Climate change and the Delta: What do we know about climate driven variability of the Bay-Delta ecosystem?

Dr. Jim Cloern briefly reviews what we know and what we don’t know about how climate change will impact the Bay-Delta

Climate change is just one of many threats to the Bay-Delta, and it has the potential to alter the ecosystem on a vast scale. The May 6 seminar, New Approaches for Responding to Climate Change in the San Francisco Bay-Delta presented by the UC Davis Center for Aquatic Biology and Aquaculture and the Delta Science Program, examined the latest research and plans to adapt the larger San Francisco Bay-Delta estuary to the impacts of climate change. Speakers during the day addressed a variety of topics, from sea level rise, and carbon sequestration of marshes to how to restore resilient landscapes that can adapt to climate change. Other sessions looked at shoreline and climate adaptation projects in the San Francisco Bay as well as permitting and funding strategies.

This week, Maven’s Notebook will be covering portions of the seminar: we’ll look at sea level rise impacts in San Francisco Bay, how marshes are responding to sea level rise, as well as carbon sequestration as well as how restoration projects in the Delta and in the San Francisco Bay can be built to be resilient in the face of climate change. But first, coverage kicks off with an introduction to climate change impacts on the Bay-Delta system, given by Dr. Jim Cloern, a research scientist at the USGS who conducts research on the ecology and biogeochemistry of estuaries to understand how they respond to climatic variability and human disturbance.

(Note: You can view all presentations from the conference at this link.)

When thinking about how the Bay-Delta ecosystem might respond to climate change in the upcoming decades, one important step we can take is to review what is already known about how the system responds to the natural variability of the climate system, began Dr. Jim Cloern. “I’m not going to tell you anything that you don’t know already – this talk is organized around a number of lessons that we’ve learned from long-term observations in this ecosystem,” he said.

Climate system variability and time scales

The climate system drives variability in the Bay-Delta ecosystem at all different time scales, from hours to centuries, he said. He then gave some examples.

The first example is a record of dissolved oxygen in one of the salt ponds in South San Francisco Bay, showing oscillations of dissolved oxygen between the dark period and the light period. “You can see these oscillations of dissolved oxygen that are cued to the diel light cycle,” he said. “So during the dark, respiration draws down dissolved oxygen and when the lights come on, photosynthesis exceeds respiration and dissolved oxygen builds. This is an example of hourly scale variability of a climatic factor, in this case, solar radiation.”

He noted that if we take the daily averages of those dissolved oxygen concentrations and do a time series over days, we see variability at the next time scale, the variability at the time scale of several days, so events can have a duration of several days.

He then presented a time series several months of dissolved oxygen daily average observations, noting there is a lot of variability in those observations, as well as three events of hypoxia (or very low dissolved oxygen) during this record. “The largest of the hypoxic events that lasted for about a week and coincided with a climatic event – a heat wave. The temperature reached very high levels for several days, and dissolved oxygen drew down in this pond,” Mr. Cloern said. “This is a response to system metabolisms, so as temperature goes up, system metabolism goes up, and the total respiration over the course of the day exceeds photosynthesis and so we have these hypoxic events.”

The other two hypoxic events weren’t related to temperature but rather were related to changes in solar radiation, Dr. Cloern said. “During cloudy conditions, there isn’t enough sunlight energy to drive net photosynthesis so the net respiratory loss of dissolved oxygen draws down DO during these events,” he said. “Here’s an example of daily scale variability that’s tied to daily scale fluctuations of the climate system, in this case air temperature and solar radiation.”

Dr. Cloern then gave another example of an event scale process tied to a weather event. “In September of 2004 we observed something that we’d never seen before – a large dinoflagellate red tide in central and southern San Francisco Bay and this event coincided with a heat wave,” he said, noting that on the graph, the gray bar represents the daily air temperature and the diamonds represent four consecutive days of record high temperatures. “This heat wave coincided with this period of unusually calm winds and really stagnant atmospheric conditions, so the combination of large heat input to the water coupled with weak wind mixing allows the water column to become thermally stratified. Phytoplankton that are in the surface layer stay in this shallow surface layer, have high light exposure, high nutrients and biomass can build very quickly. So another example of an event scale, biological response to event scale changes in local weather.”

An example of seasonal scale variability tied to the climate system is the seasonal pattern of precipitation and runoff in the watershed and freshwater inflow to the estuary. He presented a record of the Delta outflow index during the winter of 1996, noting that most of this variability especially during the wet season is associated with variability of precipitation and runoff. “Of course we manage water and have a big influence on these patterns, but these events are tied to the natural seasonal cycle of precipitation and runoff,” he said.

