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PIE

Plum Island Ecosystems LTER

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Pete Raymond, Byron Crump and Nat Weston installing a Sediment Elevation Tube (SET) in Spartina patens marsh. Photo courtesy of the Plum Island Microbial Observatory
PIE
The Plum Island Ecosystems (PIE) LTER is an integrated research, education and outreach program whose goal is to develop a predictive understanding of the long-term response of watershed and estuarine ecosystems at the land-sea interface to changes in climate, land use and sea level. The principal study site is the Plum Island Sound estuary, its coupled Parker, Rowley and Ipswich River watersheds and the adjacent coastal ocean, the Gulf of Maine. The PIE LTER focuses on how several aspects of global change influence organic matter and inorganic nutrient biogeochemistry and estuarine foodwebs. The inputs of organic matter and nutrients from land, ocean and marshes interact with the external drivers (climate, land use, river discharge, sea level) to dictate the extent and degree of nutrient and organic matter processing and determine the spatial patterns of estuarine productivity and trophic structure.

Humans are altering the ecosystems of the world at rates not previously experienced. Understanding and predicting how multiple stresses affect the sustainability of ecosystems is one of the most crucial challenges in environmental biology. The PIE LTER focuses on how several aspects of global change influence organic matter and inorganic nutrient biogeochemistry and estuarine foodwebs. The inputs of organic matter and nutrients from land, ocean and marshes interact with the external drivers (climate, land use, river discharge, sea level) to dictate the extent and degree of nutrient and organic matter processing and determine the spatial patterns of estuarine productivity and trophic structure. The overarching question is:
How will trophic structure and primary and secondary productivity in estuaries be affected by changes in organic matter and nutrient loading and hydrodynamics caused by changing land use, climate and sea level?
The project uses a combination of approaches to address research questions and hypotheses: 1) short- and long-term “core” measurements 2) short and long-term experiments, 3) comparative ecosystem studies and 4) modeling. The research integrates estuarine biogeochemistry with studies of food webs and population biology of all trophic levels. The PIE LTER data and information system provides a centralized network of information and data related to the Plum Island Sound Estuarine Ecosystem and its watersheds. The centralized network provides researchers associated with PIE-LTER access to common information and data in addition to centralized long-term storage. Data and information are easily accessible to PIE-LTER scientists, local, regional, state partners and the broader scientific community. Researchers associated with PIE-LTER are committed to the integrity of the information and databases resulting from the research.
Broader Impacts: PIE-LTER has developed links with local teachers and students, citizens, conservation organizations, and local, state and federal agencies. What started out as a “minimalist” program has grown to be a broad, well-rounded suite of activities. The education/outreach program is expected to further expand during LTER2, as additional support is obtained from other federal agencies, the Commonwealth of Massachusetts, and private foundations. The long-term goal is to establish a ‘Coastal Outreach” office at the PIE-LTER study site that will serve to integrate and promote interactions with interested parties throughout New England. During the next funding cycle, the LTER will expand its schoolyard program to provide on-going professional development for teachers; support the expansion of this project into nearby urban areas including Salem, Boston, and Revere; and facilitate the transfer of this program to New Hampshire and Maine via the Gulf of Maine Institute. There will have greater involvement with undergraduate and graduate education with the addition of PIs Mather and Pontius from UMass and Clark University. PIE-LTER has a very active outreach program in which the goal is to communicate research findings to individuals, organizations, and agencies that will use PIE-LTER research results to better manage local and regional coastal resources. The project has established partnerships on three major issues: intertidal marshes, coastal eutrophication and watershed resource management. The issues of sea level rise, marsh survival and wetland restoration will be the next outreach focal points of the project.

Biogeographical Perspective: Marine biologists have long realized the presence of distinct regional distributions of the coastal flora and fauna. On the basis of diversity and general faunal and floral distributions, 9 geographic provinces have been described for coastal waters of North America. The provinces are generally related to the 4 climatic regions of the Atlantic and Pacific Oceans: arctic, cold-temperate, warm-temperate and tropical (Hayden and Dolan 1976). Coastal and land-margin LTERs are situated in 5 of these 9 provinces.

