Natural habitats have become more fragmented due to the intensification of agricultural practices and urbanization [1]. The urbanization has also led to a need for increased local handling of stormwater because of an increased proportion of impermeable areas. To solve the issue of local stormwater handling, wet stormwater ponds, that resemble small natural lakes, is one of the most utilized practices and is in Denmark considered to be the best available technology (BAT) ^[2][3][4][5][2,3,4,5]. Generally, there are two types of stormwater ponds: dry and wet. The dry ponds are normally used only for storage during rain events ^[3][6][3,6]. The primary purpose of the ponds, both wet and dry, was originally to store stormwater, limit hydraulic load and, thus, prevent erosion in streams [3]. A more recent purpose of the wet stormwater ponds is retention by adsorption, absorption and sedimentation to limit the discharge of nutrients, metals and other xenobiotics to the natural recipients [7]. The most important process improving the water quality in wet stormwater ponds is sedimentation because most incoming nutrients, metals and xenobiotics are bound to organic and inorganic particles [8]. The incoming particles will often be in the size interval of 1100 µm and exhibit very different settling times, with the smallest particles (<63µ m) taking the longest time to settle [8]. For both particulate phosphorus (PP) and metals bound to particles, the smaller particles (<63µ m) often contain disproportionally high concentrations of bioavailable P and metals, due to the relatively higher surface area ^{[9][10][11][12]}[9,10,11,12]. Egemose et al. [11] conducted a study on outlets to Lake Nordborg and showed that 47–73% of PP settled slower than 1 m/hour and 28% of the particles and PP settled slower that 1 cm/hour. Thus, a pond with a depth of 1.5 requires a residence time of 8 h for 29.4–68.3% of PP to settle. Wet ponds are rarely designed with a depth exceeding 1–1.5 m, but with a storage volume allowing the wet volume to increase until 2–2.5 m during rain events. A depth larger than 1.5 m is not recommended due to increased risk of anaerobic conditions at the sediment surface which will limit P binding by iron compounds ^{[13][14][15][16][17]}[13,14,15,16,17]. Because of the shallow depth, it is important that the ponds are appropriately designed to accommodate the required settling velocities necessary for effective sedimentation.

Sønderup et al. [18] performed an extensive study including 39 ponds in Aabenraa and demonstrated that factors such as distance between inlet and outlet, ratio between wet volume and reduced catchment area (m³ red·ha⁻¹) and age are very important factors for an effective retention.

Sønderup et al. [18] showed a strong positive tendency towards the fact that increasing distance between inlet and outlet improved the retention (%) in the ponds. With a distance >80 m, a retention of 30–50% for suspended solids (SS), organic matter (OM), nitrate (NO₃⁻), PP and total dissolved phosphorus (TDP) was documented. A significantly higher retention of metals with greater distance between inlet and outlet has not been documented [19], though a tendency towards higher retention with greater distance was observed.

A similar positive correlation between retention and increased ratio was also documented. Sønderup et al. [18] concluded the optimal ratio to be 250 m³ red·ha⁻¹ or higher. This is cooperated by previous studies that determined satisfactory removal efficiencies with a pond ratio of 200–300 m³ red·ha^{−1 [3][16][20]}[3,16,20]. Egemose et al. [19] found a similar positive relation between increasing ratio and the retention of lead (Pb), nickel (Ni) and zinc (Zn), whereas for copper (Cu), chromium (Cr) and cadmium (Cd), there was a negative retention < 800 m³ red·ha⁻¹.

