Invertebrate Fauna of Lake Merced
Nelia White,
Graduate Student,
Geography
San Francisco State University
The climate, geology, soils, water, and vegetation of Lake Merced determine the habitats of its animals. Plants especially serve as food source, shelter, and nesting sites: tule, smartweed seeds, and coontail provide important food sources for waterfowl and songbirds; small mammals and birds are sustained by blackberry fruits and herbaceous plants and use the vegetation for nesting and cover. The same is true for the invertebrates: the San Francisco yellow bush lupine on the northwest side of Impound Lake is home for the San Francisco yellow bush lupine moth (GeoResources 1993); the phytoplankton provides a food source for the zooplankton and the midge larvae.
The invertebrates of Lake Merced are essential for the health and abundance of the prized fish and birds for which the area is locally renowned Currently no comprehensive invertebrate species list, nor is there any indication of their abundance or distribution patterns within different vegetation zones, however a report prepared for the EIP Associates Significant Natural Resource Areas Management Plan (2000), complied by Low and Conser (2000), should soon be available. Instead, the habitats for invertebrates are broadly lumped into aquatic or terrestrial. The aquatic zones are the near shore littoral, the open water limnetic, and the bottom benthic.
Historical conditions and tidal connection
Lake Merced is a natural fresh water body on the western edge of
the San Francisco
peninsula, fed by runoff, underground springs and the surrounding
Merced aquifer. From the
northeastern tip to the west, then to the southern tip, it is about 2
miles long. The
deepest point is 10 ft. below sea level, and it lies in a structural
valley, separated by
a narrow sand ridge from the ocean about a thousand feet away (Miller
1958).
The sand barrier was breached in the 1852 earthquake and likely at other times in the past (Trihey & Associates, Inc. 1999, 1-3). According to Lawson as cited in Miller (1958), salt water invaded the lake during the late Quaternary. A U.S. Coast and Geodetic Survey map from 1869 shows a tidal connection to the ocean at the north end of the lake, about where Sloat Blvd. is today. Despite this salt-water history, Lake Merced was likely dominated by freshwater, giving an advantage to euryhaline species, tolerant of a wide range of salt concentration (Miller 1958).
After sand dunes blocked the inlet for the last time in the late 1880s, the water level rose 10 ft. above the tide, and then was raised artificially another 10 ft., according to Miller (1958) quoting Lawson. In 1895, the lake was split into North Lake and South Lake by an earthen dam. A culvert connecting the two lakes is usually kept closed, creating the possibility for different biotic assemblages to develop in each lake.
An overflow tunnel connects South Lake to the ocean. Because of low lake levels, this tunnel is not a likely source for introducing marine animals (Miller 1958). Impound Lake, small and shallow at the southern end of South Lake, was separated after 1935, when a photograph shows it still connected. By that time, the coastal dune community around the lake had been changed to resemble the current urban coastal lake with exotic trees.
In concert with increased pumping of the aquifer, the lake level has dropped dramatically since 1980. The lowered lake level is likely to increase the mean water temperature, thereby lowering oxygen available within the water column, making it more difficult for the resident aquatic fauna, especially cold water fishes to live ( see water section).
Human alterations to the lake include splitting the lake into three bodies, using rotenone to kill predatory fish species so that planted game trout would thrive, and using copper sulfate to control algal blooms. Storm water, sewer overflow, herbicide, pesticide, and fertilizer residues from the golf course are all contaminants with potential to affect the lake zooplankton and invertebrates (Camp Dresser & McKee 1999). The ones that have survived all these stresses must be hardy.
The information on zooplankton is derived from limnological profiles and plankton counts for 1997 and 1998 done by the Water Quality Bureau in the Environmental Field Services Section of the San Francisco Water Department. They sampled at deep points in North Lake, East Lake and two sites in South Lake, all in the limnetic zone (Camp Dresser & McKee 1999).
The plankton counts are predominantly phytoplankton, by three to four orders of magnitude greater. Thus, for plankton tows of some 13,780 phytoplankton, there are a total of 2 copepods. In general, the data were so sparse for zooplankton, that the significance is questionable. Also, the variation between sites and dates for zooplankton was too small to be reliable. Zooplankton listed are cladocerans, nematodes, copepods, rotifers, and nauplii (Camp Dresser & McKee 1999).
Figure 1. Daphnia (John Clare 2000)
Cladocerans: Daphnia is a cladoceran, a tiny crustacean that lives in the open freshwater, known as a water flea because of the jerky motion from using its antenna. Because they can reproduce parthenogenically (asexually), their population can increase rapidly. They are a favorite fish food
Figure 2. Nematode (DeBoyd Smith 1977)
Nemotodes: These are microscopic roundworms covered with a clear cuticle, most often known as pathogens. They are also common in the soil.
