Santa Monica Bay Nearshore Zone
A. Kimo Morris
updated: 1/07/06

Graduate work of A. Kimo Morris


The physical dynamics of the very nearshore pelagic environment in the vicinity of surf zones are poorly known. This is due in part to the awkward nature of performing controlled observations near the surf zone, particularly when approached from the water, yet it is reasonable to assume that such dynamics could have a profound influence on the life histories of organisms in intertidal, shallow subtidal, and nearshore infaunal communities. Larval dispersal, retention, and recruitment are known to be tied to annual and lunar cycles (Oliver and Willis, 1987), tidal cycles (Shanks, 1983), and diurnal cycles (Shanks, 1986). Some information exists on how larvae that are primed to settle will take advantage of physical phenomena that would aid in their return to adult habitat (Shanks, 1983; Shanks, 1985; Wing et al., 1995a; Wing et al., 1995b; Shanks et al., 2000; Brubaker and Hooff, 2000). Additionally, since the early 1980’s, studies have addressed the transport of ichthyoplankton in oceanic and nearshore habitats (Cowen, 2002; Norcross and Shaw (1984) for a review on earlier literature). These have included flux and transport across fronts of different scales, from estuarine (e.g. Govoni, 1993) and coastal fronts (e.g. Nakata, 1989) to shelf break fronts (Sakamoto and Tanaka, 1986; Sabatés and Mas, 1990). Yet, wherever the larvae of nearshore and demersal fishes become aggregated, they must eventually find their way back to suitable habitat. To date, few studies have examined the behavior of larval fish returning to nearshore habitats (but see Carleton et al., 2001).

The zone between the shoreline and 2km offshore can be a chaotic environment to be negotiated by dispersing or returning larvae. However certain features may be formed in the nearshore zone that may serve to aid (or hinder) dispersal, retention or settlement of larvae. For example, Shanks (1983) observed the aggregation of crab megalopae in windrows associated with tidally forced internal waves. Another potential mechanism may exist in the creation of mixing boundaries generated by tidal processes in conjunction with surface swells that encounter the gradually sloping beach. Mixing will usually occur near the bottom boundary of the wave once it meets the sea bottom, however the motion of surface waves occurs on relatively short time scales, effectively changing direction too rapidly for mixing to propagate upward. Therefore, the mixing in the nearshore zone is usually facilitated by tidal motion. Thus, from the shoreline out to a distance just beyond where surface waves begin to break, the water will generally be of a uniform density, and the intensity of the mixing experienced in that zone will be greatest during the largest tidal gradients. Outside of this zone (say greater than 1 km offshore), vertical profiles can maintain stratification, particularly during times of intense solar insolation and an absence of upwelling. At a first order approximation, the distance offshore of the resulting threshold (inside of which is well mixed and outside of which is stratified) can be determined by calculating the stratification parameter (Beer, 1983). This parameter is a measure of the ratio of the energy required to promote mixing and the energy dispersed by tides. The equation describing this relationship is

D = log10(h/u3)
Equation 3-1

where the stratification parameter (D) equals the log of the ratio of the depth (h) to the water velocity (u) cubed. Small values of D indicate vertically well-mixed water. This general equation explains mixing best for wide linear coastlines where surface and tidal waves approaches the beach perpendicular to shore, as is the case near Dockweiler Beach, Santa Monica Bay.

This phenomenon can potentially generate a very interesting feature nearshore. The area inside and outside the threshold (for the purposes of this proposal will be termed inshore and offshore, respectively) will have different physical characteristics. For example, because of the mixing described above, the water mass will remain a uniform density, temperature and salinity. Also, this zone will have higher turbidity because of sediment stirring and a greater concentration of dissolved oxygen. Interestingly, tidal mixing in the nearshore can also result in intense phytoplankton blooms (Pingree et al., 1975), which were prevalent throughout southern California, including Santa Monica Bay, during the late summer of 2003. In contrast, the offshore water mass can be stratified, with vertical profile gradients and less turbidity. This can create a steep density gradient at the mixing threshold that could be further enhanced by cabbeling, a phenomenon where two adjacent water masses of differing temperature and/or salinity, but which have identical densities, mix forming a denser parcel of water than either of the two original water masses (Beer, 1983). Cabbeling would likely be a means by which the resulting front could persist, where any water resulting from mixing across the boundary would sink, thus helping to maintain the discontinuity.

