A practical definition to tell apart ponds from lakes and wetlands

Present scientific definitions of ponds

We compiled current scientific definitions of ponds by conducting a backwards and forwards search of papers referenced in or subsequently referencing three seminal pond papers8,17,18 (see “Strategies”). We in the end compiled 54 pond definitions from scientific literature (knowledge accessible19). The variables most frequently included in definitions have been floor space (91% of definitions), depth (48%), permanence (48%), origin (i.e., pure or human-made; 33%), and standing water (33%; Fig. 2a). When floor space or depth have been included in definitions, they have been typically talked about qualitatively (e.g., “small” and “shallow”). Of the 61% of definitions that included a most pond floor space, the vary was 0.1 to 100 ha, the median was 2 ha, and all however two definitions have been ≤ 10 ha (Fig. 2b). For depth, solely 17% of research offered a most depth cutoff, which ranged 2 to eight m (Fig. 2c). Of the 26 definitions mentioning permanence, 22 acknowledged that ponds may very well be non permanent or everlasting and solely three indicated that ponds are completely everlasting waterbodies. Of the 18 definitions mentioning origin, 17 talked about that ponds may very well be pure or human-made with the remaining examine indicating ponds can have various origins.

Determine 2
figure 2

Abstract of “pond” definitions from scientific literature together with (a) presence of varied morphological, organic, and bodily traits within the definition as blue bars (n = 54 definitions whole). Daring black strains point out the variety of definitions with floor space and depth values. Histograms of the higher limits from “pond” definitions for (b) floor space and (c) most depth.

Different essential components included in definitions associated to morphometry. For instance, 30% of definitions talked about the potential for vegetation to colonize the whole basin, which pertains to excessive gentle penetration (talked about in 11% of definitions) and/or shallow depths. For instance, Wetzel11 defines ponds as having sufficient gentle penetration that macrophyte photosynthesis can happen over the whole waterbody. As such, these situations could also be akin to the littoral area of lakes (11% of definitions). Lastly, 7% of pond definitions talked about mixing versus stratification, whereby ponds combine greater than lakes20 but lower than shallow lakes attributable to a smaller fetch16.

To evaluate if there was settlement in pond definitions amongst papers, we examined the variety of instances every definition was cited. Throughout the 54 definitions, there have been 89 citations of 48 distinctive papers. In the end, most papers (75%) have been solely cited solely as soon as, indicating no consensus in pond definition. Probably the most cited paper was Biggs et al.21, which accounted for 15% of citations. The subsequent two most cited papers have been Oertli et al.17 and Sondergaard et al.18, which have been seminal papers included in our backwards-forwards search, and every comprised 8% of citations.

Worldwide definitions

At a global degree, there isn’t a consensus on learn how to discriminate amongst ponds, lakes, and wetlands. In North America, wetlands are typically thought of to be shallow: < 2 m in Canada22 and < 2.5 m within the US23, which differentiates them from lakes. Some nations, equivalent to Australia, South Korea, and Uganda, explicitly embrace ponds and lakes in federal wetland definitions24 (see additionally22). The inclusion of ponds and a few lakes inside wetland definitions typically stems from the Ramsar Conference, a global physique interested by world wetland conservation that has been signed by 172 nations representing 6 continents as of 202125. The Ramsar Conference outlined wetlands as “areas of marsh, fen, peatland, or water” throughout marine, brackish, and freshwater with various levels of permanence and pure or synthetic states with a most depth of 6 m26, which overlaps depths discovered in lots of definitions of ponds and shallow lakes. In different nations, ponds are included in lake definitions beneath federal conservation legal guidelines. For instance, within the Danish “nature safety” regulation §3, lakes are outlined as waterbodies with a floor space of > 100 m2. As 98% of Danish ‘lakes’ are smaller than 1 ha27, this regulation protects many small waterbodies that could be thought of ponds elsewhere. Nonetheless different businesses have solely qualitative pond definitions: the European Fee merely defines ponds as “comparatively shallow” and might also be referred to as “pool, tarn, mere, or small lake,” a definition additionally utilized by the Worldwide Union for Conservation of Nature28,29. These examples underscore that waterbody definitions range globally, are typically qualitative, and are hardly ever based mostly on scientific proof regarding ecosystem construction or perform. The definitions presumably derive from totally different administration, safety, and monitoring methods; as an example, the European Union’s Water Framework Directive excludes waterbodies < 50 ha (0.5 km2) in measurement from monitoring30.

