Quality Characteristics and Construction Management Strategies of Wet Pond Habitat: A Case Study of the Pilot Chongqing Sponge City Project

Objective Domestic sponge city construction has gradually entered the stage of refined evaluation and optimized construction. At present, domestic research on sponge habitats focuses on stormwater management and biodiversity benefits. Wet ponds in sponge facilities, as sponge habitats with standing water levels, are key habitat heterogeneity patches in urban green spaces with significant ecological service potential. This research aims to enhance the construction quality of wet pond habitat, monitor and identify problems in the quality of native wet pond habitats, and propose strategies to improve the quality of wet pond habitat, and promote the enhancement of the ecological service function of this key habitat.

Methods Based on the completed wet pond facilities in the pilot sponge city area of Chongqing, this research monitored the quality of vegetation, soil, and water body in the wet pond habitat within the research area from 2021 to 2023. In this research, eleven representative wet pond facilities in Yuelai International Convention and Exhibition City are selected, and information such as plant species name, strain, and cover is recorded on site. Wet pond soil is surveyed and sampled to further clarify the physical properties and fertility quality characteristics thereof. Wet pond water samples are collected in sunny days and days with heavy rain or torrential rain, and water pollution indicators are measured. Vegetation habitat quality is assessed by the M-Godorn stability measurement method, soil habitat quality is assessed by the Nemero fertility index method, and water habitat quality by the Nemero pollution index method. Additionally, Kruskal-Wallis test is used to investigate whether there exists a significant difference in the quality of wet pond habitat between the construction land and the non-construction land, and Pearson analysis is used to investigate the correlation between the construction characteristics and water quality of wet pond.

Results A total of 176 species of plants classified into 140 genera under 77 families are surveyed in the wet pond habitats, dominated by herbaceous species. Specifically, native species and invasive plant species account for 71.64% and 3.41% respectively, 7 plants belong to Class I invasive species with very strong invasibility, and the average cover of invasive plants in each wet pond accounts for 7.07%. The Euclidean distance range for plant community stability in wet ponds is 17.58 – 27.00, and all wet ponds are featured by a low level of plant community stability. Plant community stability and invasive plant cover are significantly higher in urbanized area than non-urbanized area. The wet pond soil has serious compaction problems with a mean value of 1.445 g/cm³. 50% of the soil samples have a saturated infiltration rate of 6.604 mm/h, indicating poor infiltration performance. The composite fertility index of the soil samples ranges from 0.532 to 1.198, indicating poor fertility. The soil is alkaline overall, with low organic matter content, sufficient effective phosphorus content, sufficient quick-acting potassium content, and insufficient hydrolysable nitrogen content. During the monitoring period, the Nemero pollution index of water body ranges from 1.087 to 3.757, the overall water quality is good enough to meet the class V water standard, except for wet ponds g and k. In addition, each wet pond is featured by good water body turbidity, with NTU, DO, COD and COD₅ being up to standard, and pH value in the middle alkaline range. The water body habit is out of standard overall in terms of permanganate, with a few water bodies being out of standard in terms of TN, TP, and NH₃-N. The perimeter-area ratio of wet pond is significantly negatively correlated with the Nemero pollution index (r=–0.665, p<0.05). The narrower and longer a wet pond, the lower the pollution index. The depth of wet pond is not significantly correlated with the Nemero pollution index, but data show that the depth of wet ponds with better water quality is mostly in the range of 0.55 – 1.45 m. Pre-positioned ponds can significantly clarify various pollution indexes regarding the water body in wet pond, especially in times of heavy rain.

Conclusion The quality of wet pond habitat can be improved through rational design, construction and management. In terms of design and construction, it is possible to add a front pond, increase the length – width ratio of wet pond, separate the relative position between the inlet and outlet of wet pond, and control the depth of wet pond at 0.6 – 1.2 m. In terms of management and maintenance, it is possible to reduce the frequency of weed management on construction sites, eliminate invasive species, and strengthen supervision to avoid soil trampling. This research can enrich the content of habitat monitoring in sponge cities, and provide an important reference for the refined construction and management of sponge cities.