Sanctuary Integrated Monitoring Network
Monitoring Project

Volunteer Water Quality in Elkhorn Slough

Principal Investigator(s)

  • John Haskins
    Elkhorn Slough National Estuarine Research Reserve
  • Sue Shaw
    Moss Landing Marine Laboratories, California State University
Start Date: September 15, 1988

Elkhorn Slough National Estuarine Research Reserve , the Elkhorn Slough Foundation, and the Monterey County Resources Agency have been supporting a volunteer water monitoring program since 1988. Analysis revealed striking differences between sites and seasons but few significant long term changes over time. Salinity measurements reveal that one-way tide gates separating salt and freshwater habitats leak, resulting in strong marine influences in habitats managed for freshwater.Overall nitrate shows a trend of increased concentrations during the summer dry months with increased irrigation on fields.

Summary to Date

The seasonal pattern and magnitude of nutrient concentrations are fairly consistent
throughout most of Elkhorn Slough, Bennett Slough and Moro Cojo. The highest
nitrate concentrations occur during the rainy season, particularly between January and March.
Lowest concentrations normally occur during the summer and early fall. In Carneros Creek that
discharges into Elkhorn Slough, nitrate concentrations can exceed 14 mg/l during the winter rainy
season, although average concentrations are usually about 3 mg/l. At 11 of the 24
stations, nitrate concentrations are less than 1 mg/l for at least 50% of the sampling period. Nitrate concentrations are lowest at the three sites on the National Estuarine Research
Reserve and have a small increase in concentration during the winter rainy season.
Concentrations at these three sites never exceed 14 mg/l. Ammonium and dissolved
inorganic phosphate concentrations throughout the Slough are much lower and average 0.01 and
0.1 mg/l, respectively (data not shown). Despite potential inputs from dairy farms in watershed,
ammonium concentrations do not seem to be a problem.

In contrast, the lower Salinas River, which drains into the lower reaches of the slough, has
excessive nitrate concentrations all year round. Concentrations usually exceed 30 mg/l during the
summer months. The highest concentrations occur in Tembladero Slough and at the
old Salinas River channel at Monterey Dunes Way. In the lower Salinas River, nitrate
concentrations exceed 14 mg/l about 70% of the time and concentrations are rarely, if ever, less
than 1 mg/l.

Dissolved oxygen concentrations are highly variable within Elkhorn Slough as in normal in
estuaries. Although monthly sampling is not the most appropriate time scale to measure oxygen,
these data are useful for targeting areas which may have chronic problems with low oxygen
conditions. Although hypoxia does not seem to be widespread throughout the slough, areas with
restricted circulation have more low oxygen events than well flushed areas. This is
consistent with the results of our SWMP-I monitoring program that measures dissolved oxygen
concentrations at 15 minute intervals at two locations (Upper Pond and South Marsh). The
results of the SWMP-I monitoring program have shown that the Upper Pond becomes hypoxic
or anoxic on a daily basis for several weeks in late summer or early fall, while the South Marsh
does not.

Monitoring Trends

  • Significant increase in nitrate concentrations since the 1970s. The extraordinarily high nitrate concentrations in the lower Salinas River may be the highest recorded in scientific literature for a river or estuary
  • Concentrations of ammonium are an order of magnitude lower (or more) than nitrate.
  • Low concentrations of dissolved oxygen in some areas which have restricted circulation
  • The opening of the Salinas River lagoon leads to rapid increases in salinity in the lower river even during the rainy season when river discharge is high

Study Parameters

  • Turbidity
  • Ortho-phosphate
  • pH
  • Dissolved oxygen
  • Temperature
  • Salinity
  • Nitrates
  • Conductivity

Study Methods

1988 and 1994, salinity and temperature were measured with a YSI model 33 SCT meter. Dissolved oxygen was measured using a YSI model 57 Dissolved oxygen meter. An Orion model 211 pH meter was used to measure pH. Turbidity was measured using a Monitek model 21 PE nephelometer. All instruments were calibrated before sampling according to the manufacturers' recommendations. Starting in 1994, a Solomat water quality meter with temperature, conductivity, dissolved oxygen, pH and turbidity sensors was used to measure these parameters. Sensors were calibrated with appropriate standards and according to manufacturer's recommendations before monthly sampling.

Nutrient analyses (nitrate, ammonium and inorganic phosphate) were performed by the Monterey Bay Aquarium between 1988 and 1991. Samples were centrifuged to remove sediment and then run on a flow injection analyzer. Standard wet chemistry techniques were used: cadmium reduction for nitrate, phenol-hypochlorite for ammonium and ascorbic acid-molybdate for phosphate. The Monterey County Consolidated Chemistry Laboratory began analyzing samples in December 1991. Samples are filtered when received and either analyzed or frozen until analysis. Nitrate and dissolved inorganic phosphate are analyzed using a spectrophotometer using wet chemistry techniques very similar to the Monterey Bay Aquarium (APHA 1989 and EPA 1979). Ammonium is determined using an ion selective electrode (APHA)

Figures and Images

Elkhorn Slough NERR Water Quality Monitoring Stations (March 2004)