Present U.S. Federal and State definitions

Within the US, waterbody definitions range amongst federal businesses, with implications for each authorized safety and monitoring. The US Environmental Safety Company (EPA) and the US Military Corps of Engineers (ACE) outline wetlands based mostly on saturated soils and hydrophytic vegetation, which has the potential to incorporate ponds inside the class of wetlands. Conversely, the US Fish and Wildlife Service (USFWS) distinguishes amongst wetlands and lakes based mostly on floor space, depth, and emergent vegetation31. Lakes are ≥ 8 ha or if smaller, they should be ≥ 2.5 m in most depth. In distinction, wetlands are are dominated by > 30% emergent plant cowl; if there may be much less, the positioning should still be a wetland if < 2.5 m deep and < 8 ha in measurement. Subsequently, ponds are sometimes thought of by USFWS to be wetlands, however this isn’t at all times the case: ponds have been used for example of a waterbody that may be categorized as lake, wetland, or each32. The dearth of an specific, unified, and scientifically based mostly pond definitions throughout three federal businesses (EPA, ACE, USFWS) is complicated and contributes to ponds being underrepresented in US aquatic waterbody monitoring relative to their numerical dominance on the panorama3,8. For instance, US EPA monitoring applications embrace ponds in each the Nationwide Wetland Situation Evaluation and the Nationwide Lake Evaluation; nevertheless, “ponds” characterize a small variety of waterbodies in every of those surveys (< 12% categorized qualitatively as “pond” in 2011 wetland survey; 13% of waterbodies have been < 5 ha in 2012 lake survey).

Reflecting political and geographic variability on the nationwide scale, most US states have their very own waterbody protections33. We surveyed US state businesses to look at state definitions of ponds, lakes, and wetlands (see “Strategies”). Our survey responses included 42 of fifty (84%) states (Fig. 3). Just one state (Michigan) explicitly outlined ponds, 11 states outlined lakes (26%), and 30 states outlined wetlands (71%). Whereas just one state outlined ponds, half of the surveyed states used the time period “pond” of their laws. Particularly, ponds have been referenced as state waters (e.g., Vermont) or have been included in state definitions for lakes (e.g., Kansas) or wetlands (e.g., Rhode Island). It’s unclear how these definitions influence monitoring and safety or why the distinctions have been initially made. As an illustration, many states monitor lakes based mostly on minimal measurement thresholds, which range broadly from < 1 ha in Arizona and Alaska, 2–4 ha in lots of northeastern states, and as much as 8 ha in Washington and Nebraska. The number of definitions and monitoring measurement cutoffs don’t seem like scientifically based mostly, however might stem from arbitrary choices, historic references, mapping capabilities from a long time in the past, and useful resource limitations for monitoring; the identical rationale for definitions seemingly apply to native, regional, and worldwide organizations across the globe.

Determine 3
figure 3

US state responses to surveys indicating if the state has a definition of wetland, lake, or pond and if the state used the time period “pond” of their laws. NR = no response.

Evaluating lake, pond, and wetlands traits from literature

We in contrast organic, bodily, and chemical traits of waterbodies that scientists referred to as lakes, ponds, or wetlands in revealed research. To acquire knowledge for the pond traits, we used the identical literature search summarized above for pond definitions (additionally, see “Strategies”). From the 519 papers that we examined, we extracted knowledge on websites the authors referred to as “ponds” and different variants (e.g., ‘small ponds’, ‘fish ponds’, however NOT ‘lakes’). We filtered waterbodies that have been ≤ 20 ha floor space and ≤ 9 m depth (world distribution; n = 1327) to incorporate waterbodies barely larger than the utmost depth and most floor space used to outline ponds in prior research34,35. To match ponds to lakes and wetlands, we used current lake (US and Europe; n = 55,173) and wetland (US; n = 400) databases; waterbodies have been categorized as lake or wetland by the scientists or managers who revealed the database. Wetlands have been categorized as < 1 m in depth with no outlined floor space and lakes have been all > 0.02 ha with no outlined depth (see “Strategies” for particulars).