The Plum Island Sound System is located in the cold-temperate Acadian Province. Cape Cod represents a very significant boundary between waters to the south, which experience extreme seasonal temperature fluctuations (Virginian Province) and waters to the north, which are cold year round (Acadian Province). In general the species diversity is low in the Acadian Province. For example, we have found 18 and 28 fish species in Wells and Plum Island estuaries compared to >52 in Waquoit Bay (Virginian Province) (Ayvazian et al. 1992, Buchsbaum 1996). In contrast to the Virginian and Carolinean Province where endemism is extremely low (1% for fish,

The Plum Island Ecosystems LTER site lies at the interface of a thinly soiled, formerly glaciated New England land mass and the highly productive Gulf of Maine (Carlozzi et al. 1975). Three watersheds comprise the estuarine drainage basin: Parker (155 km2), Rowley (26 km2) and Ipswich (404 km2). The Ipswich River watershed is highly urbanized with Boston "bedroom" communities encroaching in the headwater region while the Parker is less urbanized and retains a higher proportion of forest.

Site Integrity: While the watersheds of the Plum Island Ecosystems LTER site are experiencing rapid change in population, economic activity and land use, wetlands both in the watershed and the estuary are well protected by state and federal regulations. Approximately 25% of the Ipswich River watershed has been set aside as conservation land. In addition, most of the estuarine ecosystem is included in the Parker River National Wildlife Refuge. Thus the integrity of the system is high and will serve as a valuable laboratory well into the next millennium.

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Short history: 
The Plum Island Ecosystems LTER (PIE LTER) was established in 1998. During the course of our research, we have designed and implemented a comprehensive study of a major, land-estuarine system in the Acadian biogeographic province in eastern New England. Our goal is to develop a predictive understanding of the long-term dynamics of watershed and estuarine ecosystems at the land-sea interface and to apply this knowledge to the wise management and development of policy to protect the natural resources of the coastal zone. The principal study site is the Plum Island Sound estuary, its coupled Parker and Ipswich River basins and the coastal ocean, the Gulf of Maine.
History: 

During the first 6 years of funding, we developed a foundation of research that includes long-term field experiments and measurements in watersheds, atmosphere, intertidal marshes, estuary and coastal ocean. Experiments and observations examine LTER core areas: primary production, organic matter accumulation, nutrient cycling, higher trophic levels, and ecosystem disturbance.

WATERSHED HYDROLOGIC CYCLE
Following substantial changes in land use in the Ipswich and Parker River watersheds during the 20th century (from >50% agriculture in 1900, to > 85% forest in 1950, to >35% urban in 2000) (Pontius and Schneider 2001), we were surprised to find that discharge was largely unchanged. We expected to see increases in discharge due to the increase in impervious surface and a projected decrease in evapotranspiration (ET) associated with land use change from forest to urban (Claessens et al. submitted). Instead we found that diversions and climate change decreased the percentage of rainfall exported from the watershed in rivers.
Analysis of historic climate data shows that precipitation and ET increased 19% and 25% (rates of 2.9 mm and 1.6 mm per year, respectively) between 1931 and 1998 (Claessens et al. submitted). We attribute the discrepancy between expected reduction in ET and actual increase in ET to climate change. We observed a downward trend in minimum temperature beginning in the 1950s as well as an increase in dewpoint temperature and a convergence of minimum and dewpoint temperatures. This pattern is an indication that the near-surface atmosphere has become more humid, a result of increased ET.
Diversion of water for municipal use (drinking water and sewage) has had the greatest impact on hydrologic budgets (Canfield et al. 1999). Diversions today are roughly 20% of annual streamflow. Historically populations outside watershed boundaries have withdrawn water from the Ipswich and Parker rivers for drinking water. More recently water is diverted as sewage to treatment plants outside watershed boundaries. As urban density increases, septic treatment decreases and sewage export increases.
Watershed hydrologic modifications have altered biogeochemical cycles. For instance, N export in sewage is now a major component of the watershed N cycle (Williams et al. submitted a,b). The increased importance of surface flow in urban vs. forested areas leads to reduced contact with “biogeochemically active sites” and a greater input of constituents to streams (Pellerin et al. in prep, Wollheim et al. submitted). We also find that N retention declines in urban areas due to the increased runoff associated with impervious surfaces (Wollheim et al. submitted.

