Robert R. Christian

Organizing and understanding a winter’s seagrass foodweb network through effective trophic levels

Trophic structure of ecosystems is a unifying concept in ecology; however, the quantification of trophic level of individual components has not received the attention one might expect. Ecosystem network analysis provides a format to make several assessments of trophic structure of communities, including the effective trophic level (i.e. non-integer) of these components. We applied network analysis to a Halodule wrightii community in Goose Creek Bay, St. Marks National Wildlife Refuge, Florida, USA, during January and February 1994 where we sampled a wide variety of taxa. Unlike most applications of network analysis, the field sampling design was specific for network construction. From these data and literature values, we constructed and analyzed one of the most complex, highly articulated and site specific foodweb networks to be done. Care was taken to structure the network to reflect best the field data and ecology of populations within the requirements of analysis software. This involved establishing internally consistent rules of data manipulation and compartment aggregation. Special attention was paid to the microbial components of the food web. Consumer compartments comprised effective trophic levels from 2.0 (herbivore/detritivore) to 4.32 (where a level 4.0 represents ‘secondary carnivory’), and these values were used to organize data interpretation. The effective trophic levels of consumers tended to aggregate near integer values, but the spread from integer values increased with increasing level. Detritus and benthic microalgae acted as important sources of food in the extended diets of many consumers. ‘Bottom-up’ control appeared important through mixed trophic impact analysis, and the extent of positive impacts decreased with increasing trophic level. ‘Top-down’ control was limited to a few consumers with relatively large production or biomass relative to their trophic position. Overall, ordering results from various network analysis algorithms by effective trophic level proved useful in highlighting the potential influence of different taxa to trophodynamics. Although the calculation of effective trophic level has been available for some time, its application to the evaluation of other analyses has previously not received due consideration.

Multiple states in the sea-level induced transition from terrestrial forest to estuary

In this paper we provide a conceptual model to examine changes in ecosystem state during the transition from terrestrial forest to shallow estuarine environments for coastal mainland marshes at the Virginia Coast Reserve (VCR), United States of America. Ecosystem states are characterized by plant community dominants and soil/sediment characteristics. The five states considered are upland or wetland forest, organic high marsh, intertidal mineral low marsh, autotrophic benthic with or without submersed aquatic vascular plants, and heterotrophic benthic (estuarine bottom). Transitions between states are described from the perspective of a fixed forest location undergoing transition from one ecosystem state to another. Rising sea level is acknowledged as the master variable that forces the process of change overall. Each state is hypothesized to have self-maintaining properties and thus is resistant to change from rising sea level; alternatively, transitions between states are facilitated by disturbance or exposure to acute stress. For change to occur, resistance must be overcome by events that are more abrupt than rising sea level and that appear as accentuated pulsings, which result in another self-maintaining and resistnnt state. Such events facilitate plant species replacement and alter sediment conditions. Mechanisms responsible for causing a state to cross a threshold are unique for each transition type and include brackish-water intrusion (osmotic stress and sulfide toxicity), tidal creek encroachment (redistribution of sediments), erosive currents and waves (resuspension of sediments, which increases light extinction), and increasing water depth (leads to greater bottom shading). Field experiments relevant to scales at which pulsings occur are not abundant in coastal marshes.

Water Quality and Phytoplankton as Indicators of Hurricane Impacts on a Large Estuarine Ecosystem

Three sequential hurricanes in the fall of 1999 provided the impetus for assessing multi-annual effects on water quality and phytoplankton dynamics in southwestern Pamlico Sound, North Carolina. Two and a half years of post-hurricane data were examined for short- and long-term impacts from the storms and >100 year flooding. Salinity decreased dramatically and did not recover until May 2000. Inorganic nitrogen and phosphorus concentrations were briefly elevated during the flooding, but later returned to background levels. Dissolved organic carbon concentrations declined through the whole study period, but did not appear to peak as was observed in the Neuse River estuary, a key tributary of the Sound. Light attenuation was highest in the fall to spring following the storms and was best correlated with chlorophylla concentrations. Phytoplankton biomass (chla) increased and remained elevated until late spring 2000 when concentrations returned to pre-storm levels and then cycled seasonally. Phytoplankton community composition varied throughout the study, reflecting the complex interaction between physiological optimal and combinations of salinity, residence time, nutrient availability, and possibly grazing activity. Floodwater advection or dilution from upstream maxima may have controlled the spatial heterogeneity in total and group-specific biomass. The storms produced areas of shortterm hypoxia, but hypoxic events continued during the following two summers, correlating strongly with water column stratification. Nitrogen loading to the southwestern sound was inferred from network analysis of previous nitrogen cycling studies in the Neuse River estuary. Based on these analyses, nutrient cycling and removal in the sub-estuaries would be decreased under high flow conditions, confirming observations from other estuaries. The inferred nitrogen load from the flood was 2–3 times the normal loading to the Sound; this estimate was supported by the substantial algal bloom. After 8-mos, the salinity and chla data indicated the Sound had returned to pre-hurricane conditions, yet phytoplankton community compositional changes continued through the multi-year study period. This is an example of long-term aspects of estuarine recovery that should be considered in the context of a predicted 10–40 yr period of elevated tropical storm activity in the western Atlantic Basin.

