BAY DELTA SCIENCE CONFERENCE: Suisun Marsh: On the edge of resilience in an era of rapid change

Dr. John Durand is a senior scientist at UC Davis who, along with his colleagues, Teejay O’Rear and Dr. Peter Moyle, run the Suisun Marsh fish study, a project founded over 40 years ago.  In this presentation from the Bay-Delta Science Conference, he discusses the dynamics of the Suisun Marsh and the effects of climate change on it.  He dedicated his talk to the memory of Dr. Larry Brown, whom he described as a wonderful collaborator and his academic sibling.

At 116,000 acres, the Suisun Marsh is the largest tidal wetland in the western United States and the largest contiguous tidal wetland in the San Francisco estuary.  The marsh is comprised of brackish tidal channels that provide a connective corridor through meandering sloughs to tidal marshes.

Salinity in the marsh is controlled by a set of gates installed as mitigation for the water projects back in the 1980s.  The effects of the gates are to reduce salinity in the marsh when in operation; the chart on the upper right shows the salinity gradient along Montezuma Slough before gate operation, illustrating how salinity decreases once the gates are in operation.

In between the channels are muted tidal seasonal freshwater ponds that are managed wetlands; these are duck clubs managed primarily for waterfowl hunting.  The duck clubs have formed a core conservationist unit that has preserved Suisun Marsh more or less in its present state over the past 100-150 years.

The managed wetlands are flooded up in the wintertime from September to June to promote habitat for waterfowl; the managed wetlands are then drained from June to September, during which a variety of annual plants grow.

This management combined with the sloughs creates a gradient of a diversity of vegetation types that benefit many different species on many different trophic levels, said Dr. Durand.  In the tidal sloughs, there is emergent tule and cattail; on the levees themselves, there’s eucalyptus, wild rose, and coyote brush; and in the ponds themselves, there is pickleweed, alkali bullrush, fat hen – all of which are very beneficial to waterfowl, and which recently were found to produce a lot of aquatic productivity when exchanged with the tidal slough.

Suisun Marsh morphology and nutrient exchange

The Suisun Marsh is situated between three small hilly regions: the Montezuma Hills, the Potrero Hills, and the Sulphur Hills.  There are three basins or complexes nested within the marsh: the North Suisun basin, the Cordelia complex, and the Nurse-Denverton complex; these complexes have mixed residence time and exchange with the entire marsh.  Nested within these basins are sloughs or tidal channels.  The terminal ends of these tidal channels tend to be ‘engines of productivity’ that exchange on a tidal basis with the larger basin of the larger region.  Adjacent to the sloughs are managed wetlands, which are seasonal engines of productivity. 

The managed wetlands are exchanging with the sloughs; the sloughs are exchanging into the basins; the basins are exchanging into the wider marsh, which itself is a basin that’s somewhat set apart from the larger San Francisco Estuary.

“While there is limited evidence for aquatic productivity fluxing from Suisun Marsh into the larger Suisun Bay region or Delta region, there is considerable evidence from our study that this sort of productivity creates an attractive environment for many native and naturalized fishes in the estuary,” said Dr. Durand.

To illustrate this, he presented a graph from a study of chlorophyll a and zooplankton along the Montezuma corridor up into the Nurse-Denverton complex.  The chlorophyll was measured continuously by a multi-parameter sonde, showing that nearing the terminal end of the slough, the chlorophyll density concentration increased considerably.

Zooplankton was also fairly scarce at the mouth of the slew, but higher up towards the terminal end, there were elevated levels of zooplankton; it was more variable, but at key periods, there are extremely high abundances of zooplankton.

Dr. Durand presented a map as an alternative way to illustrate this.   The map is from a study by Brian Williamson of the Blacklock Restoration Site that shows a similar transect for chlorophyll a.  Starting in the Nurse-Denverton Complex, the chlorophyll concentration increases as you go up through Denverton slough and up into a side branch of Denverton slough, Luca slough, and Luca Pond. 

The key takeaway from here is that, in addition to increased concentration up in terminal ends of sloughs, there’s also an increased concentration within these ponds,” said Dr. Durand.  “Here, concentrations were between 11 and 35 micrograms per liter, which is actually low for a lot of the periods. So we’ve seen extraordinarily high concentrations of chlorophyll at key times – not always, but there is a lot of variability.”

To demonstrate this in a different way, three pond sites were studied by Kyle Phillips, a grad student:  Luco Pond,  Meins Landing, and Wings Landing.  The chart compares the levels of chlorophyll in the pond with the levels in the adjacent slough.

You can see that a great deal of the time, chlorophyll abundance in the pond is extraordinarily high relative to the slough,” said Dr. Durand.

The graph on the lower left shows the same pattern at the same sites, but with zooplankton density, as opposed to chlorophyll.  There was much more zooplankton in higher densities than we found in the slough itself, he said.

Sophie Munger & Rusty Holleman did a study examining flux in and out of a series of tidal sloughs with varying elevation gradients during the summer months, shown on the upper right.

Sheldrake was the most impounded and has the smallest sort of area or tidal prism; Peytonia is a little bit larger and a little more complex; First Mallard is much more complex and larger, and Hill Slough is quite large,” said Dr. Durand, noting that the top set of graphs is flow.  “In Hill Slough, there’s a lot of water moving back and forth because hillslope is extremely large relative to the others.  So we wanted to measure what happens to chlorophyll and what happens to F-Dom, which is dissolved organic matter produced in these sloughs. Is it moving out? Is it fluxing out into the larger system? Is it sloshing back and forth? Or is it being sequestered?

