Chapter 2. Oceanographic Conditions
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Chapter 2. Oceanographic Conditions relative to other input sources, (3) determine the extent INTRODUCTION to which water mass movement or mixing affects the The City of San Diego monitors oceanographic dispersion/dilution potential for discharged materials, conditions in the region surrounding the Point Loma and (4) demonstrate the infl uence of natural events Ocean Outfall (PLOO) to assist in evaluating possible such as storms or El Niño/La Niña oscillations. impacts of wastewater discharge on the marine environment. Treated wastewater is discharged to the The evaluation and interpretation of bacterial Pacifi c Ocean via the PLOO at depths of ~94–98 m distribution patterns and remote sensing and at a distance of approximately 7.2 km west of observations (e.g., aerial and satellite imagery) may the Point Loma peninsula. The fate of wastewater also provide useful information on the horizontal discharged into offshore waters is determined by transport of wastewater plumes (Pickard and oceanographic conditions that impact water mass Emery 1990; Svejkovsky 2009; also see Chapter 3 movement, including horizontal and vertical mixing of this report). Thus, the City of San Diego combines of the water column and current patterns. These same measurements of physical oceanographic parameters factors can also affect the distribution of turbidity with assessments of fecal indicator bacteria (FIB) (or contaminant) plumes that originate from various concentrations and remote sensing data to provide point and non-point sources. In the Point Loma further insight into the transport potential in coastal region these include tidal exchange from San Diego waters surrounding the PLOO discharge site. Bay and Mission Bay, outfl ows from the San Diego River, the Tijuana River and northern San Diego This chapter describes the oceanographic conditions County lagoons and estuaries, storm drains or other that occurred in the Point Loma region during 2008. water discharges, and surface water runoff from local The results reported herein are also referred to in watersheds. For example, fl ows from San Diego subsequent chapters to explain patterns of FIB Bay and the Tijuana River are fed by 1075 km2 distributions (see Chapter 3) or other changes in the and 4483 km2 of watershed, respectively, and can local marine environment (see Chapters 4–7). contribute signifi cantly to nearshore turbidity, sediment deposition, and bacterial contamination (see Largier et al. 2004, Terrill et al. 2009). Overall, MATERIALS AND METHODS these different sources can affect water quality conditions either individually or synergistically. Field Sampling Because of the above, evaluations of oceanographic Oceanographic measurements were collected at fixed parameters such as water temperature, salinity, and sampling sites located in a grid pattern surrounding density that determine the mixing potential of the the PLOO (Figure 2.1). Thirty-six offshore stations water column are important components of ocean (designated F01–F36) were sampled quarterly in monitoring programs (Bowden 1975). Analysis of January, April, July, and October, usually over a 3-day the spatial and temporal variability of these and other period. Three of these stations (F01–F03) are located parameters (e.g., light transmittance or transmissivity, along the 18-m depth contour, while 11 sites are dissolved oxygen, pH, and chlorophyll) may located along each of the following depth contours: also elucidate patterns of water mass movement. 60-m contour (stations F04–F14); 80-m contour Monitoring patterns of change in these parameters for (stations F15–F25); 98-m contour (stations F26–F36). the receiving waters surrounding the PLOO can help: Eight additional stations located in the Point Loma (1) describe deviations from expected oceanographic kelp bed are subject to the 2001 California Ocean Plan patterns, (2) assess the impact of the wastewater plume (COP) water contact standards (SWRCB 2001). These 11 PLOOOceanCond2008.indd 11 6/29/2009 2:47:04 PM La Jo lla Remote Sensing – Aerial and Satellite Imagery F14 F03 F25 ! ! F36 ! ! Coastal monitoring of the PLOO region during 2008 F13 F24 ! ! o River also included aerial and satellite image analysis F35 F02 San D ieg ! ! F12 San Diego performed by Ocean Imaging of Solana Beach, CA F23 ! F34 ! ! C8 a (see Svejkovsky 2009). All usable images for the ! F11 F22 ! F33 ! monitoring area captured during the year by the ! C7 o Ba ! nt Lom eg y Di n F10 a Moderate Resolution Imaging Spectroradiometer F21 ! Poi S F32 ! ! A6 C6 Co r o na do (MODIS) satellite were downloaded, and 22 high ! ! F31 F20 F09 ! ! ! C5 clarity Landsat Thematic Mapper (TM) images and A7 ! F08 ! ! F30 F19 C4 two Aster images were acquired. High resolution ! ! Outfall ! Point Loma A1 1 ! 