Thread by @milesphd

Human activity plays a role in intensifying red tide blooms on the southwest Florida coast.
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New research from @ufcoastal, led by Miles Medina @milesphd. Free access here:
authors.elsevier.com/a/1enG8B8ccugB…
#redtide #florida

Karenia brevis (red tide) blooms occur almost annually along the coast of southwest Florida, where they adversely affect water quality, wildlife, ecology, public health, and economic activity. What drives these blooms? Is there anything we can do about them?

We know that red tide blooms develop naturally in the Gulf of Mexico, and we’ve long suspected that human activity may intensify blooms once they arrive on the coast, since coastal waters are artificially enriched with nutrients.

Linking blooms to human activity is challenging because the bloom process is complex and dynamic: Under different circumstances, the same input of nutrients might have different effects on blooms, depending on what else is going on in the system (“state dependence”).

We used methods that respect state dependence to investigate bloom dynamics and tease out systematic cause-and-effect relationships. We hypothesized (1) Nutrient-enriched flows intensify coastal blooms, and (2) This influence extends upstream to inland parts of the watershed.

Our study area—the Charlotte Harbor estuary—has suffered intense blooms during the past decade. This map shows the locations of red tide samples collected in the estuary (and within one mile of the coastline) between 2012 and 2021 (red dots).

The estuary’s watershed is huge, comprising 7 basins & Lake Okeechobee. Our analysis focused on the flow path from the Kissimmee River to the Lake to the Caloosahatchee River to the coast. As shown on the map, the basins are highly developed—predominantly urban and agricultural.

The flow path is also highly managed, and blue triangles on the map mark the control structures. This presents an opportunity: If managed nutrient-enriched discharges intensify coastal blooms, then managing nutrients and discharges differently might help to mitigate red tides.

Regarding our first hypothesis, we found that nitrogen inputs from the Caloosahatchee (S79) consistently intensified blooms (to varying extents) between 2012 and 2021. We also found that to a lesser extent S79 discharges also consistently facilitated blooms.

This plot shows the strength of the effects of S79 total nitrogen (gray curve) and S79 discharge (blue curve) on K. brevis blooms over time. Note that both curves are strictly positive, indicating consistent effects of intensification.

But there’s some nuance. Nitrogen and discharge seem to affect blooms through distinct but synergistic mechanisms: The effect of discharge was strongest during the earliest stages of blooms, and the effect of nitrogen was strongest during the growth and maintenance stages.

The strong early-bloom effect of discharge is intriguing and might be consistent with our group’s hypothesis that the river discharges can change estuarine hydrodynamics and effectively draw in and concentrate cells from the Gulf in the estuary.

That the effect of nitrogen is strongest later in the bloom cycle indicates that nitrogen inputs can help to maintain, intensify, and prolong them.

It’s important to remember that blooms occur naturally and that a whole host of factors govern their persistence/dynamics on the coast. Our results simply indicate that Caloosahatchee River nitrogen inputs have consistently played a detectable role in intensifying blooms.

Next (Hypothesis 2), we looked upstream. Can we trace S79 nitrogen (N) dynamics to N dynamics at Lake Okeechobee (S77), which discharges to the Caloosahatchee via a canal; and trace S77 N dynamics to N dynamics at the Kissimmee River (S65E), which drains into the Lake?

The answer to both questions was yes. We can trace the influence of nitrogen inputs from the Caloosahatchee River upstream to Lake Okeechobee and the Kissimmee basin. The results suggest that we need a comprehensive look at the entire watershed--coastal and inland areas.

The results also indicate some interesting time delays that are consistent with our understanding of the hydrology and biogeochemistry of the system, and you can check out the paper if you’re interested in the details.

To sum up: FL red tides occur naturally, but we can and do make them worse. The State of FL already acknowledges that nitrogen loads from the Caloosahatchee are too high, and our study points to yet another benefit of reducing nitrogen loads.

Co-author Dave Tomasko: “If we do the things that the State has already determined need to be done, we would not only benefit oxygen and water clarity and seagrass meadows in this estuary, but also likely help reduce the impacts of future red tides.”
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