From the waterbodies that scientists referred to as “ponds,” hydroperiod and origin various over a wide range of traits. Of the 608 ponds with hydroperiod knowledge, everlasting ponds accounted for 74% (n = 450) and non permanent ponds for 26% (n = 158). Out of 648 ponds with recognized origins, 65% (n = 418) have been constructed or manipulated and 35% (n = 230) have been pure. Subsequently, scientists think about ponds to be inclusive of each everlasting and non permanent hydroperiod and have pure or human-made origins.

When analyzing water chemistry, vitamins, and biotic knowledge throughout totally different waterbody varieties, as outlined by publishing scientists and managers, we discovered that ponds have been distinct from lakes and wetlands in two metrics (TN, pH), much like wetlands in a single metric (TP), and much like lakes in a single metric (chl a; Fig. 4; Tables S1, S2). For instance, ponds had distinctly excessive TN concentrations, which have been larger than both lakes or wetlands (Fig. 4b; Desk S2). Ponds and wetlands had equally excessive TP concentrations, which have been considerably larger than lakes; ponds have been additionally most variable in TP (Fig. 4a; Desk S2). Lastly, ponds chlorophyll (chl) a concentrations have been much like lakes, with wetlands being most variable however decrease, on common (Fig. 4d; Desk S2).

Determine 4
figure 4

Comparability of varied chemical and organic parameters throughout wetlands, ponds, and lakes, with waterbody class based mostly on the time period utilized by publishing scientists and managers (Desk S2). Violin plots point out distributions of waterbody traits, the white field signifies twenty fifth to seventy fifth percentile with median within the center, whiskers point out 1.5 × interquartile vary, and outliers are black closed circles. Letters contained in the plot point out vital variations in means (LSD, alpha = 0.05). Word all x-axes have logarithmic scales.

Does ecosystem construction and performance distinguish ponds from lakes and wetlands?

We evaluated the connection between key metrics of ecosystem construction or perform with three quantitative variables that usually confirmed up in pond, lake, or wetland definitions: floor space, most depth (hereafter depth), and emergent vegetation cowl. Our metrics of ecosystem construction or perform embrace vitamins (whole phosphorus (TP), whole nitrogen (TN)), water chemistry (pH), main producer biomass (chl a), metabolism (gross main manufacturing—GPP, respiration—R, internet ecosystem manufacturing—NEP), and warmth and fuel distributions and motion (diel temperature ranges—DTR, methane fluxes, fuel switch velocities). The information was collated from world surveys of literature and federal or worldwide databases (see “Strategies”) with in the end ten comparisons for floor space, six comparisons for depth, and 4 comparisons for emergent vegetation cowl with a variety of pattern sizes for every comparability (n = 67 to 7931, see Tables S3, S5, S7). We assessed every relationship for 4 totally different patterns in growing order of complexity: null, linear, segmented (nonlinear), and logistic (nonlinear) patterns and chosen one of the best match and most parsimonious relationship.