The retention is usually decreasing with higher age of the ponds. Sønderup et al. [18] showed a 30–60% retention for SS, OM, NO₃⁻, PP and TDP within the first 5 years since construction, where after the retention dropped to a maximum of 10% and could lead to a release of up to 30% [18]. Though many of the ponds in the study were also small with ratios between 150–250 m³ red·ha⁻¹. The metal retention varied depending on the specific metal. Cu, Cr, Cd were only retained the first 1–2 years after which there was a net release which increased with higher age. Lead, Ni and Zn had a positive retention even in ponds 31–40 years old, but the retention did decrease with higher age. The concentration of nutrients, organic materials, suspended solids and metals differ with both size and the land use of the catchment area ^{[3][16][18][21]}[3,16,18,21]. Sønderup et al. [18] found that that the median concentrations of SS (mg L⁻¹), total phosphorous (TP) and total nitrogen (TN) (µg L⁻¹), decreased in the order of nutrient-enriched areas>> mixed and industrial > rural and urban> developing-areas in the city of Aabenraa in Southern Jutland, Denmark. Sønderup et al. [18] and Göbel et al. [21] concluded that the areas most affected by human activities, such as industry, traffic and agricultural areas also discharge the highest concentrations of nutrients, SS and metals, respectively.

Stormwater ponds are technical facilities intended to retain stormwater, nutrients and metals, but it is a reasonable assumption that the ecosystem in the pond might be negatively affected in terms of toxic effects and hydrological stress. Studies have shown that metals accumulate to higher concentrations in both vertebrates [22] and invertebrates [23] in stormwater ponds compared to natural lakes. Though several studies have concluded that there is no significant difference between the biodiversity found in stormwater ponds and natural lakes ^{[7][23][24][25][26]}[7,23,24,25,26]. It has been hypothesized that stormwater ponds may support and increase the biodiversity in urban areas, where natural ecosystems have been strongly affected by human activities ^[7][24][25][7,24,25].

Streams receiving discharge from stormwater ponds are often immensely affected by human activities due to load from both point sources (e.g., treated wastewater, sewer outlets, overflows) and diffuse runoff (e.g., drainage water and surface runoff) often leading to lower biodiversity and higher pollution levels ^{[14][27][28][29]}[14,27,28,29]. Knowledge is still lacking concerning the effect of stormwater pond discharge on streams as limited research has been dedicated to this specific subject. Some research has been done by, e.g., Koziel et al. [30] who concluded that the invertebrate biodiversity decreased significantly downstream the stormwater outlets compared to upstream. The lower biodiversity on downstream stations compared to upstream was supported by Sibanda et al. [27] and Narangarvuu et al. [31] who found similar relations.

Analysis of the invertebrate community in lakes and other freshwater and marine environments, is a commonly practiced method to determine the ecological condition in Denmark ^[32][33][32,33]. Depending on the species composition assumptions about the ecological condition can be made. Some larvae of the Diptera families, especially the Chironomidae, and the worms in the family tubificidae are particularly pollution tolerant because their blood contains hemoglobin which bind oxygen effectively enabling these animals to live in environments with low oxygen content ^[33][34][33,34]. Some families of Caddisfly larvae (Trichoptera), mayflies (ephemeroptera) and all families of stoneflies (Plecoptera) are particularly sensitive to pollution, because they have skin respiration which requires clean and oxygen rich water to accommodate [33]. Therefore, invertebrates as an indicator of the ecological quality both in streams and ponds. The biodiversity of other fauna groups and the microbial community can also provide data on for example effects of oxygen depletion, decomposition and pollution with feces.

3. Conclusions

In conclusion, stormwater ponds can be designed to both handle and treat the water sufficiently to avoid negative effects on the recipient and simultaneously provide a basis for invertebrate biodiversity in the pond along with other ecosystem services. In the design process the catchment size and type should be taken into consideration to ensure a sufficient size of the pond. In addition, the physical design of the pond is important to ensure sufficient settling and/or uptake of substances and at the same time niches and habitats for different species. This could also include small islands, stones or obstacles in the ponds that increases the distance from inlet to outlet and thereby the treatment efficiency and at the same time inducing better conditions for wildlife. Wet and dry ponds can provide valuable new habitats in the urban areas for insects and reptiles and other animals but require different maintenance practices. Accumulated sediment should be removed when needed to ensure sufficient retention time in the pond and to avoid negative effects on biodiversity. Focus on establishment of native plant species in and around the pond and avoiding the trimming of the pond surroundings more often than necessary but letting them develop into as natural ponds as possible even though they are created as technical facilities.