He noted that the bottom figure is a contour plot showing the distributions of surface salinity along the whole estuary; dark colors are high salinity and light colors are low salinity. “During these pulses of high flow during the winter and spring, we see these light colors, so there’s dilution of salinity across the entire estuary all the way down to the South Bay, so Delta outflows have an effect all the way into the southern part of San Francisco Bay,” he said. “As river flow recedes, salinity starts to increase … so during high flow events X2 is pushed seaward; as river flow recedes, salt starts to migrate inland and is up in the Sacramento River during the low flow season.”

Much of the year-to-year variability in biological communities is tied to year-to-year variability in the climate system. Dr. Cloern presented a figure from a well-known paper published by Alan Jassby in 1995 that identified relationships between annual abundances of organisms from phytoplankton to shrimp to some species of fish and X2, this measure of the salinity distribution in the estuary. “For all of these organisms, there is a correlation between year-to-year variability in outflow, salinity, X2, and population abundance or biomass of organisms,” he said.

“We’re beginning to learn a lot now about how the Bay-Delta system responds to decadal scale variability,” he said, presenting a time series showing abundance of phytoplankton, chlorophyll, two species of shrimp, and other species in the salty part of San Francisco Bay. “For all of these communities, what we see is a two-decade period of below average biomass relative to the long-term mean, and then an abrupt shift up in the abundance or biomass of all of these communities. And that abrupt shift up took place a year or two after a shift in one of the important modes of climate variability, the North Pacific Gyre Oscillation.”

Dr. Cloern explained that the North Pacific Gyre Oscillation was in its negative phase for about seven years, and then in 1999 it abruptly shifted to its positive phase and it’s mostly been in its positive phase since then. “So the shift up in the abundance of organisms in the Bay coincided with a lag of a year or two with the shift in this important mode of climate variability across the North Pacific Ocean,” he said. “So here’s an example of really restructuring of biological communities in the salty part of San Francisco Bay that followed a restructuring in the distribution of atmospheric pressure across the North Pacific Ocean.”

There are also ties between climate and the Bay-Delta on the century time scale, which we know from the sediment record, Dr. Cloern said. “This photograph is a reminder that the climate system of this part of North America has exceptional floods with a recurrence period of every 150 or 200 years,” he said. “The last one was in 1862 when it rained for four consecutive days, the city of Sacramento was under water and San Francisco Bay was a freshwater body. The outflows from this runoff were so strong that they negated the tides, there were no tidal oscillations in the Bay.”

Mike Dettinger and Lynn Ingram examined proxies and sediment records to see how frequently these kinds of events have happened in the past, he said. “There are records in the sediment record of these events in the past going back almost two thousand years, and the recurrence frequency seems to be on the order of about 150 or 200 years. So, the time is due, and many of us are wondering, not only how will the Bay-Delta system respond to the next megaflood, but what will it do to California and the western part of the U.S.? So we can say something about century-scale variability and the climate system and how it affects the Bay.”

A variety of climate change impacts

There is more to the manifestations of climate change than just sea level rise and temperature, Dr. Cloern pointed out. “We know that the Bay responds to fluctuations in solar radiation and winds—direction, speed, seasonal patterns of wind forcing of the Bay and the Delta,” he said. “Precipitation and runoff are tied to the climate system, and these longer period processes of climate variability operating across the Pacific Ocean.”

One thing that is tied to climate change but isn’t a climatic expression is the increasing concentration of carbon dioxide in the atmosphere, he said. “When CO₂ is dissolved in water it’s a weak acid, so one of the concerns about increasing atmospheric CO₂ in marine systems is acidification. And this is important for organisms that calcify … the main point is, we can’t just focus on sea level rise and temperature.”

Different processes over ocean and river basins

It’s also become clear in the last decade that the climate system has different expressions over the Pacific Ocean than it does on land, and estuaries are a real challenge, he said. “These are really challenging systems because they’re situated at continental margins and so they’re influenced both by processes on land and processes in the connected ocean,” he said.

He presented a slide with two charts, both indices of climate variability, one is an index of coast upwelling going back to 1946, and the bottom graph of the Northern California 8 station index, a measure of precipitation in the Sacramento River watershed. He noted that there is a characteristic pattern of variability in the coastal upwelling as shown on the top graph, and the bottom graph of the precipitation index which has its own characteristic pattern of variability.

“What’s really interesting to me is that if you do a cross-correlation analysis of these two time series, they’re completely uncorrelated at any lag,” Dr. Cloern observed. “So, what that tells us is that when we think about the climate system in the Bay, there’s more than one climate system: there are expressions of climate variability operating over the coastal ocean that are independent from the expressions of climate variability operating over land like precipitation. So there’s no linkage between annual variability and coastal upwelling, which has an effect on the Bay, and precipitation which also has an effect on the Bay.”