ESTUARINE HYDRODYNAMICS
Estuarine hydrodynamics and characteristic mixing times are the result of river runoff, location of river runoff, estuarine geometry and tidal mixing (Vallino and Hopkinson 1998). Total average annual river discharge from the 610-km2 watershed is 11 m3 s-1, ranging from As a result of seasonal variations in discharge, there are strong salinity gradients within the estuary that vary over time. During high flow, salinity drops to 0 ppt in the upper 10 km while during low flow salinity can exceed 15 ppt at the very head of the estuary. Likewise there are strong gradients in water residence times along the length of the estuary that also vary in response to river discharge. During low flow ( Additional factors controlling estuarine hydrology and hydrodynamics are local precipitation (especially during summer when ET is high and river discharge is low) and sea level variation. Mean sea level exhibits strong lunar, seasonal and annual cycles and variability. The effect of sea level variation is to alter tidal excursion lengths and marsh flooding depth and frequency. Direct precipitation becomes an increasingly important factor controlling salinity distribution, especially on the intertidal marsh in periods of low river discharge and low mean sea level when marsh flooding is limited or absent.

MARSHES
The productivity of intertidal marsh plants is strongly related to variations in salinity during the growing season (Morris and Haskin 1990, Morris 2000, Morris et al. 2002). The uptake of nitrogen by salt marsh vegetation is highly inefficient, varies with salinity and MSL, and contributes to the leaky behavior of the saltmarsh nitrogen cycle. The salt gradient moves in response to changes in freshwater discharge and sea level, and this can affect the export and import of ammonium in marshes along the salinity gradient (Koop-Jakobsen and Giblin 2002). As salinity declines upriver, species composition changes (Odum 1988) and the plant community shift to one limited by phosphorus (Daoust and Morris 2003).
Increased sea level rise is likely to promote the migration of salt marshes upriver and to reduce the extent of tidal fresh and brackish marshes. We have observed substantial marsh disintegration over the past 50 years at the mouth of the estuary, due to a combination of lateral erosion and marsh ponding (Cavatorta et al. 2003), which we associate with the long-term increase in sea level at PIE and reduced sediment loads as a consequence of reforestation of the watershed following abandonment of agriculture in New England. Disintegration results in an increase in total length of the marsh-water interface (Johnston et al. 2003) where there is considerable drainage of marsh porewater. Thus, increased sea level rise is likely to increase porewater drainage which is a significant source of inorganic and organic nutrients for the planktonic sub-system (Wright et al. 1987, Raymond and Hopkinson 2003).

PLANKTONIC SYSTEM
Variations in water residence time along the estuary play a major role in phytoplankton bloom occurrence (Holmes et al. 2000), the relative importance of pelagic and benthic primary production (Hughes et al. 2000), and bacterial community structure (Crump et al. in press). Phytoplankton blooms only occur in the oligohaline part of the estuary during midsummer when river discharge is low and residence time is greater than a week. Blooms also occur offshore during spring and advect into the estuary. Shifts in bacterioplankton community composition along the salinity gradient are related to residence time and bacterial community doubling time in spring, summer and fall seasons. Freshwater and marine populations advected into the estuary represent a large fraction of the bacterioplankton community in all seasons. However, a unique estuarine community forms at intermediate salinities in summer and fall when bacteria doubling time is much shorter than water residence time (Crump et al. in press). The mid-estuary is a region of high heterotrophic activity, with O2 levels occasionally less than 50% of saturation. This reflects large inputs of organic matter substrates from adjacent intertidal marshes (Raymond and Hopkinson, 2003) that are linked via fluctuations in mean sea level and marsh flooding frequency (Wright et al. 1987). Organic matter inputs from marsh porewater drainage are likely to be 10x greater than those from the rivers.

