Where the Salt Meets the Sweet
March 2009 | Dawn Rawls, Klish Way

Lagoon provides information on the food web as well as a chance to watch fish leaping in the river. Picture by Alicia D. Rawls.

Saltwater meets sweet water in our San Dieguito Lagoon and forms a very rich brew for all kinds of plant and animal life. Fresh water flowing down the San Dieguito River watershed mixes with tidal flows from the ocean to form a salt marsh in the lagoon. Those wetland grasses and salt-loving plants may not look much like “waving fields of grain,” but scientists estimate that a healthy salt marsh produces five to ten times as much oxygen and plant material per acre as does that wheat field!

The soil in the lagoon is a fertile mix of decayed plant and animal matter that, in turn, feeds the plants and the creatures that live in the mud. These invertebrates such as worms, snails, clams and mussels then become food for birds, small fish and land mammals. The water itself is full of suspended microscopic life that also drives these complex interrelationships aptly termed a food web. Naturally, the little fish become lunch for bigger fish that eventually travel out to the ocean!

In January, 2008, the massive excavation west of I-5 was connected to the San Dieguito River so that tidal water from the ocean could flow in and out and mix with river water. Would fish travel into the new pond and use these safe waters as a hatchery? Yes! In only eight months, the pond had a burgeoning of small fishes including gobies, flatfish, pipe fish, mullet and grunion. Scientists who took a census in the pond by video camera and special nets estimated a population of nine million fish! Plants and invertebrates are also making a vibrant comeback. With all this abundance on the menu, birds are arriving to chow down at the wetlands buffet.
With natural life bursting forth once again in the San Dieguito Lagoon, memories of the past dim quickly. Lest we forget, we should note that only three years ago the tidal pond contained tons of fill dirt, remains of a World War II airfield and sewage settling ponds that were dumping raw sewage into the wetlands as late as the 1970s. The restoration of the lagoon is being completed by Southern California Edison and SDG&E as mitigation for the destruction of larval and juvenile fish in the cooling water intake of the power plant at San Onofre. The California Coastal Commission has required that part of this mitigation be several decades of monitoring and testing of the lagoon’s complex food web to insure long-term success and provide scientists and marine estuary planners with valuable knowledge for later restoration projects.