PIE-LTER Summary of Research Results through 1997

Watershed Keynote- Major upland NO₃^- loads, primarily from urban areas, are retained with 80-90% efficiency in rivers resulting in DON being the major form of N export to estuary.

Land use, economic activity and population are changing rapidly in the three river basins comprising the 585 km² upland watershed of the Plum Island Sound Estuary. We have 1) characterized historical landuse for the Parker, Rowley and Ipswich River watersheds, 2) developed digital databases (soils, land cover, slope, streams, erosivity, etc.) for use in hydrologic modeling, 3) characterized runoff chemistry for several small catchments and 4) modeled water, organic matter and nutrient discharge to the estuary (Finn and Hopkinson, 1997).

Land cover (Fig 1) in the watershed in 1985 was » 50% forest, 25% urban/suburban, 13% wetland, and 12% agricultural. During the past 25 years urban area increased substantially, largely at the expense of agricultural (down 50%) and forested land (down 8%).

Runoff characteristics and streamwater chemistry (spm, C, N and P) of streams draining various land covers differed substantially. Total N was dominated by the dissolved fractions for all land covers (Fig 2). Total dissolved N (TDN) concentration was highest in urban runoff and least in forest runoff. While TDN was dominated by inorganic N (primarily NO₃^-) in urban runoff, dissolved organic N (DON) comprised up to 90% of the total in forested streams. Agricultural runoff was intermediate in concentration, with inorganic and organic fractions of roughly equal concentration.




Material loading and export budgets were created by scaling subcatchment results to entire watersheds using a combination of hydrologic models (TR-55, Brook6, and GWLF). Our riverine budgets did not balance. For N we estimated 515-975 Mg N yr^-1 entered the river, while only 91-106 Mg yr^-1 were discharged to the estuary. Thus in stream processing removed 80-90% of inputs from land probably through a combination of processes including vegetation uptake, sedimentation, organic matter accumulation and burial and denitrification. DON was the major form of N exported from the watershed.

Estuarine Hydrodynamics Keynote- Residence time varies spatially and temporally from as long as 34 days in upper estuary to as little as 0.5 d in lower estuary depending on river flow.

Salinity distributions, dye release studies and parameter estimation techniques were used to determine longitudinal dispersion coefficients in the Estuary. Two hydrodynamic models were developed: 1) a 1-D tidally averaged, advection-dispersion model (Vallino and Hopkinson in press) and 2) a 1-D advection-dispersion model that accounts for marsh flooding (Vorosmarty and Loder 1994). These models were used to investigate characteristic mixing-time scales, average age, residence time and transit time, and to contrast nutrient dynamics during spring and neap tides. These time scales vary from hours to 2 months depending on location and freshwater discharge. Transport in the upper estuary can be either advection or dispersion dominated depending on discharge, while the lower estuary is always dominated by dispersive terms.

Microbial Utilization and Composition of Watershed-derived Organic Matter Keynotes - Novel bioenergetic model replaces Monod growth kinetics; Organic matter from various land covers differs in chemical characteristics, ability to support bacterial growth, and potential for N immobilization.

Effort focused on chemically characterizing organic matter from the variety of estuarine sources including plankton, macrophytes and runoff from several land covers. Bioassays were used to measure organic matter lability, patterns of nutrient immobilization or remineralization and bacterial growth rates and efficiency (Normann et al. 1995, Hullar et al. 1996). This work is the cornerstone of our research on organic matter - nutrient interaction effects on food web structure and efficiency.

Bioenergetic Model: A bioenergetic model was developed to examine growth dynamics associated with bacterial utilization of DOM, NH₄⁺ and NO₃^- (Vallino et al. 1996). A novel, optimization approach was taken that provides more information on bacterial growth kinetics than Monod-type models that are typically employed to describe bacterial growth. The model provides a means to predict bacterial carbon yield, growth rate and N processing and is a bridge between chemical and biological characterization of DOM: bacterial dynamics are predicted from DOM chemical characteristics. Model results demonstrate that bacterial growth can not always be explained by a single constraint (such C:N ratio of substrate) as bacteria allocate resources to maximize growth rate subject to kinetic, thermodynamic and mass balance constraints. The model suggests that bacterial growth and yield can be predicted from the degree of reduction of the substrate (y ). We have been devoting considerable time applying the model to field situations and attempting to differentially characterize labile and recalcitrant pools of bulk DOM in natural waters.