ESTUARINE BENTHOS
Watershed discharge-related spatial and temporal patterns of salinity are important controls of benthic N dynamics (Giblin et al. in prep, Weston et al. in prep) and hence productivity of the overlying water. In spring, when river discharge is high, salinity in the porewaters is low and the majority of the N remineralized in sediments is either held on exchange surfaces, or lost via denitrification. Salinity increases during the summer as discharge decreases and NH4+ is displaced from the exchange complex leading to a large benthic flux of NH4+ to the water column. This flux appears to be the major source of N supporting the mid-summer phytoplankton bloom in the oligohaline portion of the estuary. In addition, N2 losses decrease during summer as both coupled and direct denitrification rates reach minimal values in spite of high N mineralization rates. N2 losses are suppressed because nitrifiers appear to be unable to adapt to the rapid seasonal change in salinity, shutting off coupled nitrification / denitrification (Mondrup 2000) and because during mid-summer the process of dissimilatory nitrate reduction to ammonium (DNRA) appears to out compete denitrification for NO3- from the water column and water column NO3- concentration is low. In autumn, when the porewater salinities decrease, denitrification rates increase and NH4+ fluxes decrease. Preliminary work in intertidal vegetated sediments has also shown the importance of salinity in modulating the release of NH4+ (Koop-Jakobsen 2003) and measurements of denitrification are underway.

ESTUARINE HIGHER TROPHIC LEVELS
We have also found variations in river discharge and sea level to play a large role in the production of higher trophic levels. Abundance of the dominant marsh fish, mummichog, is positively related to the amount of flooded marsh in creek watersheds (Komorow et al. 1999). Factors affecting habitat quality for fish include marsh area, marsh edge length, flooding frequency, depth and duration and salinity (Haas and Deegan, in prep.). Each of these factors is directly related to sea level and/or river discharge. The species present, as well as their abundance, are related to water residence time, which sets a template that determines the pathways and fate of nitrogen in estuarine systems (Hughes et al. 2000, Holmes et al. 2000, Tobias et al. 2003a, b, Hughes et al., in prep.). When residence time is long, phytoplankton dominate nitrogen uptake and the food web has well developed benthic and pelagic communities with strong benthic-pelagic coupling controlled by the animal community. When water residence time is short, benthic microalgae are the dominant primary producers and the principal food chain is benthic.

SYNTHESIS
N export from watersheds is increasing as urbanization proceeds. The timing of N export is largely driven by variations in river discharge. Climate and land use change, as well as water diversions, are causing a greater percentage of river N export to occur earlier in the spring. Interestingly the timing of maximum N export from the watershed is also the time of minimum estuarine residence time and suggesting that the majority of N exported from watersheds is passed directly to the coastal ocean without significant estuarine processing. The greatest N processing occurs in midsummer, when residence time is longest, but when N imports are least. We presently do not know how much of the N during high flow is retained in the estuary and released later in the year. Nor do we know what the potential mechanisms of N retention might be during the high discharge period. Interestingly, the N exported from many watersheds during late winter/early spring fuels a major plankton bloom in coastal waters of the Gulf of Maine, some of which is tidally mixed into estuarine waters. Thus high algal biomass in the lower Plum Island estuary is not produced locally but externally by N not retained during estuarine passage and by oceanic N sources. The major effect of the watershed on this estuarine system appears to be the influence of freshwater discharge on the spatial and temporal pattern of water residence time. Residence time and estuarine geomorphology define the template upon which ecological processes and foodwebs operate.

Short research topics: 
The biosphere is undergoing unprecedented change as a result of human activities. Major global issues include growth of the human population, land use change, climate change, altered hydrologic cycles, and sea level rise. There are numerous ways that these globally important issues are affecting the biosphere. The PIE LTER focuses on how these issues influence organic matter and inorganic nutrient biogeochemistry and estuarine foodwebs. The inputs of organic matter and nutrients from land, ocean and marshes interact with the external drivers (climate, land use, river discharge, sea level) to dictate the extent and degree of nutrient and organic matter processing and determine the spatial patterns of estuarine productivity and trophic structure. Our overarching question is: How will trophic structure and primary and secondary productivity in estuaries be affected by changes in organic matter and nutrient loading and hydrodynamics caused by changing land use, climate and sea level?

