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 finally 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 said that ponds may very well be momentary 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 numerous origins.
Different essential components included in definitions associated to morphometry. For instance, 30% of definitions talked about the potential for crops to colonize your entire 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 your entire waterbody. As such, these situations could also be corresponding 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 as a consequence of 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. Essentially 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.
At a global stage, there isn’t a consensus on the way to discriminate amongst ponds, lakes, and wetlands. In North America, wetlands are typically thought-about 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 embody 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 international locations 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 international locations, 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 which may be thought-about ponds elsewhere. Nonetheless different businesses have solely qualitative pond definitions: the European Fee merely defines ponds as “comparatively shallow” and can also be known 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 fluctuate globally, are typically qualitative, and are not often primarily based on scientific proof regarding ecosystem construction or operate. The definitions presumably derive from totally different administration, safety, and monitoring methods; for 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 fluctuate 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 primarily based on saturated soils and hydrophytic vegetation, which has the potential to incorporate ponds throughout the class of wetlands. Conversely, the US Fish and Wildlife Service (USFWS) distinguishes amongst wetlands and lakes primarily based 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’s much less, the location should be a wetland if < 2.5 m deep and < 8 ha in measurement. Due to this fact, ponds are sometimes thought-about 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 primarily based 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 packages embody 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 affect monitoring and safety or why the distinctions have been initially made. As an illustration, many states monitor lakes primarily based on minimal measurement thresholds, which fluctuate 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 look like scientifically primarily based, however might stem from arbitrary selections, historic references, mapping capabilities from many years 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.
Evaluating lake, pond, and wetlands traits from literature
We in contrast organic, bodily, and chemical traits of waterbodies that scientists known 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 known 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 higher 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 known as “ponds,” hydroperiod and origin various over a wide variety of traits. Of the 608 ponds with hydroperiod knowledge, everlasting ponds accounted for 74% (n = 450) and momentary 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. Due to this fact, scientists think about ponds to be inclusive of each everlasting and momentary 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), just like wetlands in a single metric (TP), and just like lakes in a single metric (chl a; Fig. 4; Tables S1, S2). For instance, ponds had distinctly excessive TN concentrations, which have been higher than both lakes or wetlands (Fig. 4b; Desk S2). Ponds and wetlands had equally excessive TP concentrations, which have been considerably higher than lakes; ponds have been additionally most variable in TP (Fig. 4a; Desk S2). Lastly, ponds chlorophyll (chl) a concentrations have been just like lakes, with wetlands being most variable however decrease, on common (Fig. 4d; Desk S2).
Does ecosystem construction and performance distinguish ponds from lakes and wetlands?
We evaluated the connection between key metrics of ecosystem construction or operate with three quantitative variables that always confirmed up in pond, lake, or wetland definitions: floor space, most depth (hereafter depth), and emergent vegetation cowl. Our metrics of ecosystem construction or operate embody vitamins (whole phosphorus (TP), whole nitrogen (TN)), water chemistry (pH), major producer biomass (chl a), metabolism (gross major manufacturing—GPP, respiration—R, web ecosystem manufacturing—NEP), and warmth and gasoline distributions and motion (diel temperature ranges—DTR, methane fluxes, gasoline switch velocities). The info was collated from world surveys of literature and federal or worldwide databases (see “Strategies”) with finally ten comparisons for floor space, six comparisons for depth, and 4 comparisons for emergent vegetation cowl with a spread 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 rising order of complexity: null, linear, segmented (nonlinear), and logistic (nonlinear) patterns and chosen the very 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), gasoline trade charges (okay600; 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 gasoline trade charges (Desk S3). NEP (Fig. 5c), R (Fig. 5e), and TN (Fig. 5g) all had segmented linear relationships the place smaller techniques had steeper slopes than bigger techniques (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 techniques had steeper slopes than bigger techniques (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 techniques with extra emergent vegetation had steeper slopes than extra open techniques (Desk S8). The breakpoint in emergent vegetation cowl was 6.0% for TN, 8.2% for TP, and 26.0% for pH (Desk S7).
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 ± normal error) and the median was 1.5 ha, in step with the median of two ha from scientific definitions (Fig. 2b). The depth boundary was 5.8 ± 2.5 m (imply ± normal error) and the median was 5.2 m, throughout the vary of scientific definitions (Fig. 2c). The emergent vegetation cowl boundary was 13.4 ± 6.3% (imply ± normal error) and the median was 8.2%, each of which have been decrease than the beforehand recognized wetland decrease sure of 30%31.
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 as a consequence of small fetch and sheltering from the panorama36. We discovered that gasoline trade charges (okay600) decreased at ~ 18 ha, presumably as a consequence of diminished 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 frequent 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 gasoline 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 as a consequence of variations within the location and forms of major manufacturing throughout waterbody varieties. Whereas whole major manufacturing in deep lakes is commonly dominated by phytoplankton43, shallow waterbodies can shift towards non-planktonic major 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 could restrict gentle and cut back water column vitamins, each of that are wanted by phytoplankton and periphyton. Macrophytes can even modify water column and sediment geochemistry by offering autotrophic natural carbon and oxygen to rooting techniques within the sediments46. Consequently, these opposing drivers can clarify the excessive variability in major manufacturing we noticed (Fig. 5f, Desk S2). Distinguishing ponds from wetlands will finally be aided by extra ecosystem measurements of metabolism, greenhouse gasoline manufacturing, and extra metrics (e.g., carbon burial) throughout shallow waterbodies with a spread of emergent vegetation cowl.
A useful pond definition
Our assessment 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 stage, is just not primarily based on documented adjustments in ecosystem operate. Evaluating traits amongst waterbodies that scientists self-categorized into lakes, ponds, or wetlands, ponds have been generally distinct from lakes and wetlands (pH, TN), generally just like wetlands (TP), and generally just like lakes (chl a), suggesting ponds are an ecologically distinct sort of ecosystem. Lastly, we discovered clear nonlinear relationships after we examined relationships between ecosystem construction or operate 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 (okay600) was under 5 ha in floor space, which inserts effectively throughout the vary of most current definitions (≤ 10 ha; Fig. 2), and we recommend could also be used to differentiate ponds from lakes. For max depth, all however one ecosystem metric (chl. a) was under a 6 m depth threshold, which additionally suits effectively throughout 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 standard problem in lentic research47; we due to this fact advise additional research in waterbodies to explicitly consider this threshold. Till additional work is completed, we suggest utilizing 5 m as a most depth threshold for ponds as it’s near each threshold shifts in ecosystem operate 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, is just not 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 suggest separating ponds and wetlands utilizing the 30% protection in emergent vegetation threshold for now, however acknowledge that the Cowardin et al.31 metric is just not knowledge pushed and our evaluation was restricted by current knowledge. Future research should study how ecosystem construction and performance shifts throughout a gradient of emergent vegetation cowl to raised functionally distinguish wetlands from ponds and will finally decrease that boundary.
Our assessment of information from the literature confirmed scientists and managers view ponds as everlasting or momentary and pure or human made in origin. Due to this fact, we felt it needed 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 is just not solely mediated by depth; even within the shallowest techniques gentle could be limiting as a consequence of turbidity, dissolved natural matter, and submerged or floating crops (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 techniques51.
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 may have gentle penetration to the sediments if water readability permits and could be everlasting or momentary and pure or human-made.
Our proposed definition is predicated 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, can 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 would trigger a waterbody the scale of a pond to operate extra like a shallow lake. We due to this fact 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.