With one exception, they found that chlorophyll, in general, is mostly net zero in the smaller sloughs and even in First Mallard.  He noted that higher levels indicate flux out of the slough, and the lower levels indicate flux into the slough, so it’s possible to parse out some indication of higher flux during peak periods of tidal exchange, which would be spring tides or even King tides.  Hill Slough had perhaps some net outward flux, but every time there was flux out, it seems like there was a recoil, and there was just as much flux back into the system for a more or less net zero.

However, for dissolved organic matter (F-DOM), it’s a bit of a different story.  While there wasn’t much flux activity out of the system for Sheldrake and Peytonia, First Mallard and Hill Slough were much more productive of dissolved organic matter, and much more of that was leaving the slough system.

We think that that is really the foundation of the food web in Suisun Marsh, in addition to chlorophyll,” said Dr. Durand.  “Certainly dissolved organic matter is playing an important part.”

He noted the chlorophyll bump in Sheldrake, explaining that they think that resulted from a managed wetland dumping a lot of highly concentrated chlorophyll that affected the whole slough dynamics.  

That’s an indicator of the sorts of dynamics that go on when ponds are exchanging,” said Dr. Durand.  “This series was done in the summer. So that was a rare, anomalous event. But the sort of exception is what proves the rule, perhaps in this case.”

Nicole Aha ran a caged experiment where she found that the caged salmon placed in ponds grew at a much faster rate than fish that were placed in cages in tidal sloughs.

A similar study by Brian Williamson at the Blacklock Restoration Site found that most fishes tend to increase in abundance at the terminal end of sloughs, while Mississippi silversides and Shumofuri Gobi had the opposite trend.  Most other fishes, including native splittail and tule perch, tended to be found at the terminal ends of the sloughs.

These sorts of dynamics are what we think makes Suisun Marsh an extremely attractive habitat for fishes,” said Dr. Durand.   “In this model of fish distributions throughout the estuary by Dylan Stompe, you can see that for threadfin shad, Delta smelt, and striped bass, the locus of the fish distribution of each one of these fishes tends to be in and around Suisun Marsh, and we believe that the things that I’m discussing are driving at least part of that pattern.”

He presented two plots from the Suisun Marsh fish study; the slide on the lower left shows the results from otter trawls and beach sense conducted in the marsh.

One can see that the abundance of native fishes, shown in darker colors on both of these charts, has remained mostly stable since the 1980s, so there’s some resilience in Suisun Marsh that enables these species to persist,” he continued.  “As a result, the POD (pelagic organism decline) effects are not well detected in Suisun Marsh because as the population contracts, it tends to contract back to the marsh, and it becomes difficult to detect any kinds of declines. That’s certainly true of striped bass, which have declined estuary wide, but we have not seen that in Suisun Marsh, which seems to be prime habitat for these animals.”

Suisun Marsh vulnerable to sea level rise

Dr. Durand pointed out that Suisun Marsh is vulnerable to many risk factors, but he will only discuss the vulnerability to sea level rise for this presentation.

The plots below are of the northern end of Suisun Marsh.  The increasing dark colors are mean high or high water, mean monthly high tide, and maximum high tide in dark green.

You can see in 30 years the orange shows sites that will be breached by sea level rise,” said Dr. Durand.  “Other regions here will have all become inundated on the mean high monthly tide, but then will still drain throughout the tidal cycle so that they won’t be continuously inundated.”

In 2100, we found that the inundation pattern of the marshes such that on every high tide on a daily basis, all of these regions that were once dry or managed wetlands will be inundated,” Dr. Durand continued.  “We found that by 2150, which is very far out and extremely hard to predict, but if the patterns continue, as we predict, then the entire marsh will become more of an embayment than a marsh. And those very attributes that made Suisun Marsh desirable as habitat will be gone.”

In conclusion …

Dr. Durand then gave his takeaways.  “The POD species tend to be concentrated in the marsh as they have been for decades. Second, Suisun Marsh’s productivity derives from both naturalistic and managed landscape features. Sea level rise will wash out complex landscape features; these are the ones that support increased productivity in the marsh. And there’s limited room for uphill marsh migration because of urbanization. Finally, and notably, restoration success is always going to be context-dependent. It’s going to be dependent on elevation, connectivity, hydrodynamics, and the productivity that results from that.”

“And finally, this is something that I’ve been really pushing for many years, is that there’s a place for managed wetlands and working landscapes to support restored sites.”

QUESTION: Will sea level rise increase connectivity in the marsh?  Will that have negative or positive consequences?

Connectivity is not necessarily our friend anymore,” said Dr. Durand.  “There are certain aspects of increased connectivity that increase or improve the quality of wetland restorations. In other words, you have to have access to populations of fish that can access the resource to benefit from it. But at the same time, the Delta and Suisun Bay are increasingly dominated by invasive organisms with deleterious effects. And so, increased connectivity can increase the colonization of a region. So in some respects, increased connectivity could increase the vulnerability of that region; it could also decrease the geomorphic complexity that I hypothesize is very important in producing, sequestering, and exchanging with other areas. And so, you would lose this sort of mixed water residence time that allows these concentrations of productivity to occur. And of course, the managed wetlands would be underwater, and so I think it would become a much more depauperate system.”

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