The Plum Island Ecosystems (PIE) LTER is an integrated research, education and outreach program whose goal is to develop a predictive understanding of the long-term response of watershed and estuarine ecosystems at the land-sea interface to changes in climate, land use and sea level. The principal study site is the Plum Island Sound estuary, its coupled Parker, Rowley and Ipswich River watersheds and the adjacent coastal ocean, the Gulf of Maine. Our interests are not just local; we extend our understandings to other coastal systems through comparative studies and modeling.
Ecosystems at the land-sea interface play a major but poorly understood role in regional as well as global hydrologic, sediment, and biogeochemical cycles (Hedges et al. 1997, Aller 1998, Blair et al. 2003, McKee 2003). Coastal ecosystems play a key role in the transformation, transport, burial and exchange of water and organic and inorganic carbon and nitrogen between land, atmosphere and the ocean. With an overwhelming majority of the human population living in the coastal zone and with runoff from entire continents funneling through estuaries and ocean margins, coastal systems are among the most heavily impacted ecosystems on the globe.
The biosphere is undergoing unprecedented change as a result of human activities. Major global issues include growth of the human population, land use change, climate change, altered hydrologic cycles, and sea level rise. These are also important issues at regional scales, as well as at the PIE LTER. Human population in the U.S. continues to increase, especially in the coastal zone; we see similar dramatic increases in population in the Ipswich and Parker River watersheds. Land use has changed substantially along the entire east coast of the U.S. over the past century. The pattern of abandonment of agriculture, reforestation followed by urbanization has occurred in the PIE watershed as well (Schneider and Pontius 2001). There have been substantial changes in the frequency of storms along the U.S. Atlantic coast, with the magnitude of change in storminess increasing at more northern latitudes (Hayden and Hayden 2003). At the PIE LTER changes in century-long storm frequency are pronounced and precipitation and evapotranspiration have increased significantly (Claessens et al., submitted). In the future, we can expect to see more frequent, larger storms and higher storm frequency variability as well. There have been decadal and century-long changes in sea level at PIE as well. Therefore, the PIE LTER study can be considered a microcosm for investigating the effects of these globally important changes and it should thus be possible to transfer lessons learned from the PIE LTER to other coastal regions.
There are numerous ways that these globally important issues are affecting the biosphere. In the PIE LTER we focus on how these issues influence organic matter and inorganic nutrient biogeochemistry and estuarine foodwebs. The inputs of organic matter and nutrients from land, ocean and marshes interact with the external drivers (climate, land use, river discharge, sea level) to dictate the extent and degree of nutrient and organic matter processing and determine the spatial patterns of estuarine productivity and trophic structure. Our overarching question is:
How will trophic structure and primary and secondary productivity in estuaries be affected by changes in organic matter and nutrient loading and hydrodynamics caused by changing land use, climate and sea level?
Within the context of this overarching question, our program addresses two hypotheses about the effects of driver variability and long-term change on ecosystem dynamics and the effects of inorganic vs. organic matter inputs on estuarine foodweb structure.
Hypothesis 1. The variability in land, ocean and atmospheric forcing is a major factor controlling the fate of allochthonous and autochthonous materials and the location and magnitude of primary and secondary production.
All components of an estuarine ecosystem exhibit variation in space and time in response to external drivers (e.g., climate, river discharge and sea level) or in response to internal, biogeochemical and biological processes (e.g., nutrient cycling, population cycles and fish migrations). By comparing time-scale and intensity of processes, the relative importance of various physical or biological factors in controlling variation in process rates can be assessed and limits placed on the ability of one process to affect another (Hatcher et al. 1987). Processes that occur at similar scales are those likely to interact. Our research on the effect of changes in organic matter, nutrient and water fluxes on estuarine trophic structure and production requires that we evaluate variability in land and ocean drivers and assess the spatial and temporal scales over which the effects of these changes are likely to operate. Understanding the response of these systems to long-term changes in climate, sea level and land use first requires that we understand and can distinguish short-term variation from long-term trends.
