Nuclear power generating stations are located along lakes, rivers or coastal areas because the facilities use water to cool the reactors. Nuclear generating stations exert several negative, positive, and neutral stressors on aquatic ecosystems that are connected to the plant. These stressors revolve around their cooling water intake structures. Cooling water intake structures move water through the plant to condense and re-use the steam generated to turn the turbines which produce electricity. This water is returned to the ecosystem as discharge. Nuclear generating station discharge increases the temperature, water flow, and adds chlorine and chlorination by-products within a certain distance of the adjoined aquatic ecosystem. The variability of water condition can have both positive and negative effects on biota living nearby the outtake and intake structures as well as abiotic processes (Table 1). Cooling water intake structures also cause entrainment and impingement on local fauna which is disruptive to population structure, size and genetic flow of many species living near the plant. Understanding the overall picture of ecosystem health when attached to a nuclear generating station can aid policy in creating less invasive and disruptive cooling systems in the future. It can also lead to predictions of ecosystem health after the decommissioning of the power plant. This is important in implementing how to best aid the adjacent systems in returning to their natural state or a close approximation thereof. This would be difficult without comprehending how and to what degree the generating station influences the ecosystem as a whole as well as its individual components. 2.0 Effects of nuclear power generating stations on abiotic components of aquatic ecosystems2.1 Redirection and usage of water sourcesNuclear power generating stations require a large quantity of water for their intake system. There are two types of intakes; once-through cooling (attached to rivers, oceans, streams) and closed-looped systems (cooling towers, recirculating) (Macknick 2012). For example, the Oyster Creek Nuclear Generating Station in Forked River, New Jersey, uses a once-through cooling system and includes two surface water intake structures. The circulating water intake withdraws up to 662 million gallons per day of water from Forked River to cool the main condenser at the facility. The second intake, the dilution water intake, withdraws 748.8 million gallons per day of water into the facility from the Forked River to dilute the thermal effects of the condenser cooling water. All intake water is discharged into a discharge canal, into Oyster Creek and Barnegat Bay (Martin 2010). In regards to this system, initially Forked River and Oyster Creek were not connected. In order to create a looping system of intake to outtake a confluence was created between the two rivers (Figure 1). This confluence reversed the direction of Forked River, changing the flow from east to west due to the strong intake system of the station. 2.2 Increased Water TemperatureSaenger, et al. (1982) and Laws (1993) showed water temperatures at effluent sites can increase by as much as 8°C-8.2°C while later studies showed a higher increase. Madden, Lewis and Davis (2013) conducted a review of wastewater temperatures from nuclear power generating stations in the United States from 1996 to 2005. They found that maximum reported temperature discharges averaged 37°C (1996–2005) and were 9.5°C (1996–2000) to 10°C (2001–2005) higher than maximum reported intake temperatures during the summer. They also noted 50% of all cooling system outflow exceeded 32?°C. In a similar study, temperatures were measured at intake, mixing point and outflow of a tropical power plant, the Madras Atomic Power Station (MAPS), which uses the coastal waters of the Bay of Bengal (Table 2).2.3 Chlorination at dischargeThe release of chlorine into coastal systems vary depending on country of origin and application type. Continuous low-level chlorination (0.1 mg 1-1 total residual chlorine) is common practice in Great Britain (Beauchamp 1969) while periodic chlorination (2-3 h per day) is common practice within United States stations (Becker and Thatcher 1973). United States concentrations of chlorine in discharged cooling waters have been measured in the range of 0.05-1.00 mg 1-1 total residual chlorine (Brungs 1973, Capuzzo 1980).2.4 Flow rate increase at discharge Flow rate determines which sediment types can collect and remain at the base of the river. High flow rates lend themselves to hard packed substrate, such as clay, whereas lighter flow rate allows lighter material, such as sand, to accumulate and remain. The discharge of a nuclear power generating station can be quite strong, with average flow rates reaching 45 m3s-1 (Figure 2, Saenger 1982).High flow rate in an initially sandy substrate or other loose substrate type will eventually change to a hard packed substrate. The sand and silt cannot withstand heavy flow rates and will eventually be carried downstream, mildly changing or completely clogging up downstream ecosystems depending on sand and/or silt quantity. Saenger (1982) shows this shift in Figure 3 from sandy mud to stiff clay over a 45 day period.3.0 Effects of nuclear power generating stations on biotic components of aquatic ecosystems3.1 Increased Water TemperatureFaunaThe effects temperature change has on ecosystems depends on a variety of variables; quantity of discharge, local climate and the biological functions and parameters of the environment (Schubel et al. 1978, Lardicci et al. 1999, Chou et al. 2004). The direct and indirect effects of increased water temperature of nuclear generating stations on phytoplankton, zooplankton, bacteria, copepods, fishes, crabs and benthic organisms has been widely studied (Coutant and Brook 1970, Gibbons and Sharitz 1974, Langford 1990, Krishnakumar et al. 1991, Hung et al. 1997, Yang et. al 2002). Temperature is an incredibly important environmental variable which affects the survival, growth and reproductive rates of aquatic organisms due to its effects on organism metabolic rate and control of dissolved oxygen levels (Kinne, 1970; Langford, 1990).Discharge temperatures can be well above tolerance levels of plankton and it has been suggested temperatures be limited to around 35oC. An increase in temperature can lead to an abundance of previously rare species and a decline in naturally occurring, previously abundant species. This change in species richness can range from benthic bivalves, to plankton, to pelagic fish populations (Thomas 1986, Teixeira et al. 2012). High temperatures also change growth rates in bivalves (Mercenaria mercenaria) within approximately a 1.6km radius of the Oyster Creek nuclear generating station specifically. They exhibited lower summer growth rate (10%-25%) and a greater number of growth breaks (2 to 6 more per clam) than those away from the creek (Kennish and Olsson, 1975). Warmer temperatures can also affect gametocyte health and reproduction rate. Cooling water discharge has been shown to cause oocyte degeneration in 50% of exposed common roach (Rutilus rutilus) females and decreased gonad size (Luksiene and Sandström, 1994). These negative physiological effects lowered subsequent fish fry populations.As stated previously, high effluent temperatures can be beneficial to rare or tropical species that normally would not be found in high numbers in this region. In 1980 two subtropical ship worms (Teredo bartschi Clapp and Teredo furcifera von Martens) and a polychaete (Ficopomatus enigmaticus) were found thriving near the effluent of the Oyster Creek Nuclear Generating Station (Hoagland and Turner 1980).It has been hypothesized by Jiang et al. (2008) that copepods may react differently depending on body length and that those accustomed to an estuarine system may acclimatize better to higher temperatures. These were laboratory studies where the water temperature increased at a constant rate of 0.1°C min-1.The situation in a wild setting near an actual powerplant may not reflect laboratory conditions. Temperature variability and the tolerance level of the organism is unique to each species. Increased temperature significantly affected Acartia tonsa (Copepoda), Crangon crangon (Decapoda) and Homarus gammarus (Decapoda), with a interdependent effect on chlorine sensitivity in C. crangon, but A. tonsa were affected only at temperatures higher than would be experienced in a normally-operating station (Bamber and Seaby 2004).FloraSeagrasses communities are similarly susceptible to changes in thermal upper limits. Thalassia beds have been shown to denude when subjected to an increase of 3oC or more and as many organisms rely on the seagrass this can have a cascading effect within the system (Thorhaug et al. 1978).Rates of photosynthesis has also been tested in phytoplankton that have successfully passed through the cooling system. Morgan and Stross (1969) demonstrated a temperature increase of 8° C when natural waters were 16o C stimulated photosynthesis and inhibited photosynthesis when natural waters were 20o C. Inhibition in warm waters was furthered once phytoplankton passed through the cooling system and stimulation never happened in cooler waters. 3.2 Chlorination at intake and discharge FaunaChlorine levels at current and past recorded ranges in nuclear generating station discharge can be toxic to marine zooplankton (Roberts et al. 1975, Capuzzo et al. 1976, Capuzzo 1979). Capuzzo (1980) demonstrated that free chlorine toxicity has no significance on mortality rates when copepods are still within tolerable water temperatures, but once near their maximum thermal limit the addition of chlorine drives an increase in mortality rates. This combination of heat and chlorine shows combinations of water quality variability to be as dangerous and perhaps in some cases more dangerous to fauna than on their own. The complexity of ecosystems makes it difficult to tease out individual stressors when they are able to work in tandem.Additionally, some species can tolerate higher levels of chlorine in the water than others. Copepod and shrimp larvae have high mortality rates when exposed to low levels of chlorine, but chlorine as no effect on lobster larvae even at 1 ppm (Bamber and Seaby 2004).FloraChlorination has been shown to have a negative effect on productivity in entrained phytoplankton. Chuang et al. (2009) demonstrates 0.2 ppm chlorine significantly suppressed phytoplankton productivity whether or not the water temperature was elevated though there was little effect on periphyton. A similar study also observed a large decrease (83%) in productivity in phytoplankton at the discharge when compared to the productivity of the intake (Carpenter et al., 1972).3.3 Flow Rate increase at dischargeFaunaChanges in flow rates due to cooling intake systems can completely change benthic communities. As sediment types shift from looser, sandy, mud to stiffer clay due to unusually high flow rates, the benthic community shifts to meet the requirements of the new substrate (Saenger, et al. 1982). Flow rate can also affect phytoplankton and periphyton at the intake and discharge areas of the plant. Phytoplankton chlorophyll a has been observed at lower levels at the discharge area than at the intake, however periphyton chlorophyll a were greater at the discharge area (Chuang et al. 2009). Inhibition in one group and stimulation in another can drastically change overall community structure. Flora Flow rates, as mentioned earlier, change the substrate near station discharge areas. This change can not only shift the benthic fauna community, but also the flora. As looser silt and sand shifts to hard packed clay or mud it may make it difficult or impossible for certain species of plant to thrive or even take hold within that substrate. The seagrass Zostera marina, for example, prefers loose silty layers of sand or clas (Kenworthy and Fonseca 1977, Short 1987). Transplanting efforts for Zostera marina L. also suggest a sensitivity to changes in water movement as tidally dynamic estuary transplanting was not successful (sites had 0–15% survival) (Davis and Short 1997). The loss of seagrasses contributes to high flow rates in unchanged systems as they are responsible for velocity reduction (Fonseca et al. 1982). Seagrasses also contributes to shading and habitat stability, their absence is a significant issue for phyto-, zooplankton and fishes that would use them as shelter, food, a nursery, etc. 3.4 Impingement/Entrainment of fauna within cooling screens Carpenter et al. (1974) demonstrated an incredible loss of copepods moving through the cooling water system of a nuclear power plant on northeastern Long Island Sound (USA). 70% of copepods that entered did not leave through the effluent. This loss has significant impacts on secondary production within that system; reduction of approximately 0.1% of annual secondary production over a 333 km2 area of Long Island Sound adjacent to the power plant. This loss can have negative ripple effects throughout the ecosystem, changing the dynamics between the copepods and their food sources as well as their predators. In a similar study of nine species of larval and juvenile fish none of them made it through alive (Marcy 2011). Issues with successfully passing through a cooling system may be species specific when it comes to mortality levels. According to Bamber and Seaby (2004) the majority of individuals of A. tonsa, C. crangon .and H. gammarus would survive passage through a power-station system under normal conditions. This suggests the potential for a shift in species in favor of those that can tolerate the mechanical stressors of passing through a station’s cooling system.4.0 Conclusion Anthropogenic impact can be neutral, detrimental, or beneficial, and is dependent upon which abiotic components or populations receive the stressors and their tolerance levels. Ecosystems tied to the intake and discharge of nuclear power generating stations are dynamic, sensitive, and water quality variables are all intimately interwoven. While temperature seems to be the greatest stressor and causes the highest mortality rates in both flora and fauna there is a synergistic relationship with chlorination levels and its mortality potential. Understanding how each stressor works in tandem is crucial to the bigger picture of the health of the overall ecosystem. The stressors discussed can have far reaching effects on species, diversity, composition, and abundance. These changes can mean decline in population size or local extinction for a particular species, the creation of prime habitat for invasive and more variable tolerant species, or lead to an abundance of species already present but normally found in low quantities. These changes can happen not only immediately near the generating station, but could affect downstream locations due to sediment deposition and population shifts. However, most issues are within a short distance from the station’s intake and discharge.