Macrophytes. Golden Pond Wetlands. Tables 3 and 4 provide an indication of the changes in macrophytes plant cover between October 2000 and February 2002. Unfortunately, both wetlands were dredged of all vegetation in March 2002 by maintenance workers instructed to “tidy up and get rid of the weeds,” so monitoring was discontinued.
Table 3. Environmental indicator frameworks for measuring impacts on water resources.
|Submerged species:|| || || || || || |
|Floating-leaved species:|| || || || || || |
|Aquatic creepers:|| || || || || || |
|Paspalum distichum||-|| ||1||1||2||2|
|Emergent species:|| || || || || || |
|Typha orientalis||-||1||1||1.5||2||3|Table 4. Percentage cover of macrophytes in Wetland 2. Below-surface cover for Ceratophyllum.
When Wetland 1 (Table 3) was first surveyed in October 2000 (12 months after planting), it was dominated by waterlilies (Nymphoides and Nymphaea). Clumps of Schoenoplectus validus were restricted to the littoral margins. No plants of Baumea nor Lepironia were found, suggesting the unsuccessful survival of these species that were planted in deeper water. The greatest change in macrophyte cover occurred between October 2000 and February 2001 with the spread of the native aquatic creeper Ludwigia peploides, which took advantage of the floating-leaved species to assist in spreading from the banks towards the center. Between February 2001 and March 2001, there were two major storm events with rainfall of 86 mm and 161 mm. Flash flooding had three major impacts on the vegetation. Firstly, a large sediment load (mostly sand) “overflowed” from the small sediment basin and was deposited at the top of the wetland, smothering stands of Schoenoplectus. Secondly, scouring occurred in the flow path, causing the uprooting of Hydrilla and some Nymphoides. Thirdly, the high velocities washed away creeping stems of Ludwigia. Rapid recolonization of both Hydrilla and Ludwigia occurred post-storm. Ludwigia continued spreading. However, its smothering effect may have restricted the spread of both Nymphaea and Nymphoides. There was limited spread of emergent species along the landward margins due to continual brush cutting by maintenance workers seeking a “manicured lawn” effect (even as much as 1m into the water)!
Wetland 2 (Table 4) supported 90 percent surface cover in October 2000, dominated by Nymphaea and Nymphoides; beneath were dense stands of Ceratophyllum. Between October and February 2001, an increase in the creepers Ludwigia, Paspalum and Persicaria smothered the Nymphoides around the shallower margins. The storm events had a lesser impact due to the evenly distributed flow at the top of Wetland 2. However, some of the aquatic creepers (Paspalum, Ludwigia and Persicaria) were washed away. Post-storm recovery was rapid, and during the next 10 months, Persicaria (attenuatum and strigosa) spread to occupy 30 percent of the surface area, smothering, and displacing Nymphaea or Nymphoides.
Bridgewater Creek “Wetland”– Ponds
Table 5 provides a comparison of the temporal changes in extent of distribution of plant species in the different macrophyte zones from the initial planting in November 2001 until May 2006. The original planting scheme for the different macrophyte zones is shown in column 1 (Nov 2001). The “wetland” was designed to achieve 80 percent macrophyte cover in Ponds 2–6. However, several species did not survive the first three months: Eleocharis sphacelata and Lepironia articulata (planted in the deep marsh) and Eleocharis equisetina and Philydrum lanuginosum (planted in the marsh). Poor plant establishment occurred in all marsh zones following planting, but was worse in the deep marsh resulting in only two percent area cover. Thus, by May 2002 open water was 56 percent of the total wetland area (Ponds 2–6). Hence the wetland in effect became a pond system dominated by open water with a fringing littoral zone. During the next four years, the extent of open water increased to 80 percent as plants failed to establish and spread in the marsh zones, whereas the loss of plants in the ephemeral zone resulted in bare mud.
Table 5. Environmental indicator frameworks for measuring impacts on water resources.