Hypothesis 2. The interaction of inorganic nutrients with the quantity and quality of organic carbon and organic nitrogen plays an important role in determining the trophic structure, production and efficiency of estuarine food webs.
The delivery of organic matter and nutrients has a major effect on estuarine food webs. We know from metabolic studies that estuaries as a class are heterotrophic, consuming more organic matter than produced (Hopkinson and Smith in press) and thus dependent on allochthonous sources. This is not surprising as typically 1-2% of terrestrial NPP is exported to estuaries (Meybeck 1982). Most estuarine respiration is by bacteria (Hopkinson et al. 1989, Smith and Kemp 2003) due to the high C/N ratio and low lability of riverine organic matter (OM) (Hopkinson et al. 1998). Elevated inputs of inorganic N from watersheds have perhaps the greatest impact on estuarine condition (Bricker et al. 1999). N-enrichment (eutrophication) causes shifts in algal abundance, increases in algal productivity and standing stocks, and in extreme cases, hypoxia, anoxia and fish kills (Howarth et al. 2000a, b, Cloern 2001, Driscoll et al. 2003a, b). As watersheds become developed, organic matter export declines and inorganic nutrient export increases (Meybeck 1982, Raymond and Hopkinson 2003, Pellerin et al. submitted, Wollheim et al. submitted, Williams et al. submitted a,b,c). Major factors controlling the autotrophic-heterotrophic balance of estuaries include the balance between inputs of inorganic and organic nutrients (Kemp et al. 1997, Hopkinson and Smith, in press), water residence time and the lability of allochthonous organic matter (Hopkinson and Vallino 1995). With increased N enrichment, decreased organic matter inputs and perhaps decreased freshwater inputs to estuaries in the future, we can expect changes in foodweb structure, shifting from organic matter, microbially-based to inorganic nutrient, phytoplankton-based (Deegan et al. 1994).
CHANGES IN OUR PERCEPTIONS OF THE CONTROLS OF FOODWEB STRUCTURE
Early on we developed a trophic flow model and diagram to synthesize our understanding of the effects of organic matter and nutrient inputs on trophic structure and function (Deegan et al. 1994). We envisioned 4 basic pathways of organic matter and nutrient processing that varied over decadal time scales in relation to long-term changes in the loading of organic matter and nutrients from watersheds:
1) the classical grazing food chain: inorganic nutrients - phytoplankton > 20 µm - macrozooplankton - planktivores - piscivores, 2) the microbial loop: organic matter - bacteria - microflagellates - microzooplankton - macrozooplankton - planktivores - piscivores, 3) a hybrid of the grazing food chain and the microbial loop: inorganic nutrients - phytoplankton This conceptualization, while perhaps a good representation of long-term equilibrium conditions, over emphasized the importance of organic matter and nutrient inputs from the watershed and does not reflect the dynamic variability of estuaries. Because riverine organic N inputs are greater than inorganic N inputs (Williams et al. submitted a, b), our flow analysis model suggested a hybrid grazing -microbial loop pathway dominating in the upper estuary with keen competition for nutrients between phytoplankton and bacteria. We expected to find evidence of terrestrial organic C fueling higher trophic levels. We hypothesized that the benthic food chain would take on importance down estuary, where piscivore production would be supported from low quality organic matter inputs from the marsh. Our temporal focus was primarily decadal, the time scale over which we expected to see a shift toward the classical food chain as urbanization increased DIN loading relative to organic N.
Our perceptions of how estuarine ecosystems are structured and function have changed substantially over the past 6 years. We find that the effect of watershed OM or DIN inputs is greatly modulated by river freshwater discharge (Deegan and Garritt 1997, Holmes et al. 2000). We find that the most important input of organic matter is the marsh (Raymond and Hopkinson 2003) and that there are major recycling sources of DIN internal to the system that are controlled by salinity variations (Giblin et al. in prep). We have yet to observe the classical pelagic grazing food chain, rather phytoplankton blooms appear linked to higher trophic levels equally through both pelagic and benthic webs (Hughes et al. 2000). Benthic microalgal production is often an important base of the foodweb (Tobias et al. 2003a, b).
