorophylla suggested light limitation of phytoplankton in the tidal fresh-mesohaline transition zone. Relationships of salinity difference between surface and bottom water with chlorophylla distribution suggested the importance of tidal mixing for phytoplankton dynamics in the mesohaline zone. Accumulation of phytoplankton biomass in the mesohaline zone was generally controlled by N with the nutrient supply provided by benthic or bottom water remineralization.

A model of Zostera marina L. Photosynthesis and growth: Simulated effects of selected physical-chemical variables and biological interactions

A computer model was developed to simulate photosynthesis and growth of eelgrass (Zostera marina L.), the dominant submerged aquatic macrophyte occurring in the lower Chesapeake Bay, Virginia, U.S.A. The mathematical structure of the model is based on theoretical non-linear functions for simulating biologically controlled processes and empirical or statistical relationships for incorporating physical-chemical interactions and environmental forcing functions. Analyses of the model for 1-, 4- and 10-year simulation periods indicate that submarine light quantity (PAR) and temperature are the principal physical factors governing eelgrass photosynthesis and growth in the lower Chesapeake Bay. Typical in situ light and temperature conditions, however, constrain photosynthesis and therefore plant growth to less than physiologically capable potentials. Small changes in submarine irradiance, temperature or their combined interaction result in decreased plant productivity and eventual loss of the eelgrass community. Empirical and hypothetical relationships in the model of epiphyte colonization and growth and epiphytic grazing indicate that eelgrass growth and long-term survival are potentially governed by factors that control and limit the attached epiphytic community. Model simulations suggest that a principal factor is the interaction between epiphytic grazing intensity and ambient light levels.

Tidal exchange of nitrogen and phosphorus between a mesohaline vegetated marsh and the surrounding estuary in the lower Chesapeake Bay

Abstract A 22-month study was conducted to determine the exchange of nitrogen and phosphorus between a mesohaline vegetated marsh in the Carters Creek area of Virginia and the surrounding estuary, focusing on the role of the vegetated marsh surface in the processing of these constituents. On an annual basis there was a removal of NH 4 + , PO 4 3− , NO 3 − , dissolved organic nitrogen, dissolved organic phosphorus, particulate nitrogen and particulate phosphorus from the tidal water as it resided on the vegetated marsh. Only nitrite was transported from the marsh to the estuary. Most of the nitrogen and phosphorus species showed distinct seasonal trends with respect to the direction of transport except nitrate and orthophosphate. The ammonium flux data indicates that this nutrient was removed from the inundating water during late spring and fall, with a slight release of this constituent into the tidal water during the late summer. The transport of nitrite was from the estuary to the marsh for most of the year except during the fall. The large release of this nutrient into the tidal water at this time is associated with the senescence of the marsh vegetation. There was a large removal of DON from the tidal water during the fall, while the flux of DOP was from the estuary to the marsh for most of the year except during the summer. The largest removal of particulate nitrogen and phosphorus from the tidal water occurred during the summer months when the turbidity of the tidal water was highest, especially when wave scouring of the mudflats brings material into the water column. A loss of particulate nitrogen from the marsh to the estuary was evident during the fall and winter.

Modeling submersed macrophyte growth in relation to underwater light climate: modeling approaches and application potential

