Eutrophication in Elkhorn Slough

A small area of pickleweed has been uncovered, while an extensive area of pickleweed  is buried under a heavy mat of the alga, Ulva sp.

Coastal waters of California face many challenges such as pressure from fishing, introduction of invasive species, and eutrophication. Elkhorn Slough Estuary is no exception and although fishing and invasive species may seem more conspicuous at first, eutrophication is of increasing concern in our estuary. Eutrophication is defined as an increase in nutrients, such as nitrogen and phosphorous, which are primarily being taken up by plants or algae in the water. In a non-eutrophic system, nitrogen and phosphorous (in addition to light) can limit the growth of algae. However, provided plenty of nutrients algal growth can greatly increase, which in turn can affect many other inhabitants of the estuary.

In eutrophic systems, water quality is different from that of non-eutrophic systems. Just like plants, algae photosynthesize during the day when light is available. The algae release oxygen which becomes available as dissolved oxygen utilized by fish and invertebrates in the estuary. At night, with no light available for photosynthesis, algae (and all aquatic Slough organisms from microbes to clams to fish) respire and thus remove dissolved oxygen from the water in the estuary. Hence, in a eutrophic estuary, dissolved oxygen levels can be very high during the day and extremely low during the night. This can pose a problem for non-motile organisms such as oysters, clams, and other invertebrates, which have to limit water intake during periods of low oxygen (essentially “holding their breath”), and as these invertebrates are filter feeders, it means that they can’t feed for prolonged periods of time. Although fish have the ability to move from areas with low or extremely high levels of dissolved oxygen, an excess of algae driving dissolved oxygen levels to near 0 mg/L at night can be problematic. Furthermore, excess algal growth can result in large mats of floating algae, mainly from March to October. These algal mats get deposited on the marsh during high tides, and the mats can remain covering the marsh plants for extended periods of time.

Hence eutrophication can limit growth and reproduction of invertebrates, cause fish kills, and possibly smother marsh plants.

Our research
In order to better understand eutrophication at the slough we utilize long term monitoring data from our four permanent water quality stations, in addition to long term monitoring data from our monthly collected water samples at 18 permanent stations, spanning from the very head of the slough at Carneros Creek, to the mouth at Moss Landing Harbor, and including sites connected to Moro Cojo Slough, and Salinas River. Additionally, over the years, we and other researchers have conducted various studies at the slough, in order to better understand the effects of eutrophication on our estuarine system.

Nutrients in Elkhorn Slough
From 1997 to 2010 various researchers have examined temporal and spatial variation in nutrients at Elkhorn Slough, and the effects on water quality, Caffrey et al. 1997. For example, biogeochemical processes in the slough were affected by a 2-week foggy period. Fog that was associated with upwelling in Monterey Bay, led to increased durations of water column hopoxia, by reducing photosynthetically active radiation, Caffrey et al. 2010. Other researchers found that in general, nutrient concentratinos have increased from the 1970s to the 1990s. Dissolved inorganic nitrogen was low during the dry season in the summer and high during the wet season in winter. Additionally, nitrate concentrations peaked within a short time lag after a run-off event in the wet season and primary production was enhanced by nutrient pulses, Caffrey et al. 2007.

ESNERR water quality scientist John Haskins in collaboration with local researchers has lead efforts to better understand nutrient loads to the estuary, by measuring both nutrient concentrations and water flow in key tributaries.

Water quality at managed wetlands
ESNERR researchers support the adaptive management of wetlands behind water control structures. The Elkhorn Slough Foundation recently replaced water control structures at its North Azevedo Pond, and ESNERR researchers are examining how dissolved oxygen and marsh cover can be optimized through tide gate management.  ESNERR intern Miguel Rodriguez investigated the relationship between water depth and dissolved oxygen levels. Since eutrophication can lead to low levels of dissolved oxygen, he found that by maintaining a certain depth in the body of water, dissolved oxygen levels can possibly be managed (Rodriguez poster).  ESNERR water quality scientist John Haskins has also spearheaded an effort to decrease hypoxia at the Reserve’s North Marsh by deploying structures that increase circulation.

Detecting improvements due to restoration

Eutrophication expression is highest in areas where tidal flow is restricted by artificial structures, such as culverts or tide gates.

ESNERR researchers have also examined long-term water data to examine the effects of restoration.  They found that water quality at middle and south Azevedo marshes improved significantly (relative to adjacent Kirby Park) following restoration that included creating a broad buffer between farm fields and the wetland, Gee et al. 2010.  They also found that water quality at Porter Marsh improved following acquisition and restoration of lands along Carneros Creek.  This study also examined the role of tidal flushing through water control structures at these sites, and found that eutrophication symptoms were dramatically decreased with increasing tidal exchange – the water control structures affected local water quality more than did adjacent restoration.

Eutrophication Index
An ESNERR team analyzed spatial patterns of eutrophication in the Elkhorn Slough estuary, determining that the estuary overall is moderately eutrophic.  Eutrophication symptoms were not greatest in the areas with the greatest nutrient concentrations.  Instead, they were greatest in the areas with the least tidal flushing.  In particular, most wetlands behind water control structures were hypereutrophic, Hughes et al. 2011.

Jenna Van Parys checking fish survival at Vierra’s

Effects of low levels of dissolved oxygen on fish and oysters
In 2011, summer interns Miguel Rodriguez and Jenna Van Parys, in collaboration with slough staff, conducted a short-term experiment where they investigated the effect of low levels of dissolved oxygen on oyster growth and fish survival. They found decreased growth rates of oysters and increased rates of fish mortality in areas of low dissolved oxygen (Van Parys Poster).

Effects of floating algal mats on marsh plants
In order to better understand the effects of floating algae covering the marsh plant pickleweed, summer interns Laura Mercado, Amanda Peters, Brady Latham, and volunteer Rob Eby, in collaboration with slough staff conducted experiments where we manipulated Ulva cover on pickleweed for about two months. Results suggest that pickleweed above-ground biomass decreased after Ulva has been covering pickleweed for a few months. However, once the Ulva was removed, pickleweed seemed to recover in the time span of a few months.

In order to be able to evaluate water quality at different sites in the slough, using our continuously collected data on water quality and nutrient concentrations, intern Laura Mercado and ESNERR researchers developed a eutrophication scorecard for different sites within Elkhorn Slough. In order to calculate a eutrophication index, we incorporated water quality parameters such as total dissolved Nitrogen, total Phosphorus (Orthophosphate), chlorophyll a, free ammonia, floating algae, dissolved oxygen, pH, turbidity, and temperature.

For more information about eutrophication in Elkhorn Slough, you can contact our research staff.

In treatments where pickleweed was covered in Ulva for approximately two months, biomass was significantly lower than in treatments where pickleweed had not been covered in Ulva.