|
With eleven major watersheds draining to the Monterey Bay National Marine Sanctuary, a dynamic interaction exists between the land and the sea. As water flows through our rivers and creeks, it moves through different types of land uses, picking up pollutants and carrying them downstream. Excessive sedimentation and erosion in these coastal watersheds can affect riparian, wetland, and nearshore ecosystems.
A good example of the potential impacts is what happens to anadramous fishes such as coho salmon (endangered) and
steelhead trout (threatened), which require both freshwater and marine environments for their unique life cycle. Each year salmonids leave the ocean and enter freshwater systems to spawn. They lay their eggs in gravelly river beds, and if excessive
sedimentation has occurred, spawning areas may become buried
in fine sediments.
The Monterey Bay Sanctuary Foundation managed two watershed assessments, funded by the State Water Resources Control Board, that were completed this year. Both the Pescadero-Butano and the Upper Pajaro watersheds are identified on California’s List of Impaired Water Bodies. As part of the sanctuary’s voluntary Agriculture and Rural Lands Plan, the assessments were carried out with the assistance of the local Farm Bureaus and farmers. These completed studies provide local communities and resource agencies with important data for future watershed planning and restoration activities. The reports are available online at
www.mbnms.nos.noaa.gov/resourcepro/reportssedrep/ welcome.html
Pescadero-Butano Watershed Assessment
The Pescadero-Butano watershed is the largest coastal watershed between the Golden Gate and the San Lorenzo River. Its two
principal streams, Pescadero Creek and Butano Creek, join at Pescadero Marsh and drain about 210 square kilometers (eighty-one square miles) of watershed. The Pescadero-Butano watershed assessment, undertaken via a contract to Environmental Science Associates, characterized sediment sources and dynamics, assessed current habitat conditions for coho salmon and steelhead trout, and identified factors limiting the quality and extent of salmonid habitat.
 |
| Steelhead require both freshwater and marine environments for their unique life cycle. photo NOAA/MBNMS |
To identify relative sediment yields within this large and diverse watershed, the area was divided into distinct units defined by geology, slope, and vegetation cover. Of these units, forty field plots of forty acres each were systematically surveyed for erosional features that delivered sediment to a stream channel. In addition, twenty-three stream reaches were surveyed to determine priority stream basins for salmonid habitat. These surveys ranked physical habitat quality, biotic conditions, pool habitat conditions, and water quality conditions pertaining to salmonids. Areas recommended for protection and/or restoration were identified and ranked as high- or medium-quality salmonid habitat. The main impediments to salmonids were identified as excessive fine sediments, lack of cover, and shallow pools.
A historical study revealed that much of the damage in the watershed was done in the mid-twentieth century by clear cutting and tractor logging, abandoned agricultural fields that began forming gullies, and road construction practices and road placement. Major flooding events recorded from the 1930s to the present have flushed large quantities of sediment downstream, where it has accumulated in the lower watershed areas.
Current contributions to the sediment load include -- in order of magnitude -- roads (active unpaved, forest, and ranch land), timber harvesting, and agriculture. The assessment was done in conjunction with agricultural demonstration projects to minimize sediment erosion that were developed by the San Mateo County Farm Bureau. These projects included cover cropping, using different seed mixes to prevent erosion, and installation of three different types of livestock exclusionary fencing to protect riparian areas.
In addition to highlighting the value of these projects, useful cost data comparisons were compiled for growers to use in the future.
The Coastal Watershed Council also conducted water quality monitoring at five sites with a team of volunteers. During the short sampling season, water quality was found to be relatively good.
The assessment recommends conducting more detailed stream assessments, implementing sediment control practices in the basins identified as high priority for salmonids, and continued water
quality monitoring.
Upper Pajaro River Sediment Assessment
The Pajaro River watershed is one of the largest riverine systems entering Monterey Bay and drains an area of approximately 3,370 square kilometers (1,300 square miles) of watershed. The upper Pajaro River sediment assessment was developed for 523 square kilometers (202 square miles) in southern Santa Clara County. The assessment characterizes erosion and sedimentation dynamics in Llagas and Uvas Creeks, including an evaluation of sediment
conditions, erosion risk, and potential impacts to the watershed.
The assessment, performed via a contract to Fall Creek Engineering, was conducted in preparation for a sediment total maximum daily load (TMDL) study. The TMDL process attempts to
identify the maximum acceptable load of a pollutant for a given body of water. This pre-TMDL assessment documents current land uses
in the area and identifies sediment sources for hillside development, road ditches, agriculture, and equestrian lots; it determined which
of these sources could be controlled using management practices.
The watershed was divided into four geographic sections based
on landscape position, sub-watersheds, and current land uses. Within these sections, land-use types were categorized as rangeland, urban and residential, agricultural, and equestrian. The major reasons for impairment varied by section but collectively included vegetation removal, channel hydromodifications, concentrated hillside development, road ditches, agriculture, equestrian activities, and land use encroachment to riparian areas. The assessment was done in conjunction with landowner outreach by the Santa Clara County Farm Bureau.
The assessment recommends integrating ecological restoration activities, such as re-establishing native vegetation and buffers into the sediment TMDL. Additional water quality monitoring by local community organizations and agencies will also be necessary.
Both of these assessments show the critical need for continued work by landowners and managers to prevent erosion and sedimentation of local waters. The sanctuary’s partners in the Agriculture
and Rural Lands Plan will continue to work in these watersheds to protect water quality.