Ecosystem construction and performance have been largely nonlinearly associated to floor space (n = 9/10 variables), depth (n = 5/6 variables), and emergent vegetation cowl (n = 3/4 variables) with each segmented and logistic relationships occurring (Figs. 5, 6, 7; Tables S3–S8). For floor space, six variables had logistic relationships: TP (Fig. 5b), methane fluxes (Fig. 5d), chl a (Fig. 5f), diel temperature vary (Fig. 5h), fuel change charges (ok600; Fig. 5i), and pH (not pictured). The inflection occurred at 0.8 ha for TP, 1.1 ha for methane fluxes, 1.5 ha for chl a, 1.7 ha for pH, 4.6 ha for diel temperature vary, and 17.5 ha for fuel change charges (Desk S3). NEP (Fig. 5c), R (Fig. 5e), and TN (Fig. 5g) all had segmented linear relationships the place smaller methods had steeper slopes than bigger methods (Desk S4). The breakpoint in floor space was 1.0 ha for NEP, 1.2 ha for R, and three.8 ha for TN (Desk S3). For depth, two variables had logistic relationships: diel temperature vary (Fig. 6e) and chlorophyll a (Fig. 6f), with the inflection occurring at 5.9 m and 14.9 m, respectively (Desk S5). pH (Fig. 6b), TP (Fig. 6c), and TN (Fig. 6d) all had segmented linear relationships the place smaller methods had steeper slopes than bigger methods (Desk S6) with breakpoints occurring at 1.0, 2.1, and 5.2 m, respectively (Desk S5). For emergent vegetation cowl, TN (Fig. 7b), TP (Fig. 7c), and pH (Fig. 7d) all had segmented linear relationships the place methods with extra emergent vegetation had steeper slopes than extra open methods (Desk S8). The breakpoint in emergent vegetation cowl was 6.0% for TN, 8.2% for TP, and 26.0% for pH (Desk S7).

Determine 5
figure 5

Relationships between lentic waterbody measurement (excluding wetlands) and ecosystem construction and performance metrics: (a) gross main manufacturing (GPP), (b) whole phosphorus concentrations (TP), (c) internet ecosystem manufacturing (NEP), (d) methane fluxes (CH4 flux), (e) respiration (R), (f) chlorophyll a concentrations (Chl a), (g) whole nitrogen concentrations (TN), (h) diel temperature ranges (DTR), and (i) fuel switch piston velocity (ok600). Optimum mannequin suits from null, linear, segmented, and logistic curves in daring foreground strains. For nonlinear segmented and logistic fashions (bi), plots are ordered by boundaries between ponds and lakes, as outlined by mannequin breakpoints or inflection factors (vertical background strains).

Determine 6
figure 6

Relationships between lentic waterbody most depth (Max depth) and varied ecosystem construction and performance metrics: (a) methane fluxes (CH4 flux), (b) pH, (c) whole phosphorus concentrations (TP), (d) whole nitrogen concentrations (TN), (e) diel temperature ranges (DTR), and (f) chlorophyll a concentrations (Chl a) from literature knowledge extraction with optimum mannequin suits from null, linear or null, segmented linear, and logistic curves in daring foreground strains. For nonlinear segmented and logistic fashions (bf), plots are ordered by mannequin breakpoints or inflection factors (vertical background strains), indicative of boundaries between ponds and lakes.

Determine 7
figure 7

Relationships between lentic waterbody emergent vegetation cowl (Emergent veg.) and varied ecosystem construction and performance metrics: (a) chlorophyll a concentrations (Chl a), (b) whole nitrogen concentrations (TN), (c) whole phosphorus concentrations (TP), (d) pH from literature knowledge extraction with optimum mannequin suits from null, linear or null, segmented linear, and logistic curves in daring foreground strains. For nonlinear segmented and logistic fashions (bd), plots are ordered by mannequin breakpoints or inflection factors (vertical background strains), indicative of boundaries between ponds and wetlands.

To summarize throughout all three metrics (floor space, depth, and emergent vegetation cowl), we evaluated the place the boundaries of nonlinear relationships typically occurred, which informs boundaries between ponds, lakes, and wetlands (Desk 1). For floor space, the boundary was 3.7 ± 1.8 ha (imply ± customary error) and the median was 1.5 ha, in keeping with the median of two ha from scientific definitions (Fig. 2b). The depth boundary was 5.8 ± 2.5 m (imply ± customary error) and the median was 5.2 m, inside the vary of scientific definitions (Fig. 2c). The emergent vegetation cowl boundary was 13.4 ± 6.3% (imply ± customary error) and the median was 8.2%, each of which have been decrease than the beforehand recognized wetland decrease certain of 30%31.