Figure 3. Copepod. (DeBoyd Smith 1977)
Copepods: Most are very small, less than 1 mm, living on bacteria and one-celled organisms. There are numerous species, and they tend to occur in high population densities. They are an essential fish food item. (http://www.mov.vic.gov.au/crust/copbiol.html)
Figure 4. Rotifer (Wim van Egmond 2000).
Rotifers: These multi-celled animals have a crown of cilia like a rotating wheel, which they use for feeding and locomotion. They tend to occur in abundance.
(http://www.microscopy-uk.org.uk/mag/wimsmall/rotidr.html)
Figure 5. Nauplius (DeBoyd Smith 1977)
Nauplii, or nauplius in the singular: These are an early larval stage for many different crustaceans, often hard to distinguish by species.
Relict aquatic invertebrates
Once connected to the ocean, Lake Merced retained five species associated with marine or brackish, rather than freshwater environments: a hydroid (Cordylophora lacustris), a polychaete (Neanthes limnicola), a mysid shrimp (Neomysis mercedis), an isopod (Exosphaeroma oregonensis), and an amphipod (Corophium spinicorne). Except for the hydroid, which was most likely introduced before 1950, these invertebrates are considered relicts from the brackish environment (Miller 1958). Of these, the mysid shrimp has been found recently (personal communication, James Mackey and Thomas Neisen 2000).
Figure 6. Hydroid (Cordylophora lacustris) found
in Lake Merced.
(Smith and Carlton 1975)
Cordylophora lacustris Allman 1844. This hydroid is among the many species introduced as epizoics (animals living on other animals) with the Atlantic oyster from the 1860s to the 1910s. Only the polyp stage is present in the life cycle. It is found on pilings and boat bottoms in low salinity or fresh water (Smith and Carlton 1975). The name means "animal of a lake bearing a cudgel or headdress."
Corophium spinicorne Stimpson, 1875. This aquatic, benthic amphipod is a gammarid, which means it looks similar to a beach-hopper. It builds tubes attached to debris on the muddy bottoms of estuaries (Smith and Carlton 1975). Amphipods tend to occur in abundance and provide an important staple of the diet for larger organisms.
Gnorimosphaeroma oregonensis = Exosphaeroma oregonensis Dana 1854-1955. This isopod, which resembles a terrestrial pill bug, is found in marine and sometimes brackish waters (Smith and Carlton 1975). It was reported at Lake Merced in 1903 and 1932.
Neanthes limnicola. This is a viviparous nereid polychaete worm, meaning a live-bearing, swimming, many-bristled, segmented worm. It lives in brackish or freshwater estuarine streams, coastal lagoons, and Lake Merced. It is the same species as N. Lighti Hartman 1938. (Smith and Carlton 1975) Translation of the Latin name means "a new-blown bud living in a pond."
Figure 7. Planktonic Neomysid (DeBoyd L. Smith 1977)
Neomysis mercedis. This type of mysid shrimp is called an "opossum shrimp" because of the female’s brood pouch on her thoracic limbs. Mysids are small, translucent filter feeders and scavengers frequently found in abundant swarms in marine, brackish, and fresh waters. They are an important component of the diet for fishes in brackish waters (Smith and Carlton 1975).
Present day invertebrates
Ecology & Environment, Inc. (1993) refer to the brine shrimp and opossum shrimp as introduced, but other references (Miller 1958, Smith and Carlson 1975) recognize the opossum shrimp as native to Lake Merced.
Ecology & Environment, Inc. (1993) did a survey of aquatic and terrestrial habitats in a study to determine the effect of lead shot from the Pacific Rod and Gun Club. They took 101 soil samples, 243 sediment samples and 40 biota samples total. Ultimately they found lead concentration was elevated in benthic macroinvertebrates, those at the base of the food chain (bloodworms, clams and snails), but the effect was otherwise insignificant. The highest risk was to waterfowl ingesting lead shot directly. This study is the only systematic sampling released to date for the invertebrates of Lake Merced. However, as noted earlier, another invertebrate inventory report should soon be available (Low and Conser 2000).
Figure 8. Sampling sites (Ecology & Environment 1993).
Snails, leeches, clams, flatworms, planaria, and many types of crustacea (especially amphipods and isopods) have all been found, typical for such a freshwater lake.
Many snails, as well as leeches, were found attached to the coontail, a submerged aquatic weed (Ecology & Environment 1999). All of which are fairly tolerant of low oxygen conditions.
Lake Merced is highly eutrophic, with high algal production from excessive nutrients. The lake bottom is a mucky ooze of decomposing organic debris along the edge. Sandy bottom areas occur in some of the limnetic zone farther from shore. Benthic invertebrates, both epifauna and infauna, are good indicators of health of the aquatic ecosystem.