|Ephemeral, RL 4–4.25||14||14||14||11.5||10||8||8||1|
|Carex appressa||+||+||+||+|| || || || |
|Isolepis nodosa||+||+||+|| || || || || |
|Philydrum lanuginosum||+||+||+||+|| || || || |
|Bolboschoenus fluviatilis|| || || ||+||+||+||+||+|
|Shallow marsh, RL 3.75 – 4||23||10||14||7||7||7||7||0.1|
|Baumea rubiginosa||+||+||+|| || || || || |
|Cyperus polystachys||+||+|| || || || || || |
|Isolepis nodosa||+||+|| || || || || || |
|Philydrum lanuginosum||+||+||+||+|| || || || |
|Bolboschoenus fluviatilis*|| || ||+||+||+||+||+||+|
|Schoenoplectus validus|| || ||+||+||+||+||+||+|
|Baumea articulata|| || ||+||+||+||+||+|| |
|Marsh, RL 3.5 – 3.75||24||15||6||6||0.2||0.2||0.2||0.05|
|Baumea rubiginosa||+||+||+|| || || || || |
|Bolboschoenus fluviatilis||+||+|| || || ||+||+|| |
|Eleocharis equisetina||+|| || || || || || || |
|Schoenoplectus mucronatus|| || ||+||+||+|| ||+|| |
|Schoenoplectus validus|| ||+||+||+||+||+||+||+|
|Baumea articulata|| || ||+||+||+|| ||+|| |
|Deep Marsh RL 3 – 3.5||20||2||2||0.5||0.2||0.2||0||0|
|Baumea articulata*||+||+||+||+||+||+|| || |
|Eleocharis sphaceleta||+|| || || || || || || |
|Schoenoplectus validus*||+||+||+||+||+||+|| || |
|Potamogeton in open water|| || || || || ||8||18||10|
From December 2005 until mid April 2006 the wetland had been inundated by water depths of up to 75cm above the ephemeral zone (Relative Level 5.0m). The cause was a blockage in the outlet riser orifice that prevented draining. This extended period of inundation caused the loss of Baumea articulate and Schoenoplectus validis. Only Bolboschoenus fluviatilis survived by landward migration into the terrestrial zone. A decline in Potamogeton also occurred during this period of increased water depths.
Monitoring macrophyte establishment in these two stormwater “wetlands” has identified many hurdles facing plant survival. The greatest challenge for stormwater wetlands is successful establishment, and the hydroperiod is the key factor. The failure of macrophyte establishment at the Bridgewater Creek Wetland was due to deeper water in the marsh zones and prolonged inundation in the ephemeral zones. Thus, careful consideration must be given not only to water depth, but also to the duration of inundation. Riser and weir levels must ensure that water levels recede to the appropriate Relative Level post storm/flood event to prevent extended periods of inundation. More research needs to be undertaken into understanding the tolerance of different species to periods of inundation. Scouring, erosion and loss of topsoil also contribute to poor plant establishment. Exposure of the clay base also prevented the spread of rhizomatous species. At the Golden Pond Wetland, submerged species and aquatic creepers were washed away in large storm events. However, once established, emergent macrophytes (Schoenoplectus validus) and floating-leaved attached macrophytes (Nymphaea and Nymphoides) withstood very high flow rates without becoming uprooted and washed away. Nevertheless, the incorporation of a high flow bypass channel in wetland design can minimize the impact of increased velocities within the wetland.
Phytoplankton: Chlorophyll a. At Golden Pond, chlorophyll a values were low (3.5 ± 0.6 μg L-1 in the sediment basin, 5.5 ± 3.2 μg L-1 in Wetland 1, and 3.2 ± 0.8 μg L-1 in Wetland 2). These values are below water quality objectives (8 μg/L-1).
At Bridgewater Creek, algal blooms occurred in dry weather in Ponds 1 and 2, but chlorophyll a was reduced in Ponds 3 to 6 (Table 6). This indicates a reduction in phytoplankton biomass, despite similar soluble inorganic nitrogen concentrations in the ponds. Although the mean phosphate concentration in Pond 6 was only 0.02 mg/L compared to 0.08 mg/L in Pond 1, the N:P ratios are not limiting for phytoplankton growth (Wetzel 2001; Bayley et al. 2005). Similar light profiles in all ponds also suggest that light is not a limiting factor. Numerous microcrustaceans, in particular cladocerans, were found in Ponds 2 to 6, and may have been active predators on the phytoplankton. Phytoplankton species diversity changed with seasons and following rain events (Bayley 2008). Many of the genera identified are noted for their occurrence in eutrophic waters.
Table 6. A comparison of chlorophyll-a (μgL-1) as an indicator of phytoplankton biomass in the ponds at Bridgewater Creek “Wetland.”
|Pond 1 (Sediment Basin)||Pond 2||Pond 3||Pond 4||Pond 5||Pond 6|
|64 ± 80||54 ± 60||15 ± 20||12 ± 12||10 ± 5||12 ± 10|
Chlorophyll a exceeded water quality objectives of 8 μg/L. During the first two years of operation dissolved oxygen profiles were similar in all ponds. However, due to the large quantities of organic matter (mostly leaf litter that has washed into Pond 1), it has now become anaerobic, with surface dissolved oxygen of 1.2 mg/L and bottom dissolved oxygen of 0.2 mg/L. These conditions are limiting phytoplankton growth.