0 m aerial images were collected using Ocean Imaging’s F18 F07 F29 ! ! ! DMSC-MKII digital multispectral sensor. The F01 ! F17 F06 F28 ! DMSC’s four channels were confi gured to a ! ! specifi c wavelength (color) combination designed LA5 F27 F16 F05 ! ! ! to maximize detection of the wastewater discharge F26 F15 F04 ! ! ! signature by differentiating between the waste field 150 4 m LA4 and coastal turbidity plumes. Depth of penetration km 100 80 20 m 1 60 m 0 1 2 3 4 5 50 m m m for this sensor varies between 8–15 m depending Figure 2.1 on water clarity. The spatial resolution of the data Water quality monitoring stations where CTD casts are is dependent upon aircraft altitude, but is typically taken, Point Loma Ocean Outfall Monitoring Program. maintained at 2 m. Fifteen DMSC overflights were conducted in 2008, which consisted of one to five stations include three sites (stations C4, C5, C6) located fl ights per month during winter when the surfacing along the inshore edge of the kelp bed paralleling the potential was greatest for the wastewater plume 9-m depth contour, and fi ve sites (stations A1, A6, A7, (see below) and when rainfall was also greatest. In C7, C8) located along the 18-m depth contour near contrast, only three surveys were fl own during the the offshore edge of the kelp bed. To meet 2001 COP spring and late summer months. sampling frequency requirements for kelp forest areas, sampling at the eight kelp bed stations was conducted Data Treatment fi ve times per month. The water column parameters measured in 2008 Data for the various oceanographic parameters were were summarized by quarter in two different ways: collected using a SeaBird conductivity, temperature, and (1) means calculated over the entire water column depth (CTD) instrument. The CTD was lowered through for each station, and (2) means calculated over all the water column at each station to collect continuous stations located along each depth contour (i.e., 9-m, measurements of water temperature, salinity, density, 18-m, 60-m, 80-m, 98-m). In order to get a view of pH, water clarity (transmissivity), chlorophyll a, and the entire PLOO region for each quarterly survey, dissolved oxygen (DO). Profi les of each parameter these analyses included data from all 36 of the were then constructed for each station by averaging the offshore stations, as well as the data from the eight data values recorded over 1-m depth intervals. This data kelp bed stations that were sampled at approximately reduction ensured that physical measurements used the same time (i.e., ± one day). Each water column in subsequent analyses could correspond to discrete parameter was also summarized over all kelp bed sampling depths for indicator bacteria (see Chapter 3). stations each month for surface (≤ 2 m) and bottom Visual observations of weather and water conditions depths (10–20 m); this was done to identify seasonal were recorded just prior to each CTD cast. trends not necessarily evident in the quarterly data. 12 PLOOOceanCond2008.indd 12 6/29/2009 2:47:10 PM Finally, the spatial distributions of temperature and 5 A 2008 mean salinity values at each offshore station were mapped 4 Historical mean for each quarterly survey, with the data limited to 3 SD the discrete depths at which seawater samples are 2 collected for bacterial analysis. 1 Rainfall (in.) 0 In addition to the above, mean temperature, salinity, -1 DO, pH, and transmissivity data from 2008 were 75 compared with historical profi le plots consisting B 70 of means for 1991–2007 ± one standard deviation. Data for these historical analyses were summarized 65 at 5-m depth increments and limited to the three 60 stations located nearest the outfall discharge site 55 along the 98-m depth contour. These included station Temperature (°F) 50 l F30 located immediately offshore of the center of n y n u c eb a J the outfall wye, station F29 located 1.25 km south Ja F Mar Apr M Ju Aug Sep Oct Nov De of the southern diffuser leg, and station F31 located Month ~1.42 km north of the northern diffuser leg. Figure 2.2 Comparison of rainfall (A) and air temperatures (B) at Lindbergh Field (San Diego, CA) for 2008 compared to historical levels. For 2008, rainfall data are expressed as ESULTS AND ISCUSSION R D total inches per month, whereas temperature data are monthly averages. Historical rainfall and temperature data Climate Factors and Seasonality are expressed as monthly means ± one standard deviation (SD) for the historical period 1914 through 2007. Southern California weather can generally be classifi ed into wet (winter) and dry (spring–fall) begin to warm surface waters resulting in increased seasons (NOAA/NWS 2009a), and differences surface evaporation (Jackson 1986). Mixing conditions between these seasons affect certain oceanographic diminish with decreasing storm activity, and seasonal conditions (e.g., water column stratifi cation, current thermoclines and pycnoclines become re-established. patterns and direction). Understanding patterns of Once the water column becomes stratifi ed again by late change in such conditions is important in that they spring, minimal mixing conditions typically remain can affect the transport and distribution of wastewater, throughout the summer and early fall months.