Further offshore, where bottom depth does not influence mixing, tidal flow can also result in complete mixing of the water column if there is sufficient difference between high and low tides during spring tides. This especially occurs near the shelf break where advancing and retreating tides result in vertical movement at the shelf break (e.g. Zeidberg and Hamner, 2002; Wolanski, 1994; New and Pingree, 1990; Pingree et al., 1975). However, the zone in which the present study is being conducted lies well inside the shelf break of Santa Monica Bay, and tidal velocities in the study site are therefore of a horizontal nature.

Often nearshore, a slick (or a tightly connected series of slicks) forms just outside of the breaking waves. I hypothesize that this slick is associated with (or perhaps generated by) this mixing threshold. The aim of the present study is two fold. First, I wish to characterize the flow dynamics in a nearshore habitat. In order to address this, I have begun sampling along Dockweiler State Beach in Santa Monica Bay, California. Dockweiler is a sandy beach environment with a gradually sloping beach, where waves generally approach the shoreline from a direction perpendicular to the beach. This beach is also desirable because of its proximity to Marina del Rey Harbor and the UCLA campus. By using traditional and modern oceanographic tools (described below) and mathematical modeling, I will strictly characterize the existence of this front, and based on simple modeling, verified by in situ measurements, I hope to describe how this feature is formed.

My second aim is to address the issue of larval transport in the vicinity of this feature. In particular, I will attempt to quantify the behavior of zooplankton (meroplankton specifically), by determining the difference between the observed distribution of various zooplankton groups in the study area and the distribution that passive particles would exhibit in the same area. This difference in distribution should provide a measure of behavior not previously attained in this manner. Dye studies already performed in the vicinity of the study site (Jones et al., 1991; Jones et al., 1997) should provide enough information on passive transport, though additional distribution data will also be generated by observing where passive bodies are aggregated in the study site (e.g. fish and invertebrate eggs and poor swimming taxa). By choosing a sandy beach with no reef within 10 miles, the number of species that would successfully colonize there would be limited to sandy bottom taxa, and therefore analysis of larval forms would be tractable.

May 2002 Cruise

Drogue deployment in Santa Monica Bay. Click to enlarge.
In May 2002, I sampled the Santa Monica Bay nearshore zone threshold front aboard the RV Seaworld UCLA and a 17’ Boston Whaler. My first task was to roughly characterize the surface flow dynamics of the study site. During the three-day cruise, I deployed drogues in the morning (0800) to provide an Eulerian assessment of current velocity differences between the inshore and offshore water masses. A surface slick approximately 5 m wide, extending parallel to shore out of sight range, with free-floating debris along its length was seen approximately 500 m outside the largest breakers. The zone inside the slick was turbid, while the water outside the slick was blue-green. The location of this slick demarcated the focal point for drogue deployment. One drogue was deployed along the 9 m isobath, approximately 200 m inside the slick. Four more drogues were placed at a distance of 1, 2, 4 and 6 km offshore along a transect perpendicular to the shoreline. All drogues consisted of a 55 gal drum with large flow-through holes suspended from a float down to 5 m depth. The float was then attached to a floating mast with a numbered flag marker for visual tracking. The drogues were tracked through one tidal cycle scoring time, latitude and longitude, and then plotted. Additionally, CTD information was collected simultaneous to the drogue tracking. On the third day, plankton samples were gathered using vertically towed 333 mm plankton nets at five stations along a transect at distances of 0.5, 1, 2, 4 and 6 km offshore. This transect was repeated a second time within one hour of the first transect.