Macroinvertebrates. Wetland plant diversity is important for determining macroinvertebrate associations and wildlife diversity (Knight et al. 2001) because of the creation of habitats and food resources. Wetzel (2001) noted that the most effective wetland ecosystems “are those that possess maximum biodiversity of higher aquatic plants and periphyton associated with the living and dead plant tissue.” From Table 7, it is very evident that the constructed stormwater wetlands and ponds increased species richness compared with the channelized upstream creek bed. At Bridgewater Creek, the vegetated section of the modified creek downstream of the ponds had the highest species richness. At Bridgewater Creek, Pond 6 had the highest diversity of hemipterans and coleopterans. Macroinvertebrate species richness showed an increase during the four years since construction at Bridgewater Creek from 12 taxa to 86 taxa with 28 percent of the families having a sensitivity grade greater than five. Golden Pond Wetland maintained approximately 60 taxa with 24 percent families having a sensitivity grade of greater than 5.
Table 7. Major macroinvertebrate taxa at Golden Pond and Bridgewater Creek Stormwater Systems.
| ||Golden Pond “Wetland System”||Bridgwater Creek “Pond System”|
|Macroinvertebate Taxa||Upstream Channel||Wetland 1||Wetland 2||Downstream Natural Creek||Upstream Channel||Pond 1||Pond 6||Downstream Modified Channel|
|Acarina||3|| ||1|| || ||2||1||3|
It is interesting to note that, although water quality objectives were not being achieved, both wetland and pond treatment trains improved over all macroinvertebrate biodiversity. By comparison, Greenway et al. (2003) found 90 taxa in the Cooroy Wetland (Noosa Shire Council) which receives secondary-treated sewage effluent (25 mg/L TN, 8 mg/L NOx-N, 12 mg/L NH4-N, 0.2 mg/L TN, and 0.02 mg/L PO4-P). This demonstrates that a wide variety of macroinvertebrate species can tolerate high nutrient concentrations.
Mosquitoes. In aquatic ecosystems, mosquito larvae are an integral component of aquatic food webs. However, because mosquitoes can pose a risk to public health, there is often concern that constructed wetlands will encourage mosquito breeding. While most mosquitoes are opportunistic breeders, they will only deposit eggs if a suitable body of water is available. A critical and significant issue for successful mosquito breeding is larval survival and whether adult mosquitoes emerge from pupae. If constructed wetlands and ponds are designed to function as ecosystems with a diversity of aquatic organisms, then natural predators would control mosquito breeding (Greenway et al. 2003).
In the wetlands at Golden Pond and Pond 6 at Bridgewater Creek (Table 8), less than five percent of sampling dips during a 12-month period contained mosquito larvae. When present, they were in very low numbers (less than 10/200 mL scoop). Pond 1 recorded more larvae (14 percent of dips), but these occurred among dead vegetation, and most were only the very juvenile first and second instars. No pupae were found, indicating that the larvae did not complete their life cycle. Predation by abundant microcrustaceans and notonectids appears to be controlling mosquito larvae.
Table 8. Relative abundance of mosquito larvae.
| || ||Bridgewater Creek Wetland|
|Location||Golden Pond Wetland||Ponds 1–5||Pond 6|
|Number of Dips||130||190||30|
|Percent NO larvae present||95.2||86.3||96.6|
|Percent less than 10 larvae||4.0||11.3||3.4|
|Percent greater than 1–40 larvae||0.8||2.4||0|
A constructed wetland with a diversity of plant species and macrophyte zones subjected to wetting and drying cycles, as well as open water zones, will maximize water treatment efficiency. It will also support a greater diversity of aquatic organisms. Deeper water zones will function as refuge habitats for these organisms during dry periods and allow rapid recolonization of newly inundated zones. This is particularly important for the management of potential mosquito breeding since predators of mosquito larvae will already be present.
Golden Pond “Wetland System” Treatment Train. Suspended Solids. Water-quality data for total suspended solids (TSS) and total volatile solids (TVS), such as the organic fraction, are given in Table 9. The 12 h wet samples showed considerable variation in TSS. This reflected the problems of sampling logistics following a storm event in the absence of automated samplers, as well as differences in rainfall intensity and duration. However, for any single event, there was consistency in TSS concentration throughout the treatment train. The mean values for 12 h wet were two-to-threefold higher than the 24 h wet. Dry weather samples were similar to 24 h wet weather samples at most sites. Higher TSS and TVS were recorded at Wetland 1 outlet, whereas lower TSS and TVS were recorded downstream.
Table 9. Summary of TSS and TVS (mg/L) at Golden Pond Wetlands treatment train. (NB: 140 ha catchment drains into sediment basin: 14 ha catchment (piped) drains into Wetland 2).
| ||12-hour wet||24-hour wet||Dry Weather|
| || ||TSS||TVS|| ||TSS||TVS|| ||TSS||TVS|
| ||± SD||± SD|| ||± SD||± SD|| ||± SD||± SD|
|Upstream||6||17 ± 12||4 ± 3||8||7 ± 6||3 ± 1||14||6 ± 4||3 ± 2|
|Out Sediment Basin||6||20 ± 8||5 ± 2||8||7 ± 5||3 ± 1