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Sci Total Environ
2023 Sep 20;892:164061. doi: 10.1016/j.scitotenv.2023.164061.
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Global review reveals how disparate study motivations, analytical designs, and focal ions limit understanding of salinization effects on freshwater animals.
Walker RH
,
Belvin AC
,
Mouser JB
,
Pennino A
,
Plont S
,
Robinson CD
,
Smith LB
,
Thapa J
,
Zipper CE
,
Angermeier PL
,
Entrekin SA
.
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Global salinization of freshwaters is adversely affecting biotic communities and ecosystem processes. We reviewed six decades (1960-2020) of literature published on animal responses to increased salinities across different taxonomic and ecological contexts and identified knowledge gaps. From 585 journal articles, we characterized 5924 responses of mollusks, crustaceans, zooplankton, non-arthropod invertebrates (NAI), insects, fishes, and amphibians to salinization. Insects and fishes were the most studied taxa; Na+ and Cl- were the most studied ions-. Collectively, concentrations of the ions examined typically spanned five orders of magnitude. Species' invasiveness was a key motivation for studying mollusks, crustaceans, and fishes; threats of urbanization and road salts were key motivations for studying NAI, zooplankton, and amphibians. Laboratory studies were more common than field studies for most taxa. Focal life stages in laboratory studies varied widely but juveniles and adults were represented similarly in field studies. Studies of mollusks, NAI, and crustacean focused on adults; studies of zooplankton, insects, fishes, and amphibians focused on juveniles. Organismal- and population-level responses measuring solute uptake, internal chemistry, body condition, or ion concentrations predominated laboratory studies; population- and assemblage-level responses measuring abundance, spatial distribution, or assemblage composition predominated field studies. Negative responses to salinization predominated but positive and unimodal responses were apparent across all taxa and organizational levels. Key topics for further research include a) salinity responses by more taxa, b) responses to especially toxic ions (i.e., potassium, bicarbonate, sulfate, magnesium), c) mechanisms causing positive and unimodal responses, d) traits underpinning responses, e) effects transcending organizational levels, f) ion-specific response thresholds, and g) interactions between salinity and other stressors. Our review suggests inter-taxa variation in sensitivity to salinization reflects occurrence of certain biological traits, including gill-breathing, semi-permeable skin, multiple life stages, and limited mobility. We propose a traits-based framework to predict salinization sensitivity from shared traits. This evolutionary approach could inform management aimed at preventing or reducing adverse impacts of freshwater salinization.
Fig. 1. Trendlines showing the cumulative number of reviewed articles (N = 585) evaluating the effects of freshwater salinization on seven taxonomic groups. Silhouettes for inset plot (x-axis) represent articles focused on different taxonomic groups and are ordered as follows: mixed taxa (n = 83), mollusks (n = 63), non-arthropod invertebrates (n = 7), crustaceans (n = 60), zooplankton (n = 46), insects (n = 157), fishes (n = 128), and amphibians (n = 41).
Fig. 2. Violin (i.e., frequency distribution) plots showing the ranges in minimum (low range; yellow) and maximum (high range; blue) concentrations of specific conductivity (A; n = 3360 responses), sodium (B; n = 773), calcium (C; n = 207), magnesium (D; n = 172), chloride (E; n = 997), sulfate (F; n = 183), potassium (G; n = 85), and bicarbonate/carbonate (H; n = 158) evaluated for seven taxonomic groups based on 5924 distinct biological responses. Silhouettes (x-axis) represent responses focused on specific taxonomic groups and are ordered as follows: mollusks (n = 731 responses), non-arthropod invertebrates (n = 194), crustaceans (n = 496), zooplankton (n = 977), insects (n = 1959), fishes (n = 1202), and amphibians (n = 365). Total values may exceed the number of biological responses, as some studies included multiple ions (e.g., sodium and chloride counted in the ranges for each ion's corresponding panel).
Fig. 3. Percentages of reviewed laboratory (A, B) and field (C, D) responses (N = 5924) evaluating effects of salinity by exposure (A, C; acute: <4 days, chronic: ≥4 days) and duration of exposure (B, D) for seven taxonomic groups. Silhouettes (x-axis) represent responses focused on specific taxonomic groups and are ordered as follows: mollusks (laboratory: n = 547; field: n = 184), non-arthropod invertebrates (n = 70; n = 124), crustaceans (n = 411; n = 85), zooplankton (n = 656; n = 321), insects (n = 818; n = 1141), fishes (n = 891; n = 311), and amphibians (n = 227; n = 138).
Fig. 4. Percentages of biological responses (N = 5924) from laboratory (A) and field (B) studies evaluating effects of salinization on different life stages of seven taxonomic groups. Silhouettes (x-axis) represent responses by specific taxonomic groups and are ordered as follows: mollusks (laboratory: n = 547; field: n = 184), non-arthropod invertebrates (n = 70; n = 124), crustaceans (n = 411; n = 85), zooplankton (n = 656; n = 321), insects (n = 818; n = 1141), fishes (n = 891; n = 311), and amphibians (n = 227; n = 138).
Fig. 5. Percentages of biological responses from laboratory (A) and field (B) studies evaluating effects of salinization across seven broad biological response categories for seven taxonomic groups (N = 5924). Silhouettes (x-axis) represent responses focused on specific taxonomic groups and are ordered as follows: mollusks (laboratory: n = 547; field: n = 184), non-arthropod invertebrates (n = 70; n = 124), crustaceans (n = 411; n = 85), zooplankton (n = 656; n = 321), insects (n = 818; n = 1141), fishes (n = 891; n = 311), and amphibians (n = 227; n = 138).
Fig. 6. Percentages of biological responses (N = 5924) from laboratory (A) and field (B) studies exhibiting each of four effect directions in response to salinization. Silhouettes (x-axis) represent responses by specific taxonomic groups ordered as follows: mollusks (laboratory: n = 547; field: n = 184), non-arthropod invertebrates (n = 70; n = 124), crustaceans (n = 411; n = 85), zooplankton (n = 656; n = 321), insects (n = 818; n = 1141), fishes (n = 891; n = 311), and amphibians (n = 227; n = 138).
Fig. 7. Conceptual diagram depicting evolutionary relationships and distinguishing traits (left) among eight taxonomic groups and potential shared traits (right) that could promote (up arrows) or diminish (down arrows) salt-sensitivity across major groups of freshwater animals. Notable caveats to these traits include: 1) *unlike most Bivalvia, Sphaeriidae have direct development without a parasitic larval stage; 2) #Caenogastropoda are gilled, while Pulmonata are lunged snails; 3) ^Crayfishes, while mobile in all life stages, have limited dispersal capacity, especially for smaller individuals (0.23–2.10 m day−1), and were therefore not classified as highly mobile.