Until the Low and Conser report (2000) is released insect species must be extrapolated from existing data as well as what is know about the habitats of the area. Species likely to occur in the terrestrial and aquatic habitats of Lake Merced include species the following orders:
Chironomid midge larvae (bloodworms) were the most abundant benthic invertebrate sampled (98%), found both in mucky sediments and in sandy bottoms. A few oligochaete worms were part of the sample of 749 individual specimens. Clams were found in sandy substrate. The high proportion of bloodworms, which are very tolerant of pollution, and low species diversity indicates eutrophication in Lake Merced (Ecology & Environment 1993).
Common midges, Chironomus. Adults are large gnats with a body length up to 13 mm, wings long and narrow, legs and antennae long, and in males broad and feather-like. Their proboscis is short and non-biting. They swarm near ponds and lakes, natural or artificial. Larvae are long, slender, and worm-like, with a body length of 20-25 mm, usually curled at both ends and red with hemoglobin (so-called "bloodworms"). The head is brown and the next to last segment has 4 fingerlike gills.
Figure 9. Chironomid pupa, (Philip Rowley 2000)
Often they are extremely abundant in soft bottom mud where they build a silk-lined tube. Body undulations aid respiration by irrigating the tube. The larvae feed on phytoplankton caught in a silken net spun over the tube mouth(Powell and Hogue 1979).
Fly fishermen know that chironomids are a favorite trout food. Chironomids are not just creatures of the shallows but may live at depths of over 40 feet. (Philip Rowley 2000)
Crane flies are abundant in damp areas. The tough-skinned larvae, called "leather-jackets," live on grass roots, rotting vegetation or in the water, feeding mostly on decaying plant material.
Water boatmen are found in a broad range of habitats, but most often in fresh water where they eat algae, protozoa, and other organisms in the mucky bottom, even including mosquito larvae. They themselves are a favorite food for fish. Often their populations are so great that they provide a significant link in the aquatic food chain between tiny organisms and fish.
Backswimmers are widespread aquatic insects. They prey on other insects and sometimes small fish.
Water striders, the most widespread semi-aquatic insect, are predaceous on other insects and organisms on the water surface.
Figure 10. Damsel fly nymph (Philip Rowley 2000) Figure 11. I. gemina (Powell 1979)
San Francisco forked-tail damsel fly (Ischnura gemina) was last reported from Lake Merced in 1978 by John Hafernik (see collection label in reference section). It was first distinguished from a very similar species, I. denticollis, by Kennedy in 1917, but not until the late 1970s were several populations discovered in the Bay area by Hafernik and Garrison. The two species existed sympatrically for over 70 years, yet there is recent evidence of hybridization in Alameda and Santa Clara counties across the bay (Leong 1989). Like many damsel flies, I. gemina it is tolerant of poor conditions.
Leech’s skyline diving beetle and Ricksecker water scavenger beetle are other aquatic insects likely to occur at Lake Merced, although not reported (Hafernik 1989).
Soil
Candace Low in SFSU’s Biology Department is surveying and monitoring terrestrial invertebrates as part of the San Francisco Parks and Recreation Department’s project to restore designated Significant Natural Areas. She sets out a total of 25 pitfall traps for a week every other month: 11 at the north side of Impound Lake among the tule and willows and 14 on the north shore of East Lake, in the "mesa" or plateau area where ice plant is currently being removed.
Pitfall traps are plastic cups 4" in diameter, 4-5" deep, flush with the ground and covered by a canopy to keep out debris and rain. The animals collected have yet to be identified and classified, but she reports many small millipedes, beetles, spiders, and a bumble bee. The data they are interested in is both species lists and diversity.
Because there are so many soil invertebrates, they are so hard to identify, they are often so small, so hard to quantify, and there are few studies to compare the results with, they are frequently ignored. Soil invertebrates studied by Ecology & Environment, Inc. (1993) were earthworms (annelids) and an isopod, the wood louse (Porcellio scaber). Their report also mentioned amphipods.
Detritus eaters, predators, scavengers, root, sap, or nectar feeders are included among the soil diplopods, hexapods, isopods, gastropods, chilopods, arachnids, hymenopterans, and coleopterans. Soil macro-invertebrates enrich the food web mostly for the birds.
Other terrestrial invertebrates
A crustacean, the Tomales isopod, (Tomales asellid or Caecidotea tomalensis), and a lepidopteran, the San Francisco’s tree lupine moth (Grapholita edwardsiana) are no longer listed as species of concern by the California Department of Fish and Game (1991) since they have been found in other locations. These species, and the Blister Beetle listed below, were historically recorded but are no longer present at Lake Merced.
