SoMAS graduate student Jennifer George inspecting the contents of a net tow during the February 2010 spring bloom in Long Island Sound.
Seawater temperatures in Long Island Sound appears to be increasing over the past three decades, as has been observed elsewhere along the northeast US coast over the same period. However, the warming has not been consistent year-round. Most of the increase has taken the form of higher seawater temperatures in the winter, especially in the bottom waters; average summertime temperatures in the Sound have changed little over the past three decades. Observers generally interpret the warming in the Sound as a local manifestation of a broader phenomenon – – global warming. Because so much in the natural environment is temperature-dependent, SoMAS scientists Drs. Darcy Lonsdale and Christopher Gobler and their students are examining whether warmer winter seawater temperatures may be affecting the timing and intensity of a critical and well-documented aspect of the Sound’s environment: the spring bloom. The annual spring bloom of phytoplankton impacts a wide range of processes and characteristics in the Sound, from food web dynamics to the onset and severity of summertime hypoxia (low oxygen).
It has been demonstrated in other temperate coastal waters that the winter-spring bloom is suppressed during warm winters, and that zooplankton abundance increases while nutrient levels in the water column remain high. Is increased zooplankton grazing the mechanism behind bloom suppression during warm winters?
With support from the EPA Long Island Sound Study, the Lonsdale/Gobler research team is about to begin its second field season examining the effect of seasonal temperature on zooplankton grazing during the spring bloom in the Sound. In the first year, sampling was conducted at two sites in central Long Island Sound – – Middle Ground between Bridgeport, Connecticut and Port Jefferson, New York, and a site just northwest of Port Jefferson Harbor. This winter, additional sampling locations will be added including Execution Rocks near Port Washington in the western Sound. Once the spring bloom begins in early 2011, the research team will take weekly samples for as long as the bloom persists. In addition to the field sampling, the effect of temperature on zooplankton grazing will be assessed experimentally through controlled laboratory trials.
This is the “spring” bloom???
The spring bloom, or the seasonal growth of phytoplankton, typically occurs in most temperate ocean ecosystems between January and March. The seawater begins to heat up and phytoplankton are trapped near the water’s surface with just the right balance of light and nutrients. The convergence of these conditions leads to the rapid growth of phytoplankton and the creation of a higher than average biomass. Says Dr. Gobler, “Our hypothesis is that, in a warm winter, zooplankton grow quicker, graze (on phytoplankton) faster than they would otherwise and are able to essentially keep the spring bloom from getting too intense.”
The spring bloom is important, because it is the source of the majority of the carbon that sinks to the bottom of the Sound, where it serves as a food source for organisms. On a global level, phytoplankton act as a biological pump, capturing atmospheric carbon dioxide from the surface ocean and carrying it into the deep ocean. This process is especially important in the spring. In the summer, phytoplankton are typically smaller and consumed more quickly, leaving little carbon to sink to the bottom and feed organisms. During the winter, the relatively low amount of sunlight available to phytoplankton restricts their growth.
Phytoplankton are at the base of the marine food chain and the reproductive cycles of some species of fish and zooplankton are in sync with the timing of the spring bloom. At the spring bloom’s start, there is an abundance of nutrients. Gobler said that an important aspect of the study is to monitor nutrient levels as well as zooplankton and phytoplankton abundance to assure that it is zooplankton and not nutrient abundance that controls the spring bloom. “But, looking at historical records, the data shows that in these warm winters, when there’s not a spring bloom, there’s actually plenty of nutrients around,” said Gobler.
Dr. Lonsdale said their project is significant because it will examine how marine food web dynamics could change if global temperature increases. “The effect of temperature on plankton dynamics is an indicator of possible changes to come,” she stated.
There is no set date for the bloom in the Sound to begin; it started last year in early February. Regardless, the research team is prepared. “Whenever it happens this year, we’re ready,” Gobler said.
