Warmwater Aquatic Fauna and Climate Change

Preparers:

Craig Roghair, Fisheries Biologist, Forest Service Southern Research Station
Dr. Susan B. Adams, Team Leader and Aquatic Ecologist, Forest Service Southern Research Station

Overview

Warmwater rivers, streams, lakes, and ponds contain valuable recreational fisheries and diverse biological communities but are also among the most highly-altered aquatic habitats in the United States. Climate-induced changes in temperature and precipitation will directly and indirectly impact warmwater aquatic habitats, the animals that occupy them, and the people that use them. Retaining the essential structure and function of warmwater systems will require approaches that engage scientists, managers, and stakeholders in the creation of informed solutions that promote ecological, social, and management resilience.

An older briefing on this topic is archived here.

Issues

What is warmwater habitat?
Rivers, streams, ponds, lakes, and other waterbodies are often described as warmwater, coldwater, or coolwater. These classifications are based on the optimal thermal ranges and maximum thermal limits of the fishes they support. For example, in Wisconsin, coldwater fishes such as trout and salmon preferred waters with a maximum summer daily mean water temperature less than 20.7°C (69°F), whereas warmwater fishes such as basses, sunfishes, and catfishes preferred temperatures greater than 24.6°C (76°F) (1). Coldwater and warmwater fishes, as well as transitional species (e.g., Northern Pike, Walleye), can occur together in coolwater reaches of rivers and streams and in stratified lakes. Classification systems based on water temperature vary by management agency and region (1), but in general rivers and streams with an average daily summer water temperature greater than 20°C (68°F) are considered to be warmwater (2). While such classifications are useful in management, they fail to fully describe the intricate relationships between water temperature and aquatic biota (1).

Warmwater habitats generally occur at lower elevations and lower latitudes than their cool- or coldwater counterparts. Warmwater habitats occur in both urban and rural settings, and many have been greatly altered by channelization, damming, pollution, water withdrawals, and sedimentation (3, 4, 5, 6). Warmwater habitats support a variety of recreational activities, including valuable multispecies fisheries (2, 7), and are also biologically diverse (8, 9, 10, 11), containing numerous species of conservation concern. The combination of high recreation value, high biological diversity, and high degree of disturbance make warmwater habitats among the most challenging to manage and conserve, even before introducing the complexities of a changing climate (2).

Likely Changes

How will changes in climate impact warmwater habitats?
Changes in precipitation patterns cause changes in the quantity and quality of water delivered to warmwater habitats. Precipitation predictions are complex and vary by region, but the frequency and severity of both storms and droughts are expected to change (12). Runoff patterns associated with less frequent, but more severe, storms in the upper Midwest are impacting lake hydrology and nutrient loading (13). Recent drought in the Southeast dramatically dewatered streams, imperiling local mussel populations (14).

Air and water temperatures are correlated; therefore, as air temperatures increase, so do surface-water temperatures, and after a lag time, groundwater temperatures (12, 15, 16, 17). Many native warmwater fishes in the southwestern and central U.S. are already living near their thermal maximum limits, so predicted temperature increases will cause extirpations and extinctions (18, 19, 20). Twenty species of Great Plains and southwestern warmwater stream fishes with no avenues for northern migration are vulnerable to extinction if water temperatures rise 4°C (20).

Changes in water quantity and temperature affect the chemical characteristics of warmwater habitats, such as dissolved oxygen, salinity, and nutrient concentrations. Higher water temperatures typically decrease dissolved oxygen in surface waters (18), and low dissolved oxygen has a stronger influence than high temperatures on distributions of some aquatic species (21). Increased salinity of inland waters in arid regions of the southwestern U.S. has already occurred in some habitats and is expected to be exacerbated by climate changes due to drier conditions, more surface water diversion and groundwater extraction, and higher evaporation rates in warmer temperatures (22, 23).

As the physical and chemical characteristics of warmwater habitats change, associated biotic characteristics change as well. Pathogen and parasite distributions and virulence relate to water quality and quantity (24, 25, 26), as do the frequency and severity of harmful algal blooms (27, 28).

