December 19, 2014

Attack of the Jellifish–Is nitrogen pollution changing the food chain in the Long Island Sound?

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Is Nitrogen Pollution Changing the Food Chain in Long Island Sound?

By: Terry Backer and Julia Hyman

Ellen Thomas, PhD, a professor at Yale and Wesleyan University, released findings showing that nitrogen pollution, and perhaps warming waters, may have changed the basis of the food chain in the Long Island Sound. Her research shows that a key shift in populations of microscopic algae has been occurring over the last several decades. This fundamental shift in the Sound’s menu of who eats what is likely to cause many familiar species’ populations to decrease.

At the base of the Sound’s food chain are diatoms–single celled organisms that are found in fresh and salt water. Foraminifera, or forams, are also single celled organisms which adopt dazzling and intricate shapes with many looking like exotic microscopic seashells. Until recently the most common foram in the Sound was Elphidium excavatum, which eats diatoms. For 10,000 years diatoms were the most important microscopic algae suited to the Sound’s water temperatures and nutrient levels (including nitrogen). The marine biota that we know thrived, either directly or indirectly, on these organisms.

Historically, and up until the 1990s, E. excavatum was the dominate species of foram in the Sound. E. excavatum as well as copepods (small crustaceans eaten by many fish), rely on a diet of diatoms to maintain their population. Diatoms need not only nitrogen to grow (like other plants), but also need silica which they use to form a thin exoskeleton, just like grass contains some silica. Humans add a lot of nitrogen to the Sound from polluted runoff and sewage treatment plants, but no silica, so that ratio (N/Si) becomes unfit for diatoms to thrive so that now, other microscopic algae are out-competing diatoms. This is happening in parts of the Sound right now, especially in western Long Island Sound.

Excessive nitrogen from sewage treatment plants, polluted runoff from storm water and other sources has pushed the nitrogen-silica ratio out of whack, and thus lowered the population of diatoms available for E. excavatum. Other micro-organisms, like cyanobacteria, commonly known as blue-green algae, do not need silica to survive, and in fact many species of cyanobacteria thrive in high nitrogen environments, but these organisms are not suitable as food for many animals at the bottom of the food chain. Cyanobacteria are not a good food source for most organisms, the reason being that they’re just too small to eat.

As diatom availability drops so do populations of its consumers, eventually leaving a gap to be filled. This is where another species of foram steps up: Ammonia beccarii, which has a much wider food range. It eats diatoms and other algae. This wider diet gives A. beccarii a great advantage on E. excavatum. A. beccarii has always existed in the Sound, but in lower numbers compared to E, however, it is now becoming the dominant species in western Long Island Sound.

WHO CARES?

We do. The decreasing population of E. excavatum signifies a fundamental shift in the Sound’s food chain. Our good old, traditional E. excavatum is slipping to second place in favor of A. beccarii, the species more tolerant of excessive nitrogen levels. Small diatom-feeding organisms form the base of a food chain that begins with diatoms and ends with animals we like to eat like lobster, scallops, and many fish.

Attack of the Killer Jellyfish?

Here’s the rub: Jellyfish are one of the few organisms that have no problem eating blue green algae. Jellies can feed on smaller particles in the water. This shift in the food chain could mean jellyfish become dominant over the seafood we crave. Couple this with pH changes and warming waters and we have the stuff of a B-grade movie. Jellyfish don’t appear on many menus here…yet. They can be poisonous and, as many know, uncomfortable to swim with, to say the least. Also, jellies feed on the larval stage of many of the animals we eat, which further causes their population to drop.

These tiny forams are sending a message to us about our pollution in the Sound. Are we listening?

Foraminifera

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