by Alejandro Baladrón Julian

Urban land expansion and its multiple associated human activities cause extreme environmental disturbance in watercourses. When aquatic habitats become exceedingly polluted, not even highly tolerant organisms can survive. These organisms are important, acting as natural cleaners to maintain basic ecosystem functions. We are faced with the question: how can we establish minimum quality conditions in urban streams to protect our vital natural cleaners and ultimately our streams?

Humans: The bad guys

Freshwater invertebrates in urban streams panic every morning. Humans are using their toilets again! Stormwater pollution from urban areas is the main disturbance affecting water quality in many U.S. rivers. Physical and chemical parameters in waterbodies are adversely impacted by the periodic discharge of untreated water coming from combined sewage systems, which collect stormwater and sanitary sewage in the same pipes.

During storms, these sewer systems often reach maximum capacity and have to be bypassed, releasing untreated stormwater and wastewater directly into waterbodies. This untreated water impairs urban streams affecting water chemistry parameters, including dissolved oxygen, PH, conductivity, and turbidity. As a result, the river’s biota is threatened as organisms reach their tolerance limits beyond which they cannot survive, grow, or reproduce.

In addition to combined sewer outfalls (CSOs), there are also non-point pollution sources contributing to below standard water quality of many freshwater waterbodies. Stormwater runoff not collected by sewer systems can drain as overland flow and reach streams dragging oil, grease, litter, sediment, and other common pollutants from roads and hard surfaces. Sanitary sewer breaks and illegal wastewater connections are two other common non-point pollution sources.

But human-induced river pollution doesn’t end at storm water pollution. Degradation of the riparian corridor due to urban development (i.e., road building, stream channelization, alteration of the stream’s riparian zone, and many others) cause critical modifications in environmental conditions affecting the survival of many organisms. Urban development degrades riparian ecosystems by increasing the fine sediment deposited on the riverbeds and removing critical stream vegetation. Trees, shrubs and grasses act as a sediment trap. If they are removed, excess sediment fills the space between rocks in the riverbed reducing available space needed for reproduction and survival of biota.

In addition, fine sediments affect macroinvertebrate respiration by clogging their lungs. Removing stream vegetation also jeopardizes the health and biological integrity of rivers by eliminating the overhead canopy, which provides shade to control water temperature and supplies leaf litter that enters the aquatic food chain.

Bugs: The good guys

If you still remember the giant alien cockroach, “Edgar the bug”, in the movie, Men in Black, it will be really hard to convince you that a bug can be a good guy. The truth is there are amazingly skilled macroinvertebrates in urban streams that survive, grow, reproduce, and provide environmental services, such as reducing water pollution and maintaining the ecosystem food chain. Aquatic worms, copepods, some taxa pertaining to the order Ephemeroptera, families of Trichoptera such as Philopotamidae, and families of dipterans, such as Simuliidae, and some Chironomidae exploit organic particles entered in streams by untreated water discharges. These macroinvertebrates constitute the bulk of the “freshwater bug resistance” in polluted rivers and play an important role in nutrient recycling by consuming and transforming organic matter that would otherwise deplete the water’s oxygen level.

Likewise, macroinvertebrates serve as a link between their food and fish, as well as other higher trophic level vertebrates. If there were no macroinvertebrates resistant to water pollution and other habitat stressors, several food chain linkages between dead organic matter and primary producers such as algae and macrophytes, would be disrupted in urban streams, and the transfer of energy to higher trophic level vertebrates would be short-circuited.

Meet the “freshwater bug resistance”

Organisms that can withstand and survive (up to certain levels) in polluted streams and rivers are the key members of the bug resistance. Enrollment is completely forbidden to sensitive taxa! Thanks to tolerant macroinvertebrates we have a “natural sewage treatment system” in streams capable of digesting pollutant loads of organic material. This biological process not only reduces water pollution, but also makes nutrients in organic matter available for plants and animals. The availability of tolerant taxa and thus the water quality level will vary among streams depending on the type of pollutants and water pollution loads discharged into the water.

Blackflies (family Simuliidae) are collectors with a large, net-like feature on their heads, which they use as a sticky screen to gather food. They stick to solid surfaces, such as large rocks, and filter suspended particles in water, including those coming from sewer discharges. However, their most unique function is their ability to produce and manipulate water current movements around them. Their bodies generate two swirling currents, or vortices: one of these vortices remains in the slow-moving layer of water close to the rock surface; the other is drawn upwards along the body collecting food particles. Their ability to manipulate water current makes these animals extremely efficient as filters and cleaners of urban watercourses affected by CSOs.

