Sanctuary Integrated Monitoring Network
Monitoring Project

Comparison of discharge plumes from Elkhorn Slough and the Moss Landing Power Plant

Principal Investigator(s)

  • Andrew Fischer
    Monterey Bay Aquarium Research Institute


  • SIMoN
  • SIMoN
Start Date: June 15, 2006
End Date: December 01, 2006

There is concern that thermal discharge from coastal power stations impact coastal ocean ecosystems. The introduction of heated water from these sources, for example, can influence the aquatic environment by decreasing oxygen solubility and affecting metabolic activity of marine organisms. Here we describe and compare the general flow structure, dynamics and temperature differences between a thermal discharge from an anthropogenic point source (Moss Landing Power Plant) and the natural heat flux between two natural bodies of water, an estuary (Elkhorn Slough) and the open ocean. The data used in this analysis were collected on different occasions for two indivdual and separate studies. Data colletion of temperature, as well as other physical, chemical and biological parameters in both studies involved a variety of in situ and remote sensing techniques, from stationary temperature loggers on buoys, underway mapping systems, an autonomous underwater vehicle (AUV) and remotely sensed data collected by visible and infrared airborne sensors.

Summary to Date

The results show that tidal inertia produces a surface advected plume exiting the Elkhorn Slough through the Moss Landing Harbor channel that extends approximately 1 km in a southwesterly direction. The plume is approximately 500 m wide and extends 5-10 m in depth.

By contrast, the Moss Landing Power Plant outfall extends vertically through the water column directly over the discharge site, and at the surface a plume disperses shoreward and south of the discharge site.

Temperature measurements from each of the plumes show high levels of variability due to tidal mixing. The difference in average daily temperature between the Elkhorn Slough plume and nearshore waters was usually ~1C , but overall temperatures differences ranged from 1.3C cooler to 2.3C warmer. Temperature variability measured at the Moss Landing Power plant outfall site ranged from 3.7C cooler to 6.4C warmer than ambient ocean conditions and the overall average temperature during the sampling period was ~0.5C warmer than the surrounding nearshore waters. Temperatures cooler than surounding bay waters at the outfall discharge site occurred only 15% of the time during the sampling period, compared to 20% of the time for the Elkhorn Slough plume.

Overall, preliminary results show that these two plumes may introduce similar thermal loading to coastal nearshore waters. Future research directions are recommended to further understand the interaction of these plumes with each other, as well as their contribution to the coastal waters of Monterey Bay and the Monterey Bay National Marine Sanctuary.

Study Parameters

  • Temperature
  • Salinity
  • Turbidity
  • Conductivity
  • Nitrates
  • Transmission, Fluorescence, CDOM Fluores

Study Methods

Previous datasets were collected to examine both the spatial extent and physical characteristics of both the MLPP outfall plume and the Elkhorn Slough plume. A study to evaluate the MLPP outfall plume was conducted by Tenera Environmental in 2002 and compiled by Jeffery Paduan in 2003 (see Paduan et al., 2003). Both studies involved three seperate measurement approaches:

1) the installation of continually recording temperature sensors at a number of locations around the discharge and intake structures of the power plant, and along the beach for a period of several months (Figure 2);
2) a series of spatial surveys conducted from an instrumented boat during several phases of the tide; and
3) aerial infrared (IR) overflights conducted in conjunction with boat surveys. Refer to Paduan (2003) for further details on the sampling methods.

The discharge plume of the Elkhorn Slough was sampled by the Monterey Bay Aquarium Research Institute (MBARI) on March 17 and December 9 and 10, 2004 and on January 6, 10, 20 and 21, 2005 during the ebb stage of the tidal cycle. The surface expression of the plume was measured by a near surface underway (UW) mapping system deployed from a Boston Whaler. The surface UW mapping system is a neatly packaged set of instruments measuring water clarity (through transmission), temperature and salinity, chlorophyll fluorescence, color dissolved organic matter fluorescence, and nitrate concentration. The individual instruments used to measure these parameters were a WetLabs Cstar, SeaBird 45, WetLabs WetStar, WetLabs CDOM fluorometer and an In Situ Ultraviolet Spectrophotometer (ISUS)(Johnson and Coletti, 2002), respectively. Water samples were pumped through the instruments from one foot below the surface. The speed of the boat was regulated so as to avoid bubbling in the instruments. The ES plume was sampled between four to six times on each outgoing tide. The boat followed a serpentine path to capture the full extent of the ES plume and each sampling pass averaged 75.64 minutes (Figure 2). MBARI's autonomous underwater vehicle (AUV) was deployed from the RV Zephyr in conjuction with the underway mapping. The AUV followed a triangluar path up and down through the water column to capture vertical profiles of plume and offshore waters (Figure 2) . This pattern was repeated though the ebb stage of the tide between 4 to 7 on each sampling day. Instruments in the AUV measured physical, chemical and biological constituents of the water column including temperature, salinity, fluorescence, particle backscatter and nitrate concentrations. The average duration of each triangular sampling track was 58.4 minutes.

Drifters were deployed to determine plume surface flow patterns. The drifters, consisting of a PVC pipe with foam flotation, were attached to a drogue to ensure that the movement of the drifter coincided with the flow of the water and not that of the wind. The PVC pipe housed a Garmin GPS receiver which, in addition to logging its geographic position every 30 seconds, recorded local time, distance between logged points, speed and magnetic direction. Water samples were analyzed for chlorophyll and pigments. On March 17, 2004, the vertical structure of the plume was also measured with CTD (conductivity, temperature, and depth) profiles. Hyperspectral remotely sensed imagery was acquired on October 13, 2000 and October 7, 2002 by the National Aeronautics and Space Administration's (NASA) Airbrone Visible/Infrared Imaging Spectrometer (AVIRIS). Hourly temperature data for time series stations M1 (oceanic) and M0 (nearshore) were downloaded from the MBARI LiveAccess server ( M1 and M0 were used as reference stations to compare ES water temperatures. Similarly, hourly temperature data from time series stations L01 and L02 were downloaded from the Land/Ocean Biogeochemical Observatory (LOBO) Network Data Visualization ( website. Surface temperature data from L01 and L02 are used to help understand the temperature distribution within ES and to verify the validity of surface underway measurements in the area of the Elkhorn Slough plume.

The datasets examined here were collected independently for seperate studies. The MLPP outfall discharge data were collected between June and October, while ES plume data were collectd in winter months of December and Janaury. These time preiods represent distinct climatic and oceanographic periods in Monterey Bay (Breaker and Broenkow, 1994). Therefore, the general flow, structure, dynamics and temperature dispersion of both the ES plume and the MLPP outfall discharge may be under the influence of different seaonsal climatic and oceanographic conditions during their independent sampling periods.

Figures and Images

Figure 1. Map of the Moss Landing Power Plant (MLPP) and sites sampled as part of the physical characterization of the MLPP plume.

Figure 2. Chlorophyll, sediments, and CDOM images of the Elkhorn Slough plume exiting the harbor entrance.

Figure 3. Boat track lines through the MLPP outfall plume and the harbor entrance. Color indicates surface temperature (F).


  • October 2006 Report
    "A comparison of discharge plumes from Elkhorn Slough and the Moss Landing Power Plant ". A report submitted to the Monterey Bay National Marine Sanctuary Sanctuary Integrated Monitoring Network (SIMoN) and Monterey Bay Sanctuary Foundation October 6, 2006.