Katie Siegler
Monterey Bay National Marine Sanctuary
he physical and biological exchange between Elkhorn Slough and the Monterey Bay National Marine Sanctuary is largely unknown, yet important. The slough serves as a significant year-round link between land use activities and the coastal waters of Monterey Bay. Agricultural runoff, concentrated in the slough during periods of precipitation, can be carried by tidal exchange into the sanctuary. Tidal scouring of the slough’s banks and bed can resuspend pollutants that have accumulated in sediments over the past several decades. Increased fluxes of nutrients and pollutants into the bay can influence water quality, increase the prevalence
of harmful algal blooms and marine diseases, and have consequences for nearshore ecology.
 |
| Figure 1. An enhanced hyperspectral image, acquired by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS), reveals a discharge plume
exiting the mouth of Elkhorn Slough. |
This exchange is of particular concern because the slough’s
physical configuration has been drastically altered by human
intervention. In 1946 the Army Corps of Engineers changed the morphology of Elkhorn Slough by cutting through the dune barrier separating the slough from Monterey Bay. Since then the slough has been transformed from a sluggish backwater to a shallow, tidally forced embayment. Maximum tidal currents in the main channel have increased from approximately 1.5 knots in 1971 to 3.0 knots today, and the tidal prism -- the volume of water exchanged between the slough and the bay over a tidal cycle -- has increased significantly during the past decade. While the incoming tide introduces relatively clear water from Monterey Bay into the slough, waters discharged into Monterey Bay during the ebb tide are
laden with sediments. As the tidal currents and prism continue to increase, and the surrounding watershed is subject to growing
population pressures and land use change, there is an urgent need to understand the physical extent and the biological impacts of
the discharge plume that enters the bay from the slough.
Recent observations from high-resolution aircraft remote sensing and ship-based measurements are helping characterize the plume’s physical and biological characteristics. Recent overflights by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) have provided imagery at far greater resolution than that provided by instruments currently being flown at satellite altitudes. This imagery reveals a sediment-laden discharge plume exiting the slough at
ebb tide. (See Figure 1.) The plume extends south along the coast and eventually becomes entrained in the longshore currents. The extent of the plume varies in accordance with the tides, and previous observations have shown patches of plume water entrained in the bay’s circulation extending far offshore.
The plume’s physical characteristics, obtained through ship-based measurements, reveal a wedge of warmer, less saline water exiting the slough at maximum ebb tide, producing a sharp salinity gradient (halocline) where slough waters meet oceanic waters. This results in separation of water masses along density gradients, isolating plume waters from oceanic waters below and suppressing vertical mixing. Consequently, conditions favorable for phytoplankton growth and harmful algal blooms develop.
 |
Figure 2. a) Alloxanthin and peridinin pigment concentrations retrieved from stations in Monterey Bay along the plume track. Increasing station numbers represent progressively greater distances from the entrance of Elkhorn Slough. Carotenoid pigments were referenced against chlorophyll a, a pigment common to all algae.
b) Fatty acid concentrations of terrestrial matter, bacteria, and diatoms averaged among six stations within the slough plume (white bars) and compared with a control sample of oceanic water (dark bars) |
Water samples, collected at successive locations with increasing distance seaward of the slough, reveal distinct assemblages of
phytoplankton between plume and bay waters. Analyses of these samples for chlorophyll and carotenoids, naturally occurring plant pigments, show high concentrations of alloxanthin, a carotenoid found in cryptophytes, dominating the inland waters of Elkhorn Slough (Figure 2a). Cryptophytes are a group of phytoplankton quantitatively dominant in turbid waters. Further offshore, alloxanthin concentrations diminish and concentrations of peridinin, a
pigment indicative of the phytoplankton group dinoflagellates, increase. It is possible that a plume of inland slough water, on spring tides, can introduce rich concentrations of cryptophytes
into the bay, significantly impacting food web cycling and biogeochemical transformation rates.
The water samples were examined further using fatty acid biomarker analysis. Examination of specific fatty acids and different lipid classes will provide a better understanding of sources of input to plume waters. The results show that plume water carries indicators of terrestrial material, bacteria, and diatoms. Fatty acid concentrations of each of these markers are more abundant than those sampled from a control sample taken from nearby oceanic waters (Figure 2b). The presence of terrestrial biomarkers indicates the transport of "foreign," or land-based, materials into Monterey Bay.
Recent measurements, although preliminary, have revealed components of the physical and biological exchange between Elkhorn Slough and the sanctuary. These measurements show a stratified plume of slough water with significantly different phytoplankton diversity and abundance entering the sanctuary. Plume waters are dominated by terrestrial biomarkers, indicating the flux of land-based materials.
Further research is needed to combine on-location measurements with airborne imagery to understand the fate and consequences
of these fluxes better. Due to the dynamic nature of the plume, innovative sampling techniques are needed to capture the process during the evolution of the tide. Repeat coverage of airborne imagery coupled with concurrent on-location measurements will help further examine the ultimate fate of plume waters and what role the plume constituents play in influencing the ecology and water quality of the sanctuary.
Andrew Fischer1, Erich Reinecker1, John Ryan1, Nick Welchmeyer2, and Larry Breaker2
1Monterey Bay Aquarium Research Institute
2Moss Landing Marine Laboratories
|