How will habitat changes impact warmwater biota?
The potential for increased frequency of severe drought is particularly troubling for species with small native ranges or fragmented populations. One severe drought that dries several springs may eliminate a species. Drought dramatically reduced sizes of some isolated mussel populations in the southeastern U.S., possibly to densities too low to allow for successful reproduction, and river fragmentation by reservoirs prevents recolonization Higher salinity may become excessive for native species or may alter competitive advantages among species, even favoring some nonnative species with high salinity tolerances (22, 23, 29).

Although warmwater fishes are not expected to face the nearly-universal declines predicted for coldwater fishes, they are still impacted by warming waters. Many animals living in warmwater habitats, including fishes and invertebrates, are ectothermic, relying on external conditions to regulate body temperature. Environmental temperature strongly influences metabolic rates (30), timing of key behaviors (31), growth (32), recruitment (33), bioaccumulation rates (25, 34), and a suite of physiological processes (35) in ectotherms. Some ectotherms are adapted to a narrow range of temperatures whereas others are generalists, setting up the potential for winners and losers as temperatures increase (36, 37); however, all species have upper thermal limits beyond which physiological processes break down (35). Many native warmwater fishes in the southwestern and central U.S. are already living near their thermal maximum limits, so predicted temperature increases will likely cause extirpations and extinctions (18, 19, 20, 33, 34).

Species may shift their ranges (34) or become dominant in previously unsuitable areas (38, 39) as waters warm. However, some species are more capable than others of colonizing new warmwater habitats. Dams, culverts, waterfalls, steep gradients, watershed boundaries, and even large water bodies may present significant colonization barriers to species with limited mobility, such as freshwater mussels (40). Mobile species, such as aquatic insects capable of aerial dispersal, are more readily able to shift or expand their range (41). However, even species that are able to move may find unfavorable habitat (42), water quality (43), or novel biotic interactions (33, 44) that prevents establishment of a new population. Colonization of new habitats is not limited to native species. Invasive and non-native species will also have access to new habitats and may be able to exploit conditions better than native species (23, 45, 46).

Which warmwater habitats and species are most vulnerable?
The resilience of aquatic habitats and biotic communities determines their vulnerability to warming. An ecosystem is considered resilient if it is able to resist and recover from change, or absorb a disturbance and reorganize in ways that retain its essential ecological functions, structures, identities, and feedbacks (47, 48). Key attributes controlling resilience include connectivity, biodiversity, habitat variability, presence of refugia and support areas, natural disturbance history, human pressures, and cumulative effects (48). Therefore, species with limited distributions, narrow temperature tolerances (including those already near their thermal maxima), limited mobility, habitat specialization, or fragmented populations are least resilient and most vulnerable to changing climate.

Management Considerations

How do we manage for a changing climate in the face of uncertainty?
Climate science is evolving and new information is rapidly becoming available, but significant knowledge gaps for warmwater habitats and species remain. Key knowledge gaps and significant uncertainty create a challenging management space in which flexible and adaptive decision-making strategies are needed; inaction is not a viable management strategy (47). Managers can use the best available science as a starting point in developing proposed actions. Knowledge gaps can be incorporated into the decision-making process through a strategy robust to uncertainty, such as scenario planning (49) or structured decision making (50), that encourages scientists, managers, and stakeholders to engage in formal, collaborative relationships to systematically solve complex problems and make informed decisions. Regardless of the approach, monitoring both before and after management action is critical to understanding actual outcomes and adapting management strategies (47). Managers who engage a broad array of partners throughout the decision-making, implementation, and monitoring processes will have the greatest opportunity for success. No individual or organization has the resources or expertise required to efficiently and effectively design, implement, and monitor the large-scale projects required to address climate change (47). A broad coalition is also more likely to identify climate change messaging that resonates with diverse audiences, predict how diverse resource users might respond to proposed actions, and gather and address post-implementation feedback from those resource users (7).

Most strategies for addressing climate change focus on maintaining or improving ecological and social resilience. Ecological resilience is the ability of habitats and biotic communities to resist and recover from change, and social resilience is the ability of human groups or communities to cope with external stresses and disturbances (47,48). Resilient management approaches adapt to rapidly changing ecological and social conditions and optimize both short- and long-term outcomes (47).

