While the Elkhorn Slough and its surounding uplands host diverse habitats and species, there is strong evidence that local ecosystems are threatened, and have already undergone significant changes in the past centuries. The modifications which have occurred within the watershed are not unique, as they have occurred worldwide as a result of human use.
The threats and challenges to this landscape and its inhabitats are many. The Elkhorn Slough National Estuarine Research Reserve has identified three major areas that pose threats to the slough. These are:
- Changes in tide flow resulting in tidal erosion and marsh loss and drowning
Elkhorn Slough was once part of a vast interconnected estuarine network, including Bennett Slough, Moro Cojo Slough, Tembladero Slough, and the Salinas River channel. Early explorers report the tide rising and falling in as far-flung parts of this network as Carneros Creek, Castroville, and Salinas. Estuaries are where rivers meet the sea. As European settlers began to farm the region, they cut both elements of the life blood of estuaries -- the freshwater inputs and the marine influence -- to protect farm fields. Today, the Salinas river and Carneros creek enter the estuary through culverts and tidegates, disrupting natural connectivity between fresh and salt water. Conversely, tidal influence has been artificially cut from all the original arms of the estuary (Bennett, Moro Cojo, Tembladero Slough) exept for Elkhorn, and large portions of Elkhorn itself were diked and drained for a variety of uses: cattle grazing, railroad and road construction, and the creation of freshwater impoundments for duck hunting. This artificial restriction of tidal exchange led to massive subsidence. When tidal exchange was later returned, the mudflats were too low for marshes to return to areas they previously occupied. Tidal restriction has been by far the biggest cause of marsh loss in the estuary, and diversion of freshwater has resulted in loss of brackish plant communities. Additionally, marshes along the main channel of Elkhorn Slough that were never diked have lost significant marsh cover since the 1930s. The causes of this marsh loss are complex, but include lack of sufficient sediment due to river diversion, subsidence, and construction of an artificially large mouth to the estuary in 1946 to accomodate Moss Landing Harbor.
After the Moss Landing Harbor was constructed at the mouth of Elkhorn Slough, 50 percent of the salt marsh was lost due to the marsh drowning and the banks eroding, which continues today at dramatic rates. The accelerated rate of bank and channel erosion in Elkhorn Slough is causing tidal creeks to deepen and widen reducing functions for estuarine fish, salt marshes to collapse into the channel and die, and soft sediments that provide important habitat for invertebrates to be eroded from channel and mudflat habitats. Increases in the flooding of tidal waters on marshes are causing plants to “drown” in central areas of the marsh.
- Invasions by non-native species
Over 80 invasive non-native species have been documented in the Slough’s estuarine habitats, and the most common, conspicuous invertebrates encountered on a low tide at Elkhorn Slough are invaders. Some of these are having significant effects on native communities.
For example, the invasive Asian mudsnail now numbers around a billion individuals in mudflats in the estuary. European green crabs are more abundant than native rock crabs. In the salt marshes, upland weeds such as poison hemlock, ice plant and mustard are crowding out threatend native marsh plants.
- Non-point source pollution, including substantial agricultural run-off
Elkhorn Slough exists among strawberry and artichoke fields. The area’s climate makes for excellent farming. However, often farming makes for nutrient and pesticide run-off in nearby waterways. Remarkably high nutrient and pesticide concentrations have been documented in the Slough’s estuarine habitats. This impacts water quality, which in turn may increased the abundance of nutrient-limited producers (e.g., macroalgae such as sea lettuce) and pollution-tolerant animals, changing the natural life in the slough.
Researchers have documented strong fluctuations in oxygen levels in the estuary caused by excessive nutrients. Low oxygen is known to endanger many fish and invertebrate species.
Other possible threats to estuarine habitats at Elkhorn Slough include public access impacts, cattle trampling in marshes, and commercial uses. The role of these factors on the entire estuarine ecosystem has not been thoroughly characterized, and may be minor relative to the three major threats described above, but the development and impacts of these potential threats continue to be tracked and monitored.