The San Francisco Estuary watershed is characterized by periods of both wet and dry. But the dry seems to be occurring more frequently and intensely than in the past.
Researcher Rosemary Hartman led a collaborative team that looked at nearly five decades worth of observational information about conditions in the Estuary, from salinity to fish to flows. Equipped with an understanding of old and new data collection techniques and adding in the power of data science, this science synthesizing team is using historic data to help define drought today.
This episode is based on a special issue of San Francisco Estuary & Watershed Science Special Edition (Volume 22, Issue 1), which includes these stories:
- The Anatomy of a Drought in the Upper San Francisco Estuary
- Years of Drought and Salt
- Delta Blue(green)s
- Amazing Graze
- Dry Me a River
Science-in-Short is a quarterly podcast introducing scientists working on emerging topics in the San Francisco Estuary watershed. The podcast is written and produced by Ashleigh Papp with editing support from Ariel Rubissow Okamoto and the Estuary News Group, and music created by Peter Rubissow. Science in Short is funded by the Delta Stewardship Council. See all Science-in-Shorts here.
TRANSCRIPT
You’re listening to Science in Short, a quarterly podcast introducing you to scientists working on emerging topics in the San Francisco estuary watershed. I’m Ashleigh Papp.
Interviewer Ashleigh Papp: In today’s climate … let’s be honest, we don’t really know what to expect. Recent years have brought less and less freshwater to the San Francisco Estuary, and then other years, it seems like a nonstop deluge.
Our estuary’s watershed provides drinking water to about two thirds of Californians and irrigates millions of acres of farmland in the Delta and Central Valley. But the impacts of the three driest years on record, 2020, 2021, and 2022, are still being felt throughout this ecosystem and beyond.
Lots of people are working to tackle the challenges of prolonged drought … including a special group of scientists and resource managers. They’re looking at the past to answer questions that trouble us today. Will the Delta ecosystem survive into the next century? Will the waterways that we’ve come to depend on continue to flow? Will there be enough fresh water to go round?
Rosemary Hartman: My name is Rosemary Hartman, and I work for the California Department of Water Resources. My title is environmental program manager. I like saying that my job is to take all of the data that DWR and other state and federal agencies around the Delta collect on fish, on water quality, on invertebrates, on water flow, and use all that data in new ways to answer big important questions.
Papp: Hartman leads a collaborative science team with members drawn from a number of local agencies. Each group sends people out into the waterways and wetlands of the Delta and San Francisco Estuary to collect specific types of information … Things like temperature and salinity, water levels and flow, and of course, species population counts. And all of this information is recorded and stored.
And it turns out, dedicated researchers have been doing this type of work fairly regularly for a long time.
Hartman: Our fish data goes back to 1959 and water quality almost as far … Some of the invertebrate data since the 70s. So we really have a lot of power in that long term record.
Papp: About three years ago, Hartman got her chance to delve into this rich reservoir of data for the answers to a few really important questions… California’s Bay-Delta has been in an on-going drought for a number of years with fish populations in decline and a growing human population relying on fresh water. In early 2021, water experts realized that another dry year might be in store.
So, to better understand what was happening and how we might respond, state water managers proposed launching a drought monitoring plan, which included sending more people in the field to collect more data. But, Hartman wasn’t so sure.
Hartman: It’s like, ‘Okay, hold on, do we really need to do more? Instead of collecting new data to monitor this drought, let’s look at our old data and put this year’s data in context with our long term record … to see, what can we really expect from this drought? And what’s going to happen in the future?’
Papp: So Hartman assembled one of the largest teams she’s ever led, which became known as the Drought Synthesis Group. To get started, they met over Zoom (because the COVID-19 pandemic was still keeping most of us at home) and leveraged virtual tools to brainstorm how to go about making sense of over 70 years worth of Delta data.
Hartman: The great part is you get lots of really cool ideas, lots of really excited scientists. The not so great part is that you have a lot of scientists with a lot of really great ideas. And you get 20 people in a [virtual] room and then they have 25 different ways of doing it until like honing down everyone’s big ideas, like, ‘Okay, what can we do in a reasonable period of time?’ Because if we wanted to do absolutely everything, and look at all of the details, it would have taken forever and we had one regulatory requirement that said we need to report on what happened during the drought within a year. So I was like, okay, the guys like, let’s buckle down, let’s take six months to put all the data together, three months to analyze the data, three months to write it up. So we had a draft report within a year.
Papp: But in order to bring together so many data sets, collected by different people each doing things a little differently, the team had to normalize the data. For example, some trawling nets are bigger than others, some researchers would identify a species to a genus whereas others were more specifically tied to a genus and species.
And the stretch of time in which these data sets were collected is pretty extensive. So, how did they do it?
Hartman: So if we’re going to do a long term comparison, we need to make sure that our datasets account for the changes in those data over space and time. And the two basic ways to do that are you can use a really super fancy model with a whole lot of fancy things for temporal and spatial autocorrelation. Or you can aggregate your data into smaller bits.
We ended up taking either annual seasonal or regional averages within all of the data just to make sure that data was more balanced. And we started out just making those averages and then making a lot of graphs.
Papp: And then, Hartman took a step back and looked at the data.
Hartman: So once we made all the graphs and we figured out which ones we thought might actually be telling the story, then we had to do a little more digging to really make sure we had done all the proper quality controls, that we weren’t meshing two datasets with completely different methods.
Papp: Hartman and the team did a lot of additional research on statistical models to make sure that their methodology was robust and sound. And eventually, they published both the IEP report and a slew of papers in a special edition of the journal, San Francisco Estuary & Watershed Science.