Bioassays: Globally, changes in watershed landuse have increased levels of organic carbon transported to the sea by 3-5 times over natural levels. Yet it is unclear how much of the DOM entering estuaries is actually used by microbes and contributes to the food web and how much resists degradation and is exported offshore. To better understand this, we have taken advantage of "natural" differences in DOM composition and conducted experiments, during high and low flow periods, with runoff water from urban, agricultural and forested land covers (Uhlenhopp et al. 1995, Hobbie et al. 1996). DOM from these sites ranged in concentration from 310 to 870 µM C, C/N from 10 to 40, and DIN from <10 to > 50 µM N. Lability was measured with bacterial growth bioassays where bacterial #s, DO, DIC, DOC, DON and nutrients were monitored over time. Results (Fig 3) showed that labile DOC pools ranged from ~40 µM C in the forest to ~100 µM C in the agricultural runoff and did not vary much over the course of a year. The percent of the bulk pool that was labile was small, similar between sites (5-15%) and varied little over time. Bacterial yield during the first 3 d was compared to the bacterial yield predicted by the bioenergetic model based on the estimated degree of reduction of the DOM. Predicted bacterial yield (29, 32 and 43%) was in good agreement with that observed (29, 39, 52%: Fig 3). We further observed that N was remineralized during low flow when DIN was generally low and immobilized during high flow when DIN was high.

Organic Matter - Nutrient Interaction Effects on Higher Trophic Levels: Keynotes - Mesocosm experiments support trophic model analysis of the effect of organic matter on foodweb efficiency; stable isotope analysis of food sources and consumers indicates preferential utilization of low quality marsh detritus by benthivores.

Mesocosm Experiments: We conducted experiments in 10 m³ plastic bags to examine differential effects of phytoplankton derived organic C vs terrestrial organic C on planktonic community structure and production. To 4 mesocosms with natural, intact planktonic communities (through larval Menidia fish) were added either: A) control - nothing, B) DOM from dead leaf litter leachate: final DOC up from ~100 to 500 µM, C) daily additions of inorganic N, P and Si, and D) leaf leachate DOM as in (B) plus daily additions of nutrients as in (C). DI¹³C was added to each bag to trace phytoplankton derived C through the food web. Dramatic differences in community structure and production resulted. Gross production and net community production were highest in the nutrient enriched (C) treatment and lowest in the litter leachate (B) treatment. Whereas primary production was dominated by diatoms > 20 µm in the nutrient enriched bag, 85-90% of production was attributed to phytoplankton < 20 µm in the control (A) and DOM (B) bags. Food web efficiency was low in the DOM amendment bags (B&D) with 80% of respiration attributable to organisms < 1 µm and only 20% attributable to organisms > 20 µm. In contrast, trophic transfer was substantially greater in the nutrient addition bag as evidenced by ~50% of system respiration being attributable to organisms between 20-80 µM and >80 µm in size. Less than 50% of total respiration was attributable to bacteria. Species composition changed with treatments as well. In the DOM bag (B), Podon and Parvocalanus became the dominant zooplankton while in the DIN bag (C), Acartia quickly became dominant. A N-flow model for bag (D) illustrates the dominance of the microbial loop and the trivial transfer of N to higher trophic levels (Fig 4). These results confirm previous flow model predictions about the effects of organic matter vs nutrients on trophic structure and production (Deegan et al. 1996): inorganic nutrients support a short, efficient and productive food web while low quality organic matter without inorganic nutrients support an inefficient, microbe-dominated food web with low levels of small phytoplankton production.

