Anglès, S., Jordi, A. and Campbell, L. (2015), Responses of the coastal phytoplankton community to tropical cyclones revealed by high-frequency imaging flow cytometry. Limnology and Oceanography, 60: 1562–1576. doi: 10.1002/lno.10117
The effects of episodic climate events like hurricanes are easy to see on land: windows are blown out, boardwalks washed away, and trees knocked over. Such storms also have ramifications for aquatic ecosystems, altering the physical and chemical environment experienced by sea creatures. Unlike the aftermath of a storm we experience on shore, the oceanographic conditions, and the associated responses of the biological community, change rapidly and are often difficult to measure.
Monitoring the response of plankton populations to environmental perturbations is particularly important. Plankton are the ocean’s smallest inhabitants, with sizes ranging from microns to centimeters. These tiny organisms have an outsized impact on the rest of the ecosystem: they form the basis of the food web and influence biogeochemical cycling. Understanding how plankton respond to storms could help illuminate how the rest of the ecosystem is impacted.
Observing plankton is a challenge because of their small size and how rapidly their populations can vary. Traditional plankton sampling techniques usually involve dragging a net through the water and counting the organisms that get trapped. While effective, net sampling is labor and time intensive. Furthermore, it only gives scientists a snapshot of the community at a particular time and place. In order to study how plankton respond to a storm, researchers would have to take multiple net tows before, during, and after the event.
Scientists at Texas A&M’s Department of Oceanography used a new tool, the Imaging FlowCytobot (IFCB), to probe the plankton community response to storms. The IFCB is a specialized, in situ, imaging flow-cytometer. Once every 20 minutes, it draws in a small amount of water from the ocean, pumps it through a flow channel, and takes pictures of all the organisms in the sample (fig. 1). The instrument saves all the images and produces an hourly count of plankton that is collated into a time series of plankton abundance. An IFCB has been continuously collecting data at the entrance of the Mission-Aransas estuary in the Gulf of Mexico since 2007.
Sívila Anglès, a scientist at Texas A&M, used the resulting time series to examine the planktonic response to four tropical cyclones that made landfall in 2010. The first two storms, Hurricane Alex and Tropical Depression 2, arrived in late June and early July. Tropical Storm Hermine and Hurricane Karl arrived in September. As these storms passed, nearby research platforms recorded a suite of meteorological and hydrological data.
Anglès and her team noted that the plankton abundance rose sharply after the storms by looking at the hourly IFCB counts. They compared the development of the population increases to the environmental metrics to identify two main physical drivers: storm surges and freshwater discharges. “Storm surge” refers to elevated sea levels resulting from high onshore winds as a hurricane passes. “Freshwater discharge” is the increase in freshwater flowing to the ocean from streams driven by heavy rains from the storms.
The peak of the plankton population consistently lagged the storm surge by about two days and the freshwater discharge by about five days (fig. 2). Anglès explains that both phenomena bring an influx of nutrients, such as nitrogen and iron, which plankton need to grow. Storm surges do this by mixing the ocean and bringing nutrient rich deep water to the surface. While winds are stirring the ocean, rain from storms sweeps nutrients off the land and into streams. The streams then deposit the nutrients in the ocean as part of the freshwater discharge.
These results explained some of the overall patterns relating cyclones to plankton dynamics. But Anglès wanted to dig deeper. To do so, she turned to the images the IFCB captures. The researchers sorted all the pictures into 23 plankton categories using an automated, computer classification system. Instead of just having a single metric of plankton abundance, the group had 23 species-specific time series to examine. The scientists could then tease out the details of the population dynamics associated with the elevated nutrient levels from the storms.
Anglès and her colleagues found that the storm surges and the freshwater discharges caused different kinds of plankton to dominate the community. After the storm surge, the population was mostly comprised of a type of fast-growing phytoplankton called diatoms (fig. 3). Previous studies have shown that diatoms respond rapidly to the primarily inorganic nutrient pulses from upwelled deep water. Anglès also found that the observed number of a benthic, or bottom dwelling, diatom increased after the storm surge. That suggests that the high winds from the hurricanes also resuspended matter from the bottom of the estuary.
Dinoflagellates (fig. 4), a different kind of phytoplankton, began to dominate the system as the effects of the storm surge eased and the freshwater discharge entered the ocean. Anglès notes that this is to be expected, as dinoflagellates are known to grow well in the presence of organic compounds such as those found in river runoff. Furthermore, dinoflagellates tend to thrive in the low salinity environments such that occurs when a large freshwater pulse mixes in an estuary.
This work sheds light on how the very bottom of the marine food web responds to big atmospheric events. Rapid, short-term shifts in plankton community composition, like those observed by Anglès’ team, could alter how energy is transferred to higher trophic levels. Anglès’ study was not able to conclude whether the effect of the storms will ultimately enhance or decrease the amount of energy moving up the food web.
Anglès work underscores the need for more high-resolution studies of the marine environment. As the climate continues to change, and large storms become more frequent, it will be especially important to understand the mechanics of the marine food web. Indeed, to better understand how the whole ecosystem responds to major storms will require a larger effort involving observations at many scales.