Freshwater discharge is of paramount importance in controlling foodweb structure because of its influence on residence time of water throughout the estuary (Vallino and Hopkinson 1998). Interestingly, only Parker River flow is an important factor. Although Ipswich River flow is 10x the Parker, its input at the mouth of the estuary has no discernible effect on residence time or salinity within Plum Island Sound (Vallino and Hopkinson 1998). Residence time of water in the upper estuary decreases from 34 – 2 d as river flow increases from 0.01 to 10 m3 s-1 (Vallino and Hopkinson 1998). In the lower estuary tidal mixing has the greatest effect on residence time; seasonal increases in river flow only decrease residence time from 1.2 - 0.6 d. The relative importance of physical or biological factors depends on the time scales of physical and biological processes. The influence of organic matter and nutrient inputs on the estuarine food web is dictated by the time scales of important processes relative to water residence time. Thus, by controlling estuarine residence time, the interplay of freshwater discharge and tidal regime defines the template upon which ecological processes can operate. For example, algal blooms can only occur when water residence time is substantially longer than bloom doubling time. Hence algal blooms only occur during very low flow during summer (Holmes et al. 2000).
We also find that there are “spheres of influence” that vary over the length of the estuary and over seasonal or shorter time scales, where foodwebs develop in response to physical drivers plus DIN and OM inputs. We have modified the trophic flow diagram to include benthic microalgae and a microalgal food chain, and to include a direct link between phytoplankton blooms and benthic macrofauna. Figure 2.7 illustrates the interplay between external drivers as they define a template upon which the effects of DIN and OM inputs on foodweb structure operate. The figure shows examples of 4 foodweb structures that develop under different hydrologic templates and loading conditions. The spheres of influence identify where these foodweb structures are likely to be found. In no sense do we mean to imply that foodwebs are static communities controlled exclusively from the bottom-up. Foodwebs are dynamic and feedbacks between organisms and environment are to be expected.
One result of our altered perceptions is that the role of watershed DIN and OM inputs must be assessed within the context of the hydrologic template. The vast majority of inputs occur during late winter / early spring, when biological activity is at its lowest. We need to determine the extent and the mechanisms by which any of these inputs are retained for use later in the year. Marshes are net sinks for C and N; what enables them to export as much C and N as it appears they do and still remain net sinks? We hope to resolve some of these uncertainties as our research program continues.
PROPOSED RESEARCH QUESTIONS – To address our overarching research question and hypotheses, we organize our research around 5 interrelated questions that define Programmatic Areas. The scope of each question and the manner in which each fits in the overall program is illustrated with numbered boxes around portions of our conceptual diagram.
Q1. What is the magnitude and long-term pattern of freshwater runoff and organic carbon and nitrogen and inorganic N loading from watersheds to the estuary?
Q2. How are tidal marsh processes and their connections to estuarine waters controlled by changes in land, atmospheric and oceanic forcing?
Q3. How do planktonic community structure and production respond to short and long-term changes in watershed runoff and the inputs of organic matter and nutrients?
Q4. How do benthic recycling of nutrients and processing of organic matter respond to changes in freshwater runoff and the quality and quantity of organic matter inputs?
Q5. How do the structure and function of higher trophic levels respond to changes in land, atmospheric and oceanic forcing as well as fisheries harvest?

The Ecosystems Center
Marine Biological Laboratory
Woods Hole
MA
02543
USA
508-289-7488
508-457-1548
Estuary
elevation comment: 
Data Source: Collins/Waide. class data. 2008. not published yet.
latitude comment: 
Data Source: LTER Site Characteristics Database. http://www.lternet.edu/sites/pie
Longitude_comment: 
Data Source: LTER Site Characteristics Database. http://www.lternet.edu/sites/pie
ecosystem comment: 
Data Source: GreenLand, D., G. G. Goodin., R. C., Smith. 2003. An Introduction to Climate Variability and Ecosystem Response. p8. In Climate Variability and Ecosystem Response at Long-Term Ecological Research Sites. Oxford University Press

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