Chironomids (family Chironomidae) are commonly known as non-biting midges. These animals, related to the order Diptera, can be found in the most adverse habitat conditions, such as sandy patches with overlying fine organic material, muddy lake beds, stagnant waters or waterbodies affected by heavy metals. Blood-red Chironomini, or “bloodworms”, deserve special mention. These guys contain the red-blood pigment hemoglobin, which enables them to absorb oxygen from the water even when it is present in very low concentrations. As a result, these animals are extremely resilient and can recycle organic matter despite severely depleted oxygen levels due to high pollutant concentration.

Hydropsychids specialize in filtering fast flowing waters, living on rocks, boulders, or submerged logs. They build capture nets for filtering particles from the water column, including algae, organic particles, and small organisms. Although they are less resistant than blackflies, they are efficient filters and tolerant enough to pollutants to play an important role in cleaning moderately disturbed rivers.

Oligochaetes move by stretching and pulling their body along in a worm-like fashion, which allows them to go through nasty amounts of decomposing organic matter. Their capacity to tolerate anoxic conditions allows them to subsist inside organic detritus and its associated microflora. This ability along with the capacity to survive under extremely high water pollution levels put them at the top of the curve as nutrient recyclers.

Scuds or “side-swimmers” are crustaceans pertaining to the order Amphipoda (family Gammaridae). They survive well in a wide range of environmental conditions and use a variety of feeding methods that make them flexible in selecting habitats and food. As a result, they can easily adjust to disturbances, survive, play a role in recycling dead organic matter, and serve as food for animals in higher trophic levels of the food chain.

Bugs are waiting for back-up…

Despite the negative and often grotesque portrayal of bugs in pop culture, in reality, humans are the true villains. The members of the freshwater bug resistance described here, are essential to processing pollutants and maintaining fundamental river ecosystem functions. But, unfortunately they cannot perform miracles. We need to recast ourselves as the good guys and work to restore and protect our valuable urban rivers.

Preserving and restoring remnants of native riparian forests as well as developing “end pipe” stormwater treatments are critical to maintaining fair water quality levels in urban waterbodies and making them more self-sustaining and resilient to disturbances, as well as more suitable for sensitive biota. In this regard, vegetative buffers are fundamental in moderating the impacts of land use changes (i.e., urban development adjacent to streams) reducing water stress by removing soluble nitrogen, phosphorus, toxicants, and suspended sediments.

…and Great Ecology won’t leave them behind!

Learn how we used water quality and habitat assessments in the Ramble and North Woods of Central Park to determine the best approach to improve the health of the watercourse. We’re one step closer to restoring the natural remediation efforts that tiny bugs at Central Park perform every day.

References: 

Zinger, Y., Fletcher, T.D., Deletic, A., Blecken, G.T. and Viklander, M. 2007. Optimisation of the nitrogen retention capacity of stormwater biofiltrations systems. Novatech 2007 Sustainable techniques and strategies in urban water management. Lyon, France.

Walsh, C.J., Sharpe, A.K., Breen, P.F. and Sonneman, J.A. 2001. Effects of urbanization on streams of the Melbourne region (Victoria, Australia). Benthic macroinverte- brate communities. Freshwater Biology, 46: 535-551.

Walsh, C.J., Roy, A.H., Feminella, J.W., Cottinghamm, P.D., Groffman, P.M. and Morgan II, R.P. 2005b. The urban stream syndrome: current knowledge and the search for a cure. The North American Benthological Society, 24: 706-723.

Townsend, C.R., Begon, M. and Harper, J. L. 2008. Essentials of Ecology. Third Edition. Blackwell Publishing.

Vannote, R.L., Minshall, G.W., Cummins, K.W., Sedell, J.R. and Cushing, C.E. 1980. The river continuum concept. Canadian Journal of Fisheries and Aquatic Sciences, 37: 130-137.

PlaNYC. 2008. Sustainable Stormwater Management Plan. New York City Mayor’s Office of Long-Term Planning and Sustainability. New York, New York.

Cummins, K.W. and Klug, M. J. 1979. Feeding ecology of stream invertebrates. Annual review of ecology and systematics, 10: 147-172.

Merritt, R.W. and Cummins, K.W. 1996. An introduction to the aquatic insects of North America. Third Edition. Kendall/Hunt Publishing Company. Dubuque, Iowa.