Specific management actions that promote ecological, social, and management resilience in a warmwater management context include:

• Maintain environmental flows. The hydrological regime required to sustain aquatic ecosystems and water-dependent human activities and needs for a particular waterbody is referred to as its ‘environmental flow’ (51). Generally, more water in a system means more habitat volume for species and less susceptibility to extreme temperatures and desiccation by drought (52, 53). Maintaining environmental flows, including the natural fluctuations in timing and range of flows, tends to benefit native species (54) over non-natives (29). Establishing and maintaining environmental flows is a complex, interdisciplinary effort requiring coordination and communication among aquatic scientists, social scientists, resource managers, policy makers and the public (51).
• Maintain groundwater levels and identify and conserve critical groundwater-dependent ecosystems. As surface-water temperatures warm to species’ thermal tolerance limits, groundwater upwelling areas become increasingly important as refugia during periods of extreme temperature or low water (20) and for maintaining environmental flows (51). Drainages with more groundwater inputs appear to be somewhat more resilient to climate change impacts (55). Groundwater levels should be maintained and discharge areas (e.g., springs) identified and protected.
• Protect and restore habitat and water quality. Maintaining or re-creating high quality habitat for native species (e.g., riparian buffers, large woody debris, stable banks) will increase ecological resilience (23). Habitat protection and restoration efforts may be most effectively applied to whole ecosystems at watershed or even regional scales, particularly in diverse warmwater systems where we lack even basic knowledge about the habitat needs and environmental tolerances of many aquatic taxa (10, 56, 57). Efforts to protect or restore habitat will not fully protect aquatic biota in areas with poor water quality. Maintaining high water quality in combination with adequate flows (see 1 and 2 above) and quality habitat provides for maximum resiliency as water temperatures rise (13). Water quality protection also buffers against the increased toxicity of pollutants with increases in water temperature (25, 34).
• Manage habitat connectivity. Migration and movement barriers adversely affect numerous aquatic species by blocking or reducing access to vital habitats (5, 58). Connectivity is a prerequisite for rapid recolonization in the wake of disturbances (58) facilitates shifts in species distributions (10, 25), and for maintaining genetically robust populations (see #5 below) . Climate change is predicted to increase extreme flooding, dewatering associated with droughts and water withdrawals, and the need for species to seek new habitats (10, 25). Given these anticipated changes, the importance of connectivity cannot be overstated (48). Conversely, movement barriers such as dams or perched culverts that hinder non-native species invasions can be key to maintaining ecosystem function (47). Creating new connectivity among historically isolated drainages will undoubtedly create new invasion scenarios that will further stress some native populations.
• Manage for robust and redundant populations. Multiple, large populations provide greater resilience to extreme events, provide more effective source populations for recolonization of habitats affected by extreme events, and provide greater genetic diversity, increasing the possibility of genetic adaptation to changing conditions (14, 20, 24, 25). Selective translocation of species to new habitats has been proposed as a way to supplement natural range shifts and to provide access to suitable habitats (59). Translocation of species outside of their native ranges carries ecological risk (e.g. species invasions, disease introduction) even under stable conditions. Because recent history is no longer an appropriate guide for future ecological conditions, such practices now pose even greater risks (23).
• Reevaluate fisheries management strategies. Fisheries management traditionally focused on the stability of a single population in a particular waterbody for a narrow range of resource users, an approach that is increasingly less likely to provide desired outcomes. Management focused on protecting the mechanisms that provide for a desired structure or function, such as sustainable recreational fisheries, is more likely to be successful in the context of a changing climate (47). Furthermore, the scope and scale of projects required to maintain or improve ecological or social resilience requires engaging with a diverse array of partners and the development of collaborations that stretch beyond traditional fisheries management (49, 50).
• Unite conservation with human activities and needs. Climate change impacts on warmwater habitats are deeply intertwined with human activities such as recreational fishing and human needs such as clean water. Conservation messages are most likely to resonate with diverse audiences when delivered as part of this broader context. For example, DeWeber and Wagner (60) couched the impact of climate change in the context of increasing cost-per-fishing-trip to show a direct relationship between climate change and individual resource users. Strayer (10) suggested that aquatic invertebrate conservation may be most successful if viewed "as part of a larger problem with the dual goals of preserving freshwater ecosystems and ensuring adequate supplies of fresh water to people". Such holistic approaches may be promising for all aquatic conservation efforts as society increasingly grapples with fundamental problems of too much or not enough fresh water.