Hartman: We all stepped back and said, ‘Now, what are the most interesting bits that people are going to want to know about long term?’ And so with that, we drafted the papers that ended up in the special issue: overall water quality trends, zooplankton trends, harmful algal blooms, the jellyfish and clams work, and then the synthesis paper that kind of tied everything together.
Papp: In the first paper, Anatomy of a Drought, Hartman and the team looked at water quality data from 1975 to 2021. What they saw is that drought periods affect the delta and upper estuary by making the water warmer, saltier, and clearer. While these findings were already in line with what they were expecting, their research confirmed that this has been a trend for nearly five decades.
And they learned a few new things too:
Hartman: Some of the coolest bits of that paper were actually the velocity data that we looked at, that not a lot of people look at very much. Because, it’s sort of an assumption, that if there’s less water flowing into the Delta, water velocity will be slower, like things will move less. And it might be easier, for example, for invasive plants to establish. This is a common misconception. What we found is that when you’re in the Delta, water flowing into the delta is kind of going like this: You’re rolling a marble down your marble ramp, whereas the tide is, you’re dumping a giant bucket of marbles down your marble ramp. So, you know, one or two less marbles is, meh. And so we don’t actually see much change to velocities in the Delta unless you’re in one of those crazy big flood situations.
Papp: In case you didn’t catch that, what Hartman found was that while the inflow and outflow of water is absolutely related to drought or wet periods, the velocity of the water isn’t.
The next paper dug into tiny little animals that drift in the water, called zooplankton. These little critters are the vital link in aquatic ecosystems, connecting the small floating plants and larger consumers. So understanding how drought conditions affect these species could really help the scientists and resource managers working to improve native fish populations too.
Hartman: We’ve been collecting zooplankton data for a long time and had a number of efforts to analyze it, but it does not have as much love as the fish data certainly, or even the water quality data. Here we were looking at four different taxa and they have different reactions to drought and different reactions in different parts of the delta.
Papp: What they found is that during a drought, the distributions and population sizes of the zooplankton change. Two of the species declined while the other two increased, with salinity being the biggest determining factor. We’ll get to what that all means in a bit.
Delta Blue(green), their next paper, looked at a cyanobacteria in freshwater systems that’s blue-green in color. In an ideal world, this bacteria is in a balanced foodweb. But, when water conditions change (like, during a drought) the scales can tip and lead to harmful algal blooms that can decimate an ecosystem.
Hartman: So that one put together even more different kinds of data, we used some satellite data, continuous water quality monitors, as well as discrete water quality grabbed samples from across the area. We did show that the biggest predictors of Microcystis, which is the biggest harmful algae we have here, are temperature and water clarity.
Papp: In Amazing Graze, Hartman and the team looked at how a few invasive species respond to droughts. They keyed into two types of clams and one jellyfish, all of which have been pretty understudied for the last two decades.
Let’s start with the clams …
Hartman: They’re doing a dance back and forth in drier years and wetter years. But there’s about a year lag, because it takes quite a while for a clam to establish. They’re not just floating around there in the bottom. They only float around when they spew out their babies in the spring and the fall. So if it’s a salty spring, those babies will settle further upstream, if it’s a wet spring, they’ll settle further downstream. And then the jellyfish in contrast, they are floating around in the water. So they’ll move upstream or downstream almost immediately as things get fresher or saltier.
Papp: And last but certainly not least, Dry Me a River connected all the dots and looked holistically at what the findings together actually meant to the larger water management community.
Hartman: The last paper was the synthesis paper where we tie everything together at a high level. In this one, we looked at a lot of the water quality variables, zooplankton, nutrients, chlorophyll, and several species of fish to see ‘Okay, how do they change during long term droughts versus wet periods? And is that, multiple dry years in a row versus multiple wet years in a row, different from just the flow abundance relationship on a single year kind of basis.?’
Papp: This birds-eye view of the historic data allowed Hartman and the team to put together a conceptual model.
Hartman: So, basic drivers in a drought are flow and air temperature. We know that [in] droughts, there’s less water flowing through the river and air temperature increases. As flow decreases residence time (so the amount of time the water sits there) increases and transport decreases. So you have less things moving from place to place, they spend longer in one place.
That in turn will cause particulate matter to settle out, increasing water clarity, air temperature will increase water temperature. The decrease in flow will increase salinity and all of that will influence transport of nutrients and food. That increased temperature will also increase the amount that the critters are growing and the amount they need to eat. So predators are going to be hungrier.
And in a lot of cases, some of the smaller fish, delta smelt in particular, when the temperature increases, they cannot get enough food to eat, to continue to grow and they’ll actually die just because there isn’t enough food to compensate for the fact that they’re having to work a lot harder. All this is different across the different parts of the Delta. And that leads to the response in the ecosystem where we have a change in the phytoplankton and zooplankton communities, decrease in pelagic fishes and decrease in salmon.
Papp: Many challenges remain for this field of data science in the Delta. But as dry years become more frequent, we must understand and choose how to best respond.
Hartman: The current toolkit we have for adapting to droughts is probably not going to be sufficient when we have droughts that are longer and harder and even more people living within the Delta’s watershed.
Everything becomes more limiting during droughts, water is kind of the bottom line. And all of those conflicts between users of the water, be the human or environmental uses, become exacerbated during droughts. And so the more we can predict exactly what’s going to happen, the better we can adapt to those droughts.
Papp: To learn more about this topic and see photos of the work, check out our website at mavensnotebook.com. Science in Short is a quarterly podcast written and produced by me, Ashleigh Papp! With editing support from Ariel Rubissow Okamoto and the Estuary News Group, and with funding from the Delta Stewardship Council. For Maven’s Notebook, I’m Ashleigh Papp. Thanks for listening!