May 2002 Analysis

Figure 3-1. Drogue tracking - courtesy of John Oram. Click to enlarge.
Analysis of the preliminary data was very promising. The drogues that were furthest from shore traveled greater distances and they continued to advect in the same direction after the tidal change, although all drogues had a tendency to drift towards shore in the afternoon (Figure 3-1). The innermost drogue traveled north upcoast during flood tide and switched direction during ebb tide, moving south. With the exception of the onshore movements in the afternoon, the innermost drogue had little net directional longshore movement when averaged through the tidal cycle. Thus the inshore water had little net directional movement over the tidal cycle. In contrast, the offshore water mass beyond the slick had a clear net directional flow to the north, and while these drogues slowed somewhat during ebb tide, they continued to travel northward. This observation may be related to the phenomenon known as “sticky water” (sensu Wolanski and Spagnol, 2000), where the interplay of tides and currents produce a narrow zone of water that remains close to shore. This term was used initially to describe retention of water near coral reefs in the Great Barrier Reef, but it is also applicable to nearshore coastal zones of continents (Zeidberg and Hamner, 2002). Temperature profiles indicate strong thermal stratification at offshore stations, with weak stratification at the innermost station (Figure 3-2).
Figure 3-2. Interpolated vertical temperature profiles. Inshore, temperatures are warmer with less stratification, while offshore, temperatures are lower with a greater degree of stratification.

All plankton samples collected from the onshore-offshore transect were processed as above, and these data have been entered in tabular form in a spreadsheet where species abundances (identified to the lowest reasonable taxonomic level) were scored across stations. Early analysis indicates a strong association between water type and species assemblage.

July 2003 Cruise

Figure 3-3. Station Locations
The May 2002 cruises were repeated in Santa Monica Bay in July 2003 offshore of Dockweiler State Beach. In addition to the same equipment as before, for this cruise I also utilized an acoustic Doppler current profiler (ADCP) recently mounted on the RV Seaworld UCLA and a fluorometer. The ADCP complements the use of drogues in that the ADCP is a Lagrangian approach that looks at current velocities and vectors immediately under the boat, whereas drogues provide Eulerian information. The ADCP samples continuously every three seconds and provides velocity and magnitude in three dimensions at all depths immediately under the boat. I also decided to examine the effect of tides on the system. Hence, all physical and plankton sampling was performed along the same transect before, during and after the tidal change, and this array was repeated for three of the four sampling days (the first day was spent testing equipment). My transect was 4 km in length, perpendicular to the shoreline (Figure 3-3). Along this transect, I collected CTD data (depicted in blue) in 1 km increments. I repeated this CTD track at least four times each day through flood and ebb tides. Not shown in Figure 3-3, I also dropped drogues in the water at the blue stations each morning and tracked their movement through one tidal cycle on each of the three days. Throughout the entire cruise, the ADCP collected current information continuously under the boat. The tide changed from flood to ebb at the following times during the three days of sampling – 1214, 1257 and 1341.
Paired plankton nets. CTD (not clearly visible) is mounted between the mouth of the nets. Click to enlarge.

Each day consisted of the following logistical approach. By 0800 we reoccupied the inner-most station and begin our transect heading out toward the outer-most station, while dropping our drogues in the water on the blue stations until they were all lined up on the transect (Figure 3-3). Once all the drogues were deployed, we would reverse course and perform CTD casts at the blue stations on the same original transect from the deepest to the shallowest station. Once inshore, we would then drive to each of the drogues and mark the time and their current position. After marking the drogues, we would then proceed to the original outer-most station (where the outer-most drogue was first dropped in the morning) and perform a CTD run back into shore. This process was repeated approximately every hour for 7 hours, giving me seven drogue readings and four to seven CTD runs along the transect. In addition to this, I collected vertical plankton tows at the red stations once during flood, once during slack, and once during ebb tide (a total of three sets of samples per day). As before, I used the paired plankton net equipped with General Oceanics flow meters. All samples were fixed in 4-5% formalin.

