SCIENCE SPOTLIGHT: Making a mountain out of molehills: Synthesizing eight datasets yields Delta food web insights

The April Delta Lead Scientist’s Report included a presentation by Dr. Denise Colombano on a synthesis project that used eight different datasets from 40 years of monitoring data to explore the importance of various drivers of food web dynamics throughout the Delta.  The study is just one of the products from the Delta Science Program’s partnership with the National Center for Ecological Synthesis and Analysis, or NCEAS.

Synthesis is the process of combining disparate sources of information or data to see the bigger picture and gain new insights.  The Delta Reform Act mandates synthesis; it’s in the Delta Plan, the Delta Science Plan, the Science Action Agenda, and the State of Bay-Delta Science, an issue of papers that come out every two years.  Synthesis is a core function of the Delta Science Program.

“Synthesis has allowed us to advance informed decision-making and ecosystem-based management in support of the coequal goals,” said Dr. Colombano.  “And it’s cost-effective.  New insights gained from synthesis have the power to transform our understanding of the Delta, but it only counts for about 4% of the science funding expenditures, making it a good bang for your buck.”

The Delta Science Program has a partnership with NCEAS, a world-renowned center for advancing open science.  Open science is a global movement based on shared access, reproducibility, transparency, and inclusivity in science.  NCEAS workshops provide high-quality training in data science and statistics; participants can immediately apply new skills to synthesize existing data and produce relevant research for management.

The first cohort of participants were early career researchers from nine different agencies and universities, of which Dr. Colombano was one.  “I benefited greatly from the training that I learned there, and I jumped at the chance to apply for this position as a synthesis scientist in the science program,” she said.  Other partners from the Delta Science Program include

The findings have been published in the Journal of Ecology.  The paper, Evaluating top-down, bottom-up, and environmental drivers of pelagic food web dynamics along an estuarine gradient, integrates and synthesizes over 40 years of data from seven monitoring surveys, four regions, and 11 taxonomic groups, resulting in a single long-term dataset that allowed them to model the estuarine food web in ways that had not previously been possible.

The team sought to answer a longstanding question: What drives pelagic food web dynamics across the estuary?

There are several hypotheses as to what is driving the Delta’s food web.  Top-down control refers to the number of predators in the system, such as large striped bass, controlling the number of prey through direct consumption.  Bottom-up control is when the availability of food resources, such as phytoplankton, tiny organisms that make up the base of the food web, are controlling the number of consumers, such as zooplankton or forage fish.   An alternative hypothesis is that the environment may control organisms through direct effects based on physiology, such as tolerance to salinity or temperature.

So, the team built and tested a model of the food web based on published literature and expert opinion and included all three drivers in a single model.  Dr. Colombano explained the model as such:  “The environment may affect the food web directly, nutrients may affect phytoplankton, phytoplankton may affect zooplankton, zooplankton can affect itself because there are predatory zooplankton and there are herbivorous zooplankton, so they eat each other, and then zooplankton may affect estuarine forage fishes, and predator fishes, and so on.  We also added clams, which can filter phytoplankton and competition from other fish species.”

The team used eight different datasets from various state and federal long-term monitoring programs to create a dataset going back to 1980 that covers the ecologically distinct parts of the Delta from the freshwater to saltier areas, which were then used to test the hypotheses.

Key findings and takeaways

“All three drivers were important in the models; we didn’t necessarily expect that,” said Dr. Colombano.  “There was a top-down effect of clams on phytoplankton.  There was a bottom-up effect of phytoplankton on zooplankton, zooplankton on each other as we hypothesized, where predatory zooplankton consumed herbivorous zooplankton, and then the zooplankton on forage fishes.  There was also an environmental effect.  One example is turbidity, which is water cloudiness, on forage fishes directly.”

Although the model and the data are extensive, the team found insufficient data to determine the effects of large fishes, such as large striped bass.  “That has to do with the fact that many sampling programs don’t adequately catch them.  They’re large, they swim quickly.  A lot of the times, we’re catching smaller fish.  So that’s a data gap,” said Dr. Colombano.

“Another data gap is how we used phytoplankton data,” she continued.  “We use chlorophyll a as a proxy for phytoplankton.  We think that going forward, having phytoplankton species would provide better quality data for this model.”

The key takeaway is that by synthesizing eight sources of existing data without collecting new data, new insights were revealed that were not previously known or quantified, Dr. Colombano said.  “For instance, we didn’t know before that the effects of these drivers could be similar to each other in the Delta.  And the science community had not yet had this empirical demonstration of phytoplankton to zooplankton to fish.  We hypothesized it for a long time, but we were actually able to statistically detect this relationship, which is really helpful for the modeling and management of food webs.  So, the modeling results support the idea that ecosystem-based management can achieve desired food web responses.  And overall, these results are relevant for things such as flow releases, habitat restoration, and permitting decisions.”

Dr. Colombano closed by noting that other products will be forthcoming from the NCEAS partnership.  For more information on the NCEAS partnership, check out the blog post, Making Every Day Earth Day: Analyzing Data with the National Center for Ecological Analysis & Synthesis.

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