Desk 1 Nonlinear boundary values, parameter estimate ± customary error (SE), from comparisons between floor space, most (max.) depth, and emergent vegetation (veg.) cowl and ecosystem construction/perform metrics together with gross main manufacturing (GPP), whole phosphorus concentrations (TP), methane fluxes (CH4 flux), respiration (R), internet ecosystem manufacturing (NEP), chlorophyll a concentrations (Chl a), pH, whole nitrogen concentrations (TN), diel temperature ranges (DTR), and fuel switch piston velocity (ok600).

Pond morphology (e.g., measurement and depth) creates essentially distinct situations that govern ecosystem construction and performance. Particularly, ponds expertise much less wind-driven turbulence than bigger waterbodies attributable to small fetch and sheltering from the panorama36. We discovered that fuel change charges (ok600) decreased at ~ 18 ha, presumably attributable to lowered wind shear (Fig. 5i; additionally supported by37) and altered thermal dynamics. As an illustration, ponds and shallow lakes can heat dramatically throughout the day, inducing stratification, and funky off and blend fully in a single day38. We discovered greater diel temperature ranges have been extra widespread in waterbodies < 5 ha (Fig. 5h) and < 6 m (Fig. 6e; see additionally39). Such variations in temperature and mixing can promote inside nutrient loading40 and ecosystem respiration41, which can clarify the upper TN (Figs. 4b, 5g), TP (Figs. 4a, 5b) and ecosystem respiration (Fig. 5e) present in ponds. Lastly, variations in water column mixing, elevated vitamins, and better respiration can all contribute to the upper greenhouse fuel emissions present in ponds relative to lakes (Fig. 5d)4,42.

Metrics of phytoplankton biomass (chl a) and whole ecosystem manufacturing within the water (GPP) exhibited weak or inconsistent relationships with floor space and depth, seemingly attributable to variations within the location and varieties of main manufacturing throughout waterbody varieties. Whereas whole main manufacturing in deep lakes is commonly dominated by phytoplankton43, shallow waterbodies can shift towards non-planktonic main manufacturing like benthic algae or floating, emergent, or submerged macrophytes44. Ponds have pelagic phytoplankton, benthic algae (i.e., periphyton), and sediment rooted-submerged or floating macrophytes. In distinction, wetland productiveness typically predominantly happens above the air–water interface45. The place emergent vegetation dominates, they might restrict gentle and cut back water column vitamins, each of that are wanted by phytoplankton and periphyton. Macrophytes may also modify water column and sediment geochemistry by offering autotrophic natural carbon and oxygen to rooting methods within the sediments46. Consequently, these opposing drivers can clarify the excessive variability in main manufacturing we noticed (Fig. 5f, Desk S2). Distinguishing ponds from wetlands will in the end be aided by extra ecosystem measurements of metabolism, greenhouse fuel manufacturing, and extra metrics (e.g., carbon burial) throughout shallow waterbodies with a variety of emergent vegetation cowl.

A practical pond definition

Our evaluate of current pond definitions highlights that floor space and depth are the most typical variables used to outline ponds; but how small and the way shallow a waterbody should be to categorise as a pond is unclear, with definitions ranging by orders of magnitude. Emergent vegetation is a 3rd variable helpful in distinguishing wetlands from ponds, however the threshold worth, > 30% emergent vegetation protection for wetlands established on the US federal degree, shouldn’t be based mostly on documented adjustments in ecosystem perform. Evaluating traits amongst waterbodies that scientists self-categorized into lakes, ponds, or wetlands, ponds have been typically distinct from lakes and wetlands (pH, TN), typically much like wetlands (TP), and typically much like lakes (chl a), suggesting ponds are an ecologically distinct kind of ecosystem. Lastly, we discovered clear nonlinear relationships once we examined relationships between ecosystem construction or perform and floor space, depth, and emergent vegetation cowl; these boundaries assist to quantitatively outline ponds.