Sources of Organic Matter Supporting Higher Trophic Levels in the Estuary: The importance of phytoplankton, benthic microalgae, marsh grass and terrestrial organic matter to the estuarine food web was evaluated using stable isotopes of C, N and S (Deegan and Garritt 1997). Strong spatial differences in relative importance of organic matter sources existed within the estuary. While there is substantial spatial heterogeneity in organic matter sources within the estuary, consumers utilize sources produced in the same region. Terrestrial organic matter was not evident in the food webs of the middle and lower estuary and its importance in the oligohaline region was equivocal as there was not a distinct separation in isotopic signatures between fresh marsh grass and terrestrial organic matter. There was an interesting dichotomy in the relative importance of phytoplankton vs marsh detritus in the diets of pelagic vs benthic organisms. Benthic consumers reflected a greater dependence on marsh detritus.

Macrofaunal Community Structure Keynotes - Low number of fish species in estuary reflects nature of Acadian Biogeographic Province; Six-fold increase in forage fish density between 1965 and 93/4.

The current status of the estuary as a habitat for important marine fish and shellfish was ascertained using seine and trawl samplings (Buchsbaum 1996). Sampling sites and techniques were similar to those of earlier surveys conducted by the Massachusetts Division of Marine Fisheries (Jerome et al. 1967). The distribution of organisms varied in the different habitats of the estuary. In open water and sandy substrata, Menidia, Fundulus and Crangon dominated. In muddy salt marsh habitats the same species were present but Fundulus was the dominant and Palaemonetes was also common. In brackish, riverine habitats Morone, Alosa and Rhithropanopeus were the most common organisms. In comparison to the 1965 survey, the average density of fish was about 6-fold higher in 1993-94. In both studies Fundulus and Menidia were dominants in the community but species richness based on seine sampling was slightly higher in 1993-4. Explanations for the differences in density between years is not obvious, but may be related to marked differences in average sea level or river discharge during critical periods in the development of these organisms. Long term data are needed to adequately explain whether the differences between 1965 and 1993/4 are caused by regional and environmental factors or whether fish populations naturally exhibit short term variability.

Nutrient Cycling and Metabolism Keynotes - Temporal variability in tidal amplitude and river discharge affect spatial patterns of nutrient cycling, benthic nutrient fluxes and primary production; Estuarine waters are heterotrophic and therefore net sources of CO₂ and inorganic nutrients.

Temporal and spatial aspects of metabolism and nutrient cycling were documented, including primary production (Alderman et al. 1995, Balsis et al. 1995), spring-neap tidal contrasts in marsh-water organic matter and nutrient exchange (Vorosmarty and Loder 1994), benthic metabolism and nutrient exchange (Hopkinson et al. submitted), and inorganic nutrient and dissolved organic matter dynamics. High primary production occurs in the lower estuary during winter in association with the Gulf of Maine spring bloom. Production is high during summer in the oligohaline region but only when mixing time scales are long relative to phytoplankton turnover time. The benthos is a major site of organic matter degradation and because of denitrification and the high C/N nature of organic matter processed in the benthos, it exerts a net heterotrophic influence on the system (ie, C production from recycled N << organic carbon remineralized). Seasonal and annual variations in porewater salinity of oligohaline and brackish sediments alters the timing of NH₄⁺ release/storage in sediments. The overall system is heterotrophic with DO often depressed to 60% of saturation in mid-estuary (Fig 5). The region of highest heterotrophy coincides with organic matter inputs from marshes as evidenced by dDO¹³C gradients in the water column (Fig. 5).

Whole System Synthesis Keynote - Generalized estuarine metabolism model evaluates patterns of production, nutrient cycling and autotrophy/heterotrophy caused by variations in river discharge and material loading from land.
Flow model analysis and simulation models have been used from the outset of our research program to guide research, help in designing experiments, synthesize results of microcosm and mesocosm experiments and to extend experimental and process based research results to the field. In addition to models described above, we have used flow model analysis to understand the effects of organic matter - nutrient interactions on estuarine trophic dynamics (Deegan et al. 1995). We developed a generalized estuarine metabolism model to explore the effects of land use change and river modifications on patterns of production, respiration and nutrient cycling (Hopkinson and Vallino 1995). The model was useful for evaluating the effect of residence time, organic matter quality, and DON:DIN and POM:DOM loading ratios on patterns of net ecosystem metabolism.