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

8.0 Eelgrass
sonar imagery and marking each location with a weighted buoy for subsequent diver surveys.
This transect method provided an accurate representation of mean eelgrass turion density
within the delineated eelgrass bed using repeatable methods that documented spatial variations
in turion densities as well as maximum turion densities within the individual eelgrass beds.
Maximum turion densities collected during the 2008 survey were lower than those reported from
the 2000 surveys but remained within expected variation based on seasonal differences and
sampling methods. However, because the present methods also cover the full range of
densities, the highest turion densities recorded in quadrats sampled in 2008 were consistent
with densities reported in the 2000 baseline study among the surveyed areas (Table 8.3-9).
The importance of mean turion (shoot) density and the associated standard deviation is to
establish the continuity and consistency of surveyed eelgrass beds within the Ports. By
examining the relatively low mean shoot densities and high standard deviations reported earlier
in this section, in conjunction with documented visual observations, it is apparent that the
majority of eelgrass beds delineated in 2008, with the exception of Cabrillo North, are of low
density interspersed with higher density patches (Figures 8.3-1 through 8.3-4), and that the
plants varied in size and spacing. Several areas exemplified dense, well-defined eelgrass beds
while the majority of the areas appeared to border on ephemeral or fringe habitat during the
2008 evaluations.
8.4 SPATIAL AND TEMPORAL VARIATION
8.4.1 Seasonal Eelgrass Dynamics within Los Angeles Harbor
Eelgrass beds or meadows are highly dynamic systems that exist in a constantly changing state
where eelgrass density and survival rate is directly affected by numerous environmental factors
(Durance 2002). It is common for the density and survival rate of eelgrass meadows to fluctuate
on a seasonal basis. Therefore, monitoring of eelgrass should consider the natural and
seasonal variation of the eelgrass growth cycles. These dynamic systems can change with the
seasons or remain unchanged for decades (Fonseca et al. 1983).
Eelgrass exhibits seasonality in growth throughout its range depending on the physical and
biological factors acting on the individual areas. Eelgrass in the North Pacific, including
southern California, becomes dormant during the winter to a varying degree, depending on
location, leaves, and sustaining reserves within its underground rhizome system (Backman
1991). Eelgrass beds at the southern extent of their distribution (Sea of Cortez) typically die off
during mid-summer and are reestablished by recruitment in the fall. Considering the extensive
distribution range of eelgrass (Zostera spp.), seasonal declines and expansions are variable
based on the environmental conditions experienced within specific geographical locations. In
southern California, including the Ports, the seasonality of growth and reproduction is less
pronounced and eelgrass often grows year around. For example, flowering may occur during
any month, although it is most pronounced in the spring (Ruckelshaus 1996). The presence
and extent of eelgrass fluctuates yearly based on localized and regional conditions, but early
winter typically marks the end of the growth period and the initiation of die off or senescence of
eelgrass populations, as associated with lower water temperatures and increased turbidity from
storms or runoff. During the summer, eelgrass begins vegetative growth, expanding from
dormant rhizomes and increasing in shoot density. Meanwhile, recruitment from seed
production provides potential recolonization or expansion to areas previously devoid of eelgrass
or to fringe areas containing marginal habitat conditions. The inconsistency and variability of
winter conditions experienced in southern California and the dynamics of coastal circulation
determines the level of effect that different eelgrass beds or areas experience on a yearly basis.
2008 Biological Surveys of Los Angeles and Long Beach Harbors 8–11
April 2010