This has biological meaning, he said, presenting result from a recently published study of over three decades of fish sampling in the San Francisco Bay. “One of the results in this paper is that there’s a collection of species of fish that have high population abundance during years of strong upwelling, species like English sole and Bay goby; there are others, like white croaker and Pacific halibut, that have high population abundance during years of weak upwelling,” he said. “The same is true for these expressions of climate variability over land. So there are species of fish like longfin smelt and jack smelt that have higher than normal population abundances during wet years, and other species like Pacific sardine and Northern anchovy that have higher population abundances during dry years. So, these different expressions of climate variability over ocean and land have biological relevance for the estuary. So when we think about how the Bay-Delta system is going to respond to decades of climate change, we need to think about how this process of upwelling is going to change and how this process of precipitation is going to change.”

It’s more than just climate change

Climate change is not the only thing we need to be thinking about, Dr. Cloern said. “This ecosystem in particular has been transformed by a number of human disturbances,” presenting a slide with a number of graphs of things to be considered: one depicting the cumulative capacity of water behind dams that flow into the estuary showing that water is highly managed, another showing the increase in nitrate concentrations by a factor of ten, making this is a highly nutrient-enriched estuary; a graph showing how suspended sediment concentration has been halved in Suisun Bay; and the shift in the relative biomass of native versus introduced species of copepods in the low salinity part of the estuary, an expression of the community responses to species introductions.

“So when we think about the future, we need to think not only about climate change but all of these other human manipulations, and I want to mention one in particular,” he said, presenting a graph of the projection of the increase in the number of days in each decade in the upcoming century when water temperature in the Delta will exceed 25°C. “Days where water temperature exceeds 25° is relatively rare now, and that rare occurrence at the beginning of this millennium will progressively become more common over time. So what does that mean to the ecosystem?”

We have a lot to learn

“There are many different potential biological responses to this manifestation of global warming, increasing frequency of warm events, and as an ecologist when I think about this, I want to share with you the kinds of things that I think about when I wonder what this means,” he said. “The context for this is that we’ve enriched the estuary, the context for this is that by pumping water and putting barriers in place and taking them out, we greatly modify the flow patterns and the flow rates through the estuary and can have a big effect on things like water residence time.”

“I wonder when we couple the effects of increasing frequency of heat waves with a system that’s already nutrient-enriched where we are greatly modifying flows and increasing residence time, what are the implications of that for things like blooms of toxic cyanobacteria, like Microcystis?” continued Dr. Cloern. “This is an organism that does well in high-nitrogen environments, in warm water, in quiescent conditions with long residence time. So, when we think about effects of climate change we need to put them in this context of all the other changes that are taking place, and ask questions like, what does this mean for the potential for increased frequency, duration, magnitude of these toxin-producing blooms in the Delta, and then what does all of this mean for future balances between native and introduced species?”

So while we already know a lot about how this system responds to climate variability, we also have a lot left to learn, Dr. Cloern said, presenting a graph plotting Delta outflow against chlorophyll concentration during the spring months, and showing the size of the chlorophyll bloom in the south San Francisco Bay.

“In 1991 I published a paper where I claimed that the magnitude of the spring bloom is set by spring Delta outflow,” he said. “The explanation was that Delta outflow brings low-density fresh water into the South Bay, and that’s a source of buoyancy that can create salinity stratification that reduces vertical mixing, so phytoplankton contained in the surface low-salinity layer are exposed again to high light, high nutrients, their biomass builds. The more freshwater inflow, the stronger the stratification, the weaker the mixing, the bigger the blooms.”

“Since this paper was published, we’ve collected twenty-seven more years of data, and those are a robust test of this hypothesis that I published in 1991 and you can see that there is no longer a relationship between Delta outflow and size of the spring bloom,” he said “Two things stand out here to me: one is, almost all of the red points fall above that line. So that makes me wonder if something has changed in the system in the connection between flows, stratification, and biomass; and the other really intriguing thing is that we’ve seen years of large spring blooms during low or moderate Delta outflow. So there’s some other process going on here in addition to the spring source of fresh water that stratifies the water column. And I’m really scratching my head over this. I have not got this figured out. We have a lot to learn.”

“The last point that I want to make, is that all of these things that we’ve learned about how the Bay-Delta system responds to climate variability is the result of the fact that we have magnificent observational records, so now more than ever, it’s essential for us to keep these observational records going,” concluded Dr. Cloern.