Figure 12. Blister beetle larvae cluster (Hafernik,
SFSU).
Figure 13. Blister beetle larvae on bee (Hafernik, SFSU).
Blister Beetle, Meloe franciscanus, was originally described from a collection taken at Lake Merced in the 1920s by Van Dyke from the California Academy of Sciences (Van Dyke 1928). In those days, Lake Merced likely had more elements of its previous coastal dune environment and the only trees would have been a few willows. The beetle has not been reported at Lake Merced since the 1930s, and not locally since the 1950s when McSwain mentioned a single clump of 600 individual larvae in Berkeley (Hafernik, personal communication, May 2000).
A recent article in Science News describes the unusual strategy of mimicry that John Hafernik and Leslie Saul-Gershenz of San Francisco State University suggest this flightless beetle uses to increase dispersal in the Mojave Desert. Larval clusters resemble a female solitary bee Habropoda pallida, to attract the male. They hitch a ride on the male, move onto the female bee later during mating, and then slip into the bee nurseries packed with pollen. There they gorge, become grub-like, pupate and transform into wingless adult beetles (Milius 2000).
Most species of invertebrates found, both aquatic and terrestrial, are hardy species able to withstand poor conditions. Developing a species list of the invertebrate fauna, with some indications for abundance and distribution in the area will help elucidate patterns both of the physical parameters (vegetation, water quality, elevation, slope and aspect, soils) and the patterns of the vertebrate distributions. This area was once a coastal sand dune community. With the long shore drift, the lagoon closed off and became a freshwater lake. Soon afterwards, human alterations and introductions changed it from the coastal scrub with low growth and open, sunny exposures to more urban lakeside vegetation with more shade and trees. With this drastic change, came the loss of certain invertebrates, while still others have survived
REFERENCES
Camp Dresser & McKee Inc. 1999. Lake Merced
Watershed Sanitary Survey.
Report for San Francisco Public Utilities Commission.
Clare, John. 2000. Daphnia: An Aquarist’s Guide. clarej@tcd.ie.
Ecology & Environment, Inc., 1993. Environmental Lead Characterization. Pacific Rod and Gun Club, Lake Merced, San Francisco, California.
EIP Associates 2000. Significant Natural Resource Areas Management Plan. Prepared for Natural Areas Program, San Francisco Recreation and Parks Department.
Feasibility Evaluation of Alternatives of Raise Lake Merced. Draft. October 19, 1998. Lake Merced Technical Memorandum No. 1
Hafernik, J.E., Jr. 1989. Surveys of potentially threatened Bay Area water beetles and the San Francisco forktail damselfly. Report prepared for U.S. Fish & Wildlife Service, Sacramento Endangered Species Office.
Lake Merced Comprehensive Management Plan Appendixes. Revised Draft May, 1998. Prepared by San Francisco Public Utilities Commission and San Francisco Recreation & Park Department.
Leong, Joan M. 1989. Morphological Variation and Hybridization in Two Damselflies, Ischnura denticollis and Ischnura gemina. Masters Thesis, San Francisco State University.
Low, C. and C. Conser 2000. Unpublished preliminary data from terrestrial invertebrate sampling at Lake Merced. On file at San Francisco State University.
Miller, Robert C. 1958. "The Relict Fauna of Lake Merced, San Francisco." Journal of Marine Research 17:375-381.
Milius, Susan. 2000. "Ah, my pretty, you’re...#&! a beetle pile!." Science News 157 (19):295.
Powell, Jerry A. and Charles Hogue. 1979. California Insects. Berkeley: University of California Press.
Rowley, Philip. 2000, Chironomid Tips "Getting That Sinking Feeling" http://www.bcadventure.com/adventure/angling/protalk/rowley/sinking.phtml accessed may 8, 2000
Smith, DeBoyd L. 1977. A Guide to Marine Coastal Plankton and Marine Invertebrate Larvae. DuBuque: Kendall/Hunt Publishing Company.
Smith, Ralph I. and James Carlton, editors. 1975. Light’s Manual: Intertidal Invertebrates of the Central California Coast. Third Edition. Berkeley: University of California Press.
Trihey & Associates, Inc. 1999. Lake Merced 1998 Baseline Natural Resources Inventory. Report for San Francisco Public Utilities Commission through Camp Dresser & McKee.
Van Dyke, E.C. 1928. A reclassification of the genera of North American Meliodea. Univ. Calif. Publ. Ent., 4:395-474, 3 pls.
Biology Department people interviewed:
John Hafernik, Thomas Neisen, James Mackey, Neo Martinez, Candace Low
Figure 14.Collection card for Ischnura gemina.