Dynamics of NH4+ and NO3− uptake in the water column of the neuse river estuary, North Carolina

In an attempt to more fully understand the dissolved inorganic nitrogen dynamics of the Neuse River estuary, 15NH4+ and 15NO3− uptake rates were measured and daily depth-integrated rates calculated for seven stations distributed along the salinity gradient. Measurements were made at 2–3-wk intervals from March 1985 to February 1989. Significant dark NH4+ uptake occurred and varied both spatially and seasonally, accounting for as much as 95% of light uptake with the median being 33%. Apparent NH4+ uptake ranged from 0.001 μmol N 1−1 h−1 to 4.2 μmol N 1−1 h−1, with highest rates occurring during late summer-fall in the oligohaline estuary. Apparent NH4+ uptake was significantly related to NH4+ concentration (p<0.01); however, the regression explained <3% of the variation. Daily-integrated NH4+ uptake ranged from 0.1 mmol N m−2 d−1 to 133 mmol N m−2 d−1 and followed the trend of apparent uptake. Annual NH4+ uptake of the estuary was significantly lower in 1988 than for any other year. Dark uptake of NO3− was only 14% of maximum light uptake. Apparent NO3− uptake rates ranged from 0.001 μmol N 1−1 h−1 to 1.84 μmol N 1−1 h−1 with highest rates occurring in the oligohaline estuary. Apparent NO3− uptake was significantly related to NO3− concentration (p<0.01); however, the regression explained <5% of the variation. In general, NO3− uptake was only 20% of total dissolved inorganic nitrogen (DIN) uptake. Daily-integrated NO3− uptake ranged from 0.1 mmol N m−2 d−1 to 53 mmol N m−2 d−1 and followed similar patterns of apparent uptake. Annual NH4+ uptake was 11.39 mol N m−2 yr−1, 10.28 mol N m−2 Yr−1, 10.93 mol N m−2 yr−1, and 7.38 mol N m−2 yr−1, and 1.84 mol N m−2 yr−1, with the 4-yr mean being 10.0. Annual NO3− uptake was 3.12 mol N m−2 yr−1, 3.40 mol N m−2 yr−1, 1.96 mol N m−2 yr−1, and 1.84 mol N m−2 yr−1, with the 4-yr mean being 2.6. The total annual DIN uptake was more than twice published estimates of phytoplankton DIN demand, indicating that there is an important heterotrophic component of DIN uptake occurring in the water column. The extrapolation of nitrogen demand from primary productivity results in serious underestimates of estuarine nitrogen demand for the Neuse River estuary and may be true for other estuaries as well.

Network Analysis of Nitrogen Inputs and Cycling in the Neuse River Estuary, North Carolina, USA

The Neuse River estuary, North Carolina, United States, has demonstrated various symptoms of eutrophication during the past 20 yr. We contributed to an environmental assessment program, through ecological network analysis, a group of algorithms to evaluate networks of material flows within a structured system. Networks of nitrogen (N) cycling for 16 consecutive seasons were constructed based on previous field and laboratory studies. Network analysis provided understanding of the relationship between N loading and recycling, the fates of N and the expected interseasonal variation of both model inputs and outputs. Various indices indicated that recycling of imported N was very high, supporting measured observations. There was little correlation between estimates of loading and N uptake by phytoplankton, although loading of total and particulate N did correlate positively with export. Because of the high degree of recycling of N, the rate of loading of new N is a small fraction of the total processing of N or of the needs for primary production alone. We predict that on a short-term basis the controls on primary production tend to be associated with conditions in the estuary rather than import. This condition is likely to postpone easily observable responses to loading reduction over the entire estuary and in the short term, although improvements in water quality should occur over time.

Nitrogen cycling networks of coastal ecosystems: influence of trophic status and primary producer form

Abstract We have used ecological network analysis to compare nitrogen cycles from five well-researched coastal ecosystems. These included a representative ricefield and two lagoons (Tancada and Encanysada lagoons) in the Ebro River delta, Spain; a region of the Sacca di Goro, a lagoon at the mouth of the Po River, Italy; and a drowned river estuary in North Carolina, USA, the Neuse River estuary. We constructed networks for the various systems and ranked them by trophic status (i.e., degree of eutrophication) using four indices. We then considered the importance of (1) trophic status, (2) growth form of dominant primary producer and (3) water residence time to the intensity and pattern of recycling and to the manner in which the systems can “filter” N. Three indices of flux (rate of import, primary producivity and total systems throughput) gave similar rankings of trophic status among ecosystems with the Italian and U.S. systems being most eutrophic, ricefields next, and then the two Spanish lagoons. Patterns of N export and of cycling within the systems were most closely related to the growth form of dominant primary producers. Phytoplankton, with their rapid turnover rate, foster rapid recycling within the water column and continuous transfer to sediments and export. Submersed and emergent aquatic vegetation and macroalgae create lags and pulses within systems by sequestering N during growth and releasing it during senescence, death and decomposition. Trends in cycling among systems relative to trophic status or water residence appear largely secondary to primary producer growth form.