July 2003 Analysis

Data analysis for this chapter has just begun. A slick was observed throughout the day along the 10 m isobath, approximately 700 m outside the largest breakers. It persisted for most of the day, though the afternoon onshore winds (in excess of 10 kn) made it difficult to visualize. Preliminary profiles of CTD information for each transect are presented in Figure 3-4 for one of the three sampling days. Comparing these images over time has allowed me to detect changes in thermocline depth and possibly internal waves associated with the changing tide. Preliminary results of the drogue data indicate that the paths traveled by the drogues agree exactly with the information generated by the ADCP, and somewhat with the results of the May 2002 field effort. The inner-most drogue appeared to have moved almost entirely by tidal motion (Figure 3-5). However, the offshore drogues appear to also have been driven mostly by tides. This may be explained by the fact that all drogues were only submerged to a depth of 5 m, and the ADCP results (Figure 3-6) demonstrate the strongest water velocities were within the top 10 m of the water column. During flood tide, the surface water traveled in a northward direction (depicted in red), and then switched to a southward direction (depicted in blue) after the tide change. Velocities experienced in the vertical direction were negligible, with the highest values being much smaller than horizontal speeds by more than an order of magnitude.

Figure 3-6. Continuous vertical profile track of water velocity in the north direction from acoustic Doppler current profiler (ADCP) aboard the RV Seaworld UCLA from 07:47 to 17:17. Red color indicates water traveling to the north while blue color indicates water flowing to the south. White, black and gray color indicates the ocean bottom. Driving the boat repeatedly offshore and onshore gives the multi-humped shape to the graph.

The ADCP clearly indicated that tides affected both the nearshore and offshore zones. However averaging the velocities in the nearshore zone (which was no more than 10m deep) and comparing them to averages of the upper 10 m of the offshore zone supports the drogue data in revealing that tides affect surface waters (in the upper 10 m) in a similar way regardless of distance from shore. However, since I did not submerge drogues in deeper water offshore, I cannot make a Eulerian comparison to the ADCP results.

Figure 3-7. Scatterplot and trend line of average longshore water velocity in the nearshore zone (averaged over entire depth - maximum depth = 10m). Positive values indicate longshore movement to the north.

Figure 3-8. Scatterplot and trend line of average longshore water velocity in the offshore zone in four depth bins (yellow = 1-10m, orange = 11-20m, blue = 21-30m, and purple = 31-40m). Positive values indicate longshore movement to the north.
In Figure 3-7, I have plotted the average velocities of the nearshore zone over one tidal cycle for the last day of sampling. The solution to the integral of this curve is approximately zero, indicating that a parcel of water in this nearshore zone will, on average, have no net directional flow. In Figure 3-8, I have plotted the average velocities of the offshore zone over one tidal cycle for four depth bins (1-10m, 11-20m, 21-30m, and 31-40m). Integrating over the deeper three curves results in a positive number, indicating that at these depths there is a net directional flow to the north. It is unclear whether or not the shallowest 1-10m depth bin of the offshore zone exhibits any net directional flow. Despite the r2 value of 0.9712 (Figure 3-8), better time averaging will be necessary to resolve this zone. Also, each sampling day lasted only 8.5 hours, and thus I did not sample through an entire tidal cycle. However, these preliminary results are compelling as they may reveal a profound effect on the dispersal of larvae of benthic invertebrates and demersal fish. Further refinement of my analysis, and elaboration of the physical results will occupy the next two months of my time. A future field effort may be necessary to fully understand the dynamics of this zone. This would include the deployment of drogues in deeper water in the offshore zone, and to sample for more than a 12-hour period of time.

Processing of plankton samples has commenced, and volumetric measurements have been recorded for all samples. Identification and enumeration of zooplankton will begin by mid-October, and data will be scored with multivariate statistics being utilized to establish any correlations between physical parameters and plankton abundance and diversity.

Non-parametric statistics will also be utilized to examine trends not addressable by multivariate statistics. For example, dissimilarity dendrograms generated for each day will illustrate degrees of dissimilarity between stations with respect to water type and species assemblage. The data will also be arranged in a grid for temporal analysis of changes in abundance and assemblage throughout a daytime tidal cycle.

Literature Cited
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