The underwater light climate is one of the most important determinants of submersed aquatic vegetation. Because of the recent, large-scale, declines in aquatic vegetation, largely attributed to deterioration of the underwater light climate, interest in tools to predict the wax and wane of aquatic macrophyte populations has greatly increased. This paper summarizes two modeling approaches that can be applied to assess impacts of changes in underwater light climate on submersed vegetation. The first, stand-alone, model type focuses on metabolism and biomass formation of submersed freshwater macrophytes with difference in phenologies. This type is illustrated by examples from various sites using models developed for the freshwater macrophytes Hydrilla verticillata (L.f.) Royle (HYDRIL) and Myriophyllum spicatum L. (MILFO), and also by an example ecological risk assessment. The models (HYDRIL and MILFO) track carbon flow through the vegetation in meter-squared (m2) water columns. The models include descriptions of various factors that affect biomass dynamics, such as site-characteristic changes in climate, latitude, light attenuation within the water column, carbon assimilation rate at light saturation, temperature, wintering strategies, grazing and mechanical control (removal of shoot biomass). Simulated biomass, net assimilation and maintenance respiration over a relatively short (1–5 year) period agree well with measured values. The models are, therefore, believed to be suitable for predicting plant community production, growth and survival characteristics over relatively short periods over a large range of sites. The feasibility of using a macrophyte growth model of the HYDRIL type for ecological risk assessment is demonstrated. It is used to evaluate the consequences of management changes in large rivers for the survival of submersed vegetation. The current assessment evaluates the potential impact of increased commercial navigation traffic on the growth of Potamogeton pectinatus L. in Pool 4 of the Upper Mississippi River, U.S.A. In this case, navigational traffic scenarios were translated into suspended solids concentrations and underwater light climate, with the latter being used as inputs into the aquatic plant growth model. Model results demonstrate that the scenario increases in commercial traffic cause minimal decreases in growth and vegetative reproduction. Results indicate that this growth model can be a useful tool in ecological risk assessment, since the required stress-response relationships could be established. The second, integrated, model type focuses on the role of seagrass and other primary producers in estuarine littoral zone material cycling (carbon and nitrogen) at the Goodwin Islands, Virginia, U.S.A. The latter model was used to explore the effects of changes

A metabolism-based trophic index for comparing the ecological values of shallow-water sediment habitats

We determined fluxes of oxygen and nutrients between water and sediments at 21 sites primarily in Virginia and North Carolina estuaries, over the past 15 yr. These sites represented broad ranges in salinity, tidal amplitude, hydrology, nutrient availability, turbidity, light availability, depth, sediment grain size, and anthropogenic disturbance. In general, we found that heterotrophically dominated sediments had the potential to degrade water quality, whereas photoautotrophy in the sediments ameliorated this impact. We propose a benthic trophic state index as a management tool to make general assessments of the degree to which sediments support ecological processes related to photoautotrophy. The index can be based on simple measurements of metabolic parameters. We also evaluated the relative significance of variability in the index across a number of spatial and temporal scales. Reduced photoautotrophy and/or enhanced heterotrophy tended to be associated with finer-grained, organic-rich sediments. This sediment type was common in oligohaline areas at water depths exceeding 2 m. Temporally, autotrophy declined from winter to spring particularly at sandy sites, while interannual variability was more pronounced for mud sites. *** DIRECT SUPPORT *** A01BY074 00011

Dynamics of epiphytic photoautotrophs and heterotrophs inZostera marina (eelgrass) microcosms: Responses to nutrient enrichment and grazing

The combined effects of nutrient enrichment and grazing by isopods and amphipods on abundances of seagrass epiphytes were tested inZostera marina L. (eelgrass) microcosms. Using epifluorescence microscopy, densities of epiphytic diatoms, cyanobacteria, heterotrophic flagellates, and heterotrophic bacteria were enumerated after 1 mo and 2 mo of treatment. In general, numbers of diatoms decreased, in the presence of grazers and showed little response to nutrient enrichment, whereas numbers of cyanobacteria increased with nutrient enrichment and showed little response to grazing. Thus, macrofaunal grazing maintained a photoautotrophic community domainated by cyanobacteria, particularly under nutrient enriched conditions. Following 2 mo of treatment, dense macroalgal growth under nutrient-enriched conditins with grazers absent appeared to limit populations of both epiphytic autotrophs. Patterns of abundance of heterotrophic bacteria suggested that the original bacteria population was nutrient limited. Bacteria populations may have been limited by organic carbon supplies at the end of the experiment. Abundances of heterotrophic flagellates and bacteria were strongly correlated on both sampling dates. Results suggest that heterotrophic flagellates might serve as a link between heterotrophic bacterial production and higher trophic levels in seagrass epiphyte food webs.