Particularly, we discovered that throughout accessible ecosystem metrics, ecosystems shift in construction and performance at common (± SE) values of three.7 (± 1.8) ha in measurement, 5.8 (± 2.5) m in depth, and 13.4 (± 6.3) % emergent vegetation cowl (Desk 1). For floor space, all however one ecosystem metric (ok600) was under 5 ha in floor space, which inserts nicely inside the vary of most current definitions (≤ 10 ha; Fig. 2), and we propose could also be used to tell apart ponds from lakes. For optimum depth, all however one ecosystem metric (chl. a) was under a 6 m depth threshold, which additionally suits nicely inside the vary of depths reported in pond definitions (Fig. 2), and matches the revealed threshold of 5 m most depth for shallow lakes44. Our depth evaluation was much less strong than floor space as a result of we had much less depth knowledge, a typical problem in lentic research47; we subsequently advise additional research in waterbodies to explicitly consider this threshold. Till additional work is finished, we advocate utilizing 5 m as a most depth threshold for ponds as it’s near each threshold shifts in ecosystem perform and matches with the shallow lake literature44,48. We had the fewest ecosystem metric comparisons for emergent vegetative cowl, and noticed three nonlinear boundaries starting from 6 to 26% cowl. The imply (13.4%), although smaller, shouldn’t be statistically totally different than the 30% emergent vegetation cowl (one pattern t-test, t2 = − 2.6, p = 0.12) proposed by Cowardin et al.31 to separate wetlands from lakes. We advocate separating ponds and wetlands utilizing the 30% protection in emergent vegetation threshold for now, however acknowledge that the Cowardin et al.31 metric shouldn’t be knowledge pushed and our evaluation was restricted by current knowledge. Future research should look at how ecosystem construction and performance shifts throughout a gradient of emergent vegetation cowl to raised functionally distinguish wetlands from ponds and will in the end decrease that boundary.

Our evaluate of knowledge from the literature confirmed scientists and managers view ponds as everlasting or non permanent and pure or human made in origin. Subsequently, we felt it vital to offer the inclusivity of those ideas in a pond definition. Different definitions additionally hyperlink depth to gentle availability, the place gentle penetrates to the sediments throughout the pond (e.g.,11). Nonetheless, gentle availability shouldn’t be solely mediated by depth; even within the shallowest methods gentle could be limiting attributable to turbidity, dissolved natural matter, and submerged or floating vegetation (e.g.,49,50). For instance, floating duckweed can cowl most of a pond’s floor space and cut back gentle penetration to < 1% relative to the sunshine above the water’s floor49, and dramatically change the ecology of shallow methods51.

As our analyses point out that ponds are functionally distinct from lakes and wetlands, we suggest the next scientifically knowledgeable pond definition (Fig. 8):

Ponds are small and shallow waterbodies with a most floor space of 5 ha, a most depth of 5 m, and < 30% protection of emergent vegetation. Ponds can have gentle penetration to the sediments if water readability permits and could be everlasting or non permanent and pure or human-made.

Determine 8
figure 8

Conceptual mannequin to outline lentic waterbodies based mostly on three totally different standards (depth, floor space, and emergent vegetation). Boundaries for all three axes come from our evaluation and are knowledgeable by current pond, lake, and wetland definitions. Determine by Visualizing Science.

Our proposed definition relies on the present state of the science; we anticipate that future analysis will additional resolve variations amongst these 5 classes. For instance, we name for future analysis to look at how ecosystem construction and performance shift throughout our proposed boundaries, significantly for depth and emergent vegetation, which had smaller pattern sizes and fewer ecosystem metrics than floor space. Further variables equivalent to basin geometry (e.g., space:quantity ratios), sheltering from wind, water residence time, water readability, and geographic location, might also have an effect on a waterbody’s ecosystem construction and performance, creating some overlap between classifications particularly alongside the higher and decrease bounds of our pond definition. As an illustration, a panorama with little wind sheltering will increase water column mixing that might trigger a waterbody the scale of a pond to perform extra like a shallow lake. We subsequently advocate for added sampling of lentic waterbodies, particularly in places the place lentic waterbodies are understudied or being quickly constructed like tropical and subtropical areas52, to assist resolve boundaries amongst waterbody varieties and additional refine the pond definition.

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