Urban water ecology

Sewage overflows are a fact of life in urban areas, and in many cities, excess sewage water enters streams and lakes with rain runoff. Although this pollution is harmful to most organisms, there is one group of insects that thrives on it: mosquitoes. Luis Fernando Chaves, a post-doctoral researcher at Emory University, and his team discovered mosquitoes in abundance in a sewage-contaminated stream in Atlanta, but rarely in a nearby clean stream. They also found that mosquitoes were largest in streams with high levels of organic minerals - in this case, nitrogen and phosphorous - that originated from the sewage treatment plants. Likewise, in laboratory experiments, mosquitoes reared in sewage overflow were larger than those reared in clean water.

To make matter worse, it's possible that other aquatic insects that would feed on mosquito larvae are poisoned in sewage overflow water. Many aquatic insects breathe underwater through gill-like structures, so excess nitrogen and phosphorous could be toxic to them. Mosquito larvae, however, breathe air at the water's surface through a specialised siphon. Chaves says that this combination of an increase in food and a potential decline in predators could be the key to these mosquitoes' success.

Freshwater fish, especially stream fish, rely on terrestrial insects as a portion of their food supply. But little is known about their importance to fish in lakes, where the size and shape of a lake can determine how much its fish rely on shoreline food sources. Tessa Francis, a post-doctoral researcher at the National Oceanographic and Atmospheric Administration (NOAA), wanted to know if the urbanisation of lakeshores affects the amount of terrestrial insects available as food to the lake's fish inhabitants. To answer her question, she analysed fish stomach contents over the course of a year in four Pacific Northwest lakes, surveyed fish in 28 Pacific Northwest lakes and compiled published data on fish populations in lakes across North America.

At undeveloped lakes, insect outbreaks often happen in pulses, in which insects emerge over a short time period, but Francis and her team found no signs of these pulses in highly developed areas. This disparity was apparent in fish food availability: In the four lakes, terrestrial insects comprised up to 100 percent of the diet of fish in undeveloped lakes, in contrast to a maximum of 2 percent in developed lakes, a pattern that was also apparent at the regional and national scale.

Francis' research also showed that trout in developed lakes had a 50 percent lower daily intake of energy. Lower energy intake can slow growth and compromise fish reproduction, she says, which will ultimately lead to population declines. But she emphasizes that even a small amount of shoreline vegetation can serve as insect habitat.

Having a garden on your roof isn't just nice for a garden party; it can make your city more environmentally friendly. Many American cities are beginning to incorporate greenroofs into their planning ordinances because they recognise that, planting a rooftop garden can offset heat, increase city biodiversity and decrease stormwater runoff. This runoff can be problematic in cities where rainwater is funneled by streets and parking lots directly into streams, carrying with it chemicals and debris and increasing the risk of flash floods.

But the plants on greenroofs can absorb some of this water - 'like a sponge being saturated,' says Olyssa Starry, a graduate student at the University of Maryland at Baltimore County. Starry studied a greenroof atop a Baltimore building in comparison to a similar building without a greenroof to determine how well the roof would absorb water from frequent storms. By measuring water flowing out of building downspouts, she found that the greenroof retained from 30 to 75 percent of water from storms, compared to a negligible amount retained by the building with no greenroof.

Although her results are preliminary, Starry thinks that cities can reap benefits from making greenroofs a part of their building requirements, as cities like Toronto and Berlin have recently done. Using GIS satellite imagery, she estimated the number and area of buildings that could hold greenroofs within one watershed in the Baltimore area. If all these roofs were greened, she says, the city could save the watershed 8 million gallons of water per year, or about 10% of its yearly water loss.

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