Lignocellulose and lignin in the salt marsh grass Spartina alterniflora: initial concentrations and short-term, post-depositional changes in detrital matter

Lignocellulose was found to comprise the bulk of various anatomical structures of the salt marsh grass Spartina alterniflora, as well as of detritus derived from this plant; although concentrations of both lignin and lignocellulose varied with plant height-form, age, and anatomical structure. Changes in the relative concentrations of lignin and polysaccharide in lignocellulose due to long-term degradation by natural marsh microflora were determined using standard gravimetric assay procedures and a new procedure utilizing specifically-radiolabelled 14C-(cellulose)-lignocellulose and 14C-(lignin)-lignocellulose prepared from S. alterniflora. Results obtained with the two methods agreed, thus validating the new procedures. Mineralization of the cellulose moiety was more rapid than mineralization of the lignin moiety resulting in relative enrichment of S. alterniflora detritus in lignin. Rates of mineralization of both moieties decreased over time such that extent of mineralization was a function of the logarithm of aging time.

Significance of subtidal sediments to heterotrophically-mediated oxygen and nutrient dynamics in a temperate estuary

Sediment-water exchanges of ammonium (NH4+), nitrate + nitrite (NOx−), filterable reactive phosphorus (FRP, primarily ortho-phosphate), and oxygen (O2) under aphotic (heterotrophic) conditions were determined at 2–5 stations in the Neuse River Estuary, from 1987 to 1989. Shallow (1 m), sandy stations were sampled along the salinity gradient. Fluxes from deep (>2 m) sites were compared to the shallow sites in two salinity zones. Grain size became finer and organic content increased with depth in the oligohaline zone but not in the mesohaline zone. Net release of NH4+ and FRP occurred at all sites. Fluxes varied from slight uptake to releases of 200–500 μmol m−2 h−1 (NH4+) and 150–900 μmol m−2 h−1 (FRP). Net NOx− exchange was near zero, but were ±100 μmol m−2 h−1 over the year. Release of NH4+ and FRP from the shallow sandy stations decreased with distance down the estuary, but O2 uptake did not change. The deeper oligohaline site had twofold higher rates of NH4+ and FRP release and O2 uptake than the shallow site, but no differences occurred between depths in the mesohaline zone. Temperature and organic content were important controls for all fluxes, but water column NOx− concentration was also important in regulating NOx− exchanges. Ratios of oxygen consumption to NH4+ release were near the predicted ratio (Redfield model) at oligohaline sites but increased down estuary at mesohaline sites. This may be due to greater nitrification rates promoted by autotrophy in the sediments.

Ecological Response to Hurricane Events in the Pamlico Sound System, North Carolina, and Implications for Assessment and Management in a Regime of Increased Frequency

Since the mid 1990s, the Atlantic and Gulf Coast regions have experienced a dramatic increase in the number of hurricane landfalls. In eastern North Carolina alone, eight hurricances have affected the coast in the past 9 years. These storms have exhibited individualistic hydrologic, nutrient, and sediment loading effects and represent a formidable challenge to nutrient management aimed at reducing eutrophication in the Pamlico Sound and its estuarine tributaries. Different rainfall amounts among hurricanes lead to variable freshwater and nutrient discharge and variable nutrient, organic matter, and sediment enrichment. These enrichments differentially affected physical and chemical properties (salinity, water residence time, transparency, stratification, dissolved oxygen), phytoplankton primary production, and phytoplankton community composition. Contrasting ecological responses were accompanied, by changes in nutrient and oxygen cycling, habitat, and higher trophic levels, including different direct effects on fish populations. Floodwaters from the two largest hurricances, Fran (1996) and Floyd (1999), exerted, multi-month to multi-annual effects on hydrology, nutrient loads, productivity, and biotic composition. Relatively low rainfall coastal hurricanes like Isabel (2003) and Ophelia (2005) caused strong vertical mixing and storm surges, but relatively minor hydrologic and nutrient effects. Both hydrologic loading and wind forcing are important drivers and must be integrated with nutrient loading in assessing short-term and long-term ecological effects of these storms. These climatic forcings cannot be managed but should be considered in the development of water quality management strategies for these and other large estuarine ecosystems faced with increasing frequencies and intensities of hurricane activity.

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