by David J. Yozzo, PhD
Climate change is impacting the health and biological integrity of marine and estuarine waters throughout the United States, and globally. Rising average air and water temperatures, more frequent and extreme weather events, and steadily rising sea levels are changing baseline environmental conditions, and may alter the distributions and life history patterns of marine/estuarine organisms, including fish, invertebrates, sea birds, sea turtles and marine mammals. The magnitude of these ecological changes is expected to increase in the future, with important implications for strategic, effective management of marine and coastal resources, including sustainable fisheries and swimmable waters. One especially widespread (global) indicator of the effects of climate change (specifically increased sea surface temperatures) on marine resources is the increasing magnitude of change in the distribution of marine and estuarine fish species. (Roessig et al. 2004, Nye et al. 2009, Koenigstein et al. 2016). For example:
However, discerning climate-driven changes in marine fish distributions is challenging – the signal from climatic effects may be confounded by other factors such as forage availability, changes in inshore habitat structure and commercial overharvesting. In addition, marine fish populations can undergo cyclic patterns of abundance associated with multi-decadal natural changes in oceanic currents, such as the North Atlantic Oscillation (NAO), the Pacific Decadal Oscillation (PDO), and the El Nino-Southern Oscillation (ENSO) (Crozier and Hutchings 2014). Even under nearly constant environmental conditions, fish distributions are not static. Fish populations occupy optimal habitats under low abundances, but also disperse into less optimal habitats at high abundances (Sinclair 1988, MacCall 1990). This means that species that are only rarely or periodically seen in temperate estuaries may be driven there in response to higher densities/competition for resources in more tropical waters and not necessarily because of favorable temperatures.
Many aquatic and marine species are sensitive to temperatures just a few degrees higher than those they are generally adapted to in nature (Kennedy et al. 2002). Oceanic warming simultaneously reduces the total amount of dissolved oxygen that can be held in water and increases demand for oxygen in cold blooded aquatic animals. Even modest increases in ocean temperatures may affect growth/metabolism, determine behavior and alter distribution patterns. The Intergovernmental Panel on Climate Change (IPCC 2014) has documented an average global temperature increase among land and ocean surfaces of 0.85 °C (1.53 °F) between 1880 and 2012. The upper ocean (0 to 75 m) has, on average, warmed by 0.11 °C (0.20 °F) every decade since the early 1970s.
Increased surface water temperature, along with changing patterns of precipitation and riverine hydrology may alter the timing and magnitude of phytoplankton production in estuaries, favoring production by species known to form harmful algal blooms (HABs) (Pyke et al. 2008)—such as the notorious “red tides” currently occupying a large expanse of the southwestern Florida coastline, resulting in massive fish kills, and respiratory distress to humans on beaches. Toxic effects of HABs vary; some forms may exhibit toxicity to fish and aquatic biota even at low cell concentrations, while others may be essentially non-toxic but present a nuisance through high biomass production – they interfere with grazing by zooplankton and alter patterns of nutrient supply and elemental recycling (Gobler et al. 2017).
Along the U.S. Atlantic coast, warm-temperate fish species fish assemblages may benefit from climate changes that are impacting cooler-water species, by expansion of their range to more northern estuaries. One of the most compelling examples of this phenomenon is Narragansett Bay, Rhode Island. Nye et al. (2009) documented changes in the abundance and latitudinal distribution for several bottom-dwelling species, which were historically abundant and characteristic of the Narragansett Bay winter fish community, including red hake (Urophycis chuss), and silver hake (Merluccius bilinearis). Simultaneously, the abundance of warm water species that migrate into the Bay during summer such as butterfish (Peprilus triacanthus) and scup (Stenotomus chrysops) increased. These changes coincided with a 90% decline in winter flounder (Pseudopleuronectes americanus) abundance in the Bay (Oviatt 2004, Jefferies et al. 2011). Winter flounder spawn in estuaries at temperatures ranging from 1 to 10 °C, with optimal spawning conditions at 2 to 5 °C. The evolution of cold water spawning in winter flounder is a mechanism for avoiding predation on newly emerged/metamorphosing larvae, principally by sand shrimp (Crangon septemspinosa). Winter flounder eggs hatch when sand shrimp have historically been absent or dormant in the Bay. However, as winter water temperatures increased, sand shrimp remained active and consumed flounder larvae (Taylor and Collie 2003). Warmer waters are also associated with greater egg mortality rates, reduced larval growth rates, and diminished larval condition (Keller and Klein-McPhee 2000). Winter flounder have historically exhibited long-term cyclical abundance patterns; however, abundance peaks have diminished in recent decades.
Further south, faunal shifts have also been documented for the Hudson-Raritan Estuary, including the extirpation of rainbow smelt (Osmerus mordax). Smelt abundance in Hudson River tributaries began to decline during the 1970s, and the last recorded specimen from the Hudson drainage was collected in 1998 (Waldman 2006). Another cold-water species, Atlantic tomcod (Microgadus tomcod), has also diminished in the lower Hudson River; a contributing factor may be the species’ naturally short lifespan at this extreme southern portion of their distributional range (most Hudson River tomcod only live one year compared to 3 to 4 years in the northern reaches of their range). It is expected that the Hudson River population will further diminish, and perhaps become extirpated entirely in the coming decades (Waldman 2014).
In contrast, gizzard shad (Dorosoma cepedianum), historically rare north of Sandy Hook, New Jersey, colonized the Hudson River during the 1970s and has become established as far north as the Merrimack River, Massachusetts. Channel catfish (Ictalurus punctatus), another species most often associated with aquatic habitats (including large coastal river basins) to the south of the Hudson drainage, became increasingly abundant in the tidal Hudson river during the mid- to late-1990s (Daniels et al. 2005). Another recent faunal shift in the Hudson-Raritan Estuary is the increasing presence of species in the drum family (Sciaenidae), including Atlantic croaker (Micropogonias undulatus), spotted seatrout (Cynoscion nebulosus), and red drum (Sciaenops ocellatus). These species are most often associated with estuaries to the south such as Delaware Bay and Chesapeake Bay, and Albemarle-Pamlico Sound (Waldman 2014).
The coastal management community is paying close attention to changes in the distribution and abundance of marine and estuarine biota, as well as other climate-related impacts on coastal habitats, water quality, and recreation. Future climate projections and vulnerability may require re-assessing present-day federal, state, and local water quality (e.g., temperature and dissolved oxygen) standards for estuaries and coastal waters. For example, meeting existing thermal standards may represent an increasing challenge for electrical power generating facilities and other industries which discharge heated effluent into estuaries and coastal bays. Maintaining compliance will likely require the development of newer, more efficient technology and operational procedures, especially if regulators were to adopt more stringent (protective) criteria to protect coastal resources.
The U.S. Environmental Protection Agency’s (EPA) National Estuary Program (NEP) has identified projected Increases in ocean surface temperature as a key vulnerability of its program to protect and restore the water quality and ecological integrity of estuaries of national significance. EPA’s Climate Ready Estuaries Program (https://www.epa.gov/cre) provides resources to support individual NEP component programs, and the coastal management community, in identifying climate vulnerabilities, developing adaptation measures/strategies and educating and engaging local stakeholders affected by climate change impacts in coastal areas throughout the U.S.
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