appendix b final 2008 biological surveys of los angeles and long ...

8.0 Eelgrass
formation evident. While this bed appears persistent from previously reported observations,
seasonal differences in eelgrass bed extent were apparent from the side-scan surveys.
The Mitigation Site located in the center of the Pier 300/Seaplane Lagoon Area consisted of
large, evenly spaced plants with occasional dense patches that were less than 3 m 2 . Red algae
(Chaetomorpha spp. and Gracillaria spp.) was commonly observed as potential competitors for
space and light among the eelgrass plants and was particularly evident adjacent to the rock
revetments and in areas with hard substrate (rock) or shell fragments. The southern portions of
the Pier 300 site contained higher densities of Gracillaria spp., to the extent that it was much
more common than the eelgrass plants. Considering the Mitigation Site is composed of rock
revetments constructed to support the dredge fill of various sizes, the occurrence on marine
algae is not surprising based on the available substrate. Considering the location of the
Mitigation Site between the small sandy beach embayment to the west, comprised of mostly fine
sand and mud, and the deep dredged channel leading into the Seaplane Lagoon, circulation
and scouring from tidal water movement likely contribute to the occurrence of courser sediments
and the persistence of competitive algal species. Previous eelgrass transplantation that was
conducted in 2003 and 2007 still appears spatially distinct and the associated eelgrass
community is likely being shaped by the complex interactions of competition and hydrology that
affects sediment distribution.
The eelgrass bed within the Mitigation Site displayed very little season change, decreasing from
15.4 acres in the spring to 15.1 acres in the fall (approximately 2.5%). The low density and
evenly spaced plants observed throughout this site coupled with dense aggregations of
Gracillaria spp. made evaluations of the differences in sonar surveys difficult. For example,
although the imagery shown in Figures 8.3-4 and 8.3-5 depicts a continuous low density bed,
the actual configuration of the site is likely more fragmented at a finer scale.
The Terminal Site in the western most portion of the Pier 300/Seaplane Lagoon area is the most
consistent eelgrass habitat within this portion of the Port and consists of uniform sand/mud
substrate supporting a continuous, healthy bed of eelgrass with only intermittent occurrences of
algae (Chaetomorpha spp. and Gracillaria spp.). The Terminal Site extends from the sandy
beach adjacent to the Mitigation Site to the south along the riprap. The spatial extent of the
Terminal Site bed decreased from 10.4 acres in the spring to 8.9 acres in the fall (approximately
15%), with the bed contracting throughout the offshore edge and the densest areas decreased
overall (Figures 8.3-2 and 8.3-4). Seasonal differences in plant sizes were evident with the
eelgrass bed along the beach shifting from small and large plants in the spring to mostly large
(>2 ft) plants in the fall, with a low epiphyte load. During both spring and fall surveys the
eelgrass was evenly spaced, forming a continuous bed just inshore of a noticeable berm at
approximately -5 to -8 ft MLLW. Extending south along the riprap adjacent to the terminal, the
substrate contained several dark areas of fine mud where the eelgrass bed became increasing
discontinuous and sparse moving away from the beach and the riprap.
In some cases considerable annual (seasonal) variation in abundance has been documented in
southern California (e.g., winter die-off and spring/summer re-growth) due to a variety of factors,
including but not limited to physical and biological disturbance, changes in nutrient availability,
and changes in water quality parameters such as turbidity and salinity. These factors can result
in long-term changes in eelgrass abundance depending on the frequency and intensity of
unfavorable conditions. Eelgrass density also can vary substantially depending on the bottom
depth of plant attachment. This is due to variations in the size of plant turions at different
depths. For example, at shallower depths eelgrass density is typically greater than at deeper
depths (Durance 2002 and Gussett 2002). Eelgrass density and morphology vary with respect
to depth, exposure, substrate, and water clarity (Durance 2002). As a result, it can be difficult to
define characteristics for a “normal” eelgrass bed.
2008 Biological Surveys of Los Angeles and Long Beach Harbors 8–13
April 2010