|Santa Monica Bay Nearshore Zone
A. Kimo Morris
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 1980s, 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
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
May 2002 Analysis
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
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.
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.
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