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

8.0 Eelgrass
Seasonal patterns among all surveyed areas were inconsistent. Cabrillo Beach eelgrass beds
displayed expansion and growth while the Pier 300/Seaplane Areas had a slight decrease in
overall eelgrass acreage. Diver surveys completed in the spring of 2008 recorded density
differences with respect to depth at Cabrillo South transect CS1 and Cabrillo North transect
CN2 (Table 8.3-1 and 8.3-2). The eelgrass beds associated with Cabrillo North and South
expanded in spatial extent and density over the summer growth period, consistent with
expected trends and with previously reported seasonal patterns within the Ports (MEC 2002).
Decreases in eelgrass area and density within all three sites in the Pier 300/Seaplane Lagoon
Area, although relatively small, were not expected. Observations documenting seasonal growth
of individual eelgrass plants progressing from small (4-6 in) to >2 ft tall were recorded at all sites
as expected.
8.4.2 Regional Eelgrass Dynamics within the Ports
In conjunction with anticipated seasonal and temporal variability in eelgrass bed distribution,
eelgrass distribution can also be affected by localized or regional episodic events. For example,
El Niño Southern Oscillation (ENSO) events can have negative effects on eelgrass health and
persistence as a result of elevated sea surface water temperatures and increases in sea level.
Eelgrass beds throughout southern California declined 50 to 70% during ENSO events in
1997/1998 and subsequently recovered by 2000 (Merkel & Associates 2000). Large regional
episodic events that disrupt water quality can increase effects to eelgrass by limiting growth
conditions or altering the substrate. Considering that habitat and conditions conducive for
eelgrass recruitment and growth are relatively narrow within the Ports, minor perturbations could
have significant implications for eelgrass distribution, persistence, and health.
Events such as El Niño/La Niña have broad implications on regional biological production and
especially eelgrass growth. The trend of higher sea surface temperatures and corresponding El
Niño conditions recorded between 2002 and the early portion of 2007 could be the source of
decreased spatial extent and lower densities observed within the Ports during the 2008 baseline
surveys (Figure 8.3-6). While 2007 and the early portion of 2008 were considered a mild La
Niña condition, introducing cooler surface waters to the SCB, eelgrass growth and bed
consistency likely require greater than one season to recover or experience a lag effect for
regrowth or recruitment. Lower average monthly sea surface temperatures for the Central
Pacific ocean waters in the late portion of 2007 and early 2008 were short lived with warmer sea
surface temperatures returning in early summer and fall of 2008 (Table 8.3-12). Sea surface
temperatures within the SCB followed a similar trend to the ones reported in the time series
depicted in Figure 8.3-7.
Inter-specific competition from marine algae, phytoplankton blooms, and fouling from epiphytes
can also have considerable effects on eelgrass distribution and health. Increases in nutrients
from runoff and elevated water temperatures can be associated with algal blooms and
increased epiphytic growth that reduces light availability. Diebacks are frequently observed in
eelgrass communities when the epiphytic load reduces the amount of light reaching the plant to
a level where photosynthesis can no longer balance metabolic demands (Hanson 2000).
Species utilizing eelgrass beds and the associated benthic infauna as food sources can also
have localized effects on eelgrass distribution. As examples, various waterfowl and benthic
invertebrates graze directly on eelgrass blades while several species of rays can cause
bioturbation and increased sediment cover within eelgrass beds when foraging for prey species
in the substrate.
8–14 2008 Biological Surveys of Los Angeles and Long Beach Harbors
April 2010