PIE-LTER
Summary of Research Results, 2003
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populations outside watershed boundaries have withdrawn water from the
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 <0.001 up to 100 m3 s-1. Average discharge is
about 67 times lower in volume than a single tidal prism (tidal cycle Δt). It
is the balance between discharge (advection) and tides
(dispersion) that controls mixing along the length of the estuary (24 km).
Mixing is dominated by dispersion in the lower 10 km of the estuary (Plum
Island Sound proper), while advection becomes increasingly important up estuary
and as discharge increases. Interestingly the
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 (<0.1 m3 s-1) residence time is about 2 weeks in the
upper estuary and less than a day in the lower estuary. With increasing
discharge residence time decreases throughout the estuary and the region of
longest residence time shifts downstream.
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
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
