2011 Mugu Lagoon Benthic Infauna Report 3
MBC Applied Environmental Sciences, 3000 Red Hill Avenue, Costa Mesa, CA 92626 (714) 850-4830
(round worms) and phoronids (horseshoe worms), were each represented by a single taxon.
Arthropods were the most abundant phylum with 1,352 individuals, 49% of the individuals in the
samples. Nematodes, annelids and mollusks were next most abundant with 717 (26%), 580
(21%), and 84 (3%) individuals, respectively. The two remaining phyla comprised less than 1% of
the total abundance.
Species Richness. Species richness averaged 17 species per station, and ranged from
8 species at Station ML3, the easternmost station in the lagoon, to 23 species at both Station
ML6 in the middle of the western arm of the lagoon, and Station ML74 in the central lagoon
(Figure 2, Table 2 and Appendix B). Species richness was somewhat variable by station in the
lagoon.
Abundance. Abundance averaged 551 individuals per station (a density of 5,508
individuals/m
2
) and ranged from 42 individuals at Station ML3, to 1,209 individuals at Station ML6
(Figure 2, Table 2 and Appendix B).
Species Diversity. Shannon-Wiener species diversity (H') averaged 1.51 per station and
ranged from 1.21 at Station ML14, in the western arm, to 1.68 at Station ML74 (Figure 2, Table 2
and Appendix B).
Table 2. Infaunal community parameters and community indices.
2011 Mugu Lagoon Benthic Infauna Report 4
MBC Applied Environmental Sciences, 3000 Red Hill Avenue, Costa Mesa, CA 92626 (714) 850-4830
Biomass. Infauna biomass totaled 22 g and averaged 4.4 g per station (44.1 g/m
2
)
(Figure 2, Table 2 and Appendix B). Values ranged from 1.0 g at Stations ML3 and ML74 to 17.4
g at Station ML6. Mollusks contributed 65% to the biomass, a larger share than their proportion of
the abundance due to occurrence of a few large individuals. Annelids contributed another 26% to
the total followed by arthropods, the most abundant group, with 8%. The remaining phyla together
contributed 1% to the total biomass.
Benthic Response Index. The Southern California Benthic Response Index (BRI) is the
abundance-weighted average pollution tolerance of species occurring in a sample. For the
evaluation the pollution tolerance scores (p
i
) for northern (Point Conception to Newport Bay) bay
and harbor habitats were used. BRI values averaged 51.5 for the study area, and ranged from
32.6 (Category 1, Reference Level) at Station ML74 to 77.2 (Category 4, High Disturbance) at
Station ML15, the station closest to Calleguas Creek in the eastern arm of the lagoon (Table 2,
Appendices A and B).
Index of Biotic Integrity. The Index of Biotic Integrity (IBI) uses species diversity and
abundance of key taxa to distinguish impacted benthic communities from reference benthic
conditions. In Mugu Lagoon, IBI scores ranged from 1 (Category 2, Low Disturbance) at Station
ML14 to an IBI score of 3 (Category 4, High Disturbance) at Station ML3 (Table 2, Appendices A
and B).
Relative Benthic Index. Relative Benthic Index (RBI) values are calculated as a
weighted sum of four community parameters (number of species, number of crustacean species,
number of crustacean individuals and number of mollusk species) and abundances of positive
and negative indicators. RBI values in the lagoon ranged from 0.09 (Category 3, Moderate
Disturbance) at Stations ML6 and ML14 to 0.01 (Category 4, High Disturbance) at Station ML3
(Table 2, Appendices A and B).
River Invertebrate Prediction and Classification System. The River Invertebrate
Prediction and Classification System (RIVPACS) uses presence or absence of expected species
based on habitat variables including station location and depth. RIVPACS scores in the lagoon
ranged from 0.86 (Category 2, Low Disturbance) at Station ML6 to 0.00 (Category 4, High
Disturbance) at Stations ML3 (Table 2, Appendices A and B).
The Benthic Community Index Integration is the
median value (rounded up in value for all fractions) of the Category scores for the four indices at
each station. Integration values at Mugu Lagoon ranged from 3 (Moderate Disturbance) at
Stations ML6 and ML14, both in the western arm, and Station ML74 in the central lagoon, to 4
(High Disturbance) at Stations ML3 and ML15 in the eastern lagoon (Table 2, Appendices A and
B).
Community Composition. Twelve species each comprised 1% or more of all individuals
collected; together they totaled about 24% of the species but 93% of the individuals in the infauna
collection (Table 3, Appendix B). They included six arthropods, four annelids, and one each of
mollusk and nematode. Nematodes were the most abundant taxa accounting for 26% of all
individuals collected. Though taken at all stations, abundance was overwhelmingly highest at
Station ML6. The cumacean Oxyurostylis pacifica, the second most abundant species in 2011
with more than 17% of all individuals, occurred almost exclusively at Stations ML6 and ML14 in
the western arm of Mugu Lagoon, and was not found at eastern arm stations. Similarly, the
amphipod Monocorophium acherusicum, which accounted for 11% of all individuals were nearly
all from Station ML15, with only two individuals reported at Station ML6. Oligochaetes, accounting
for another 11% of abundance, and the amphipod Grandidierella japonica with 10% of the total,
also showed preference based on location in the lagoon. The remaining taxa each contributed
less than 10% to the total abundance, with most taxa showing strong preference for a station or
region of the lagoon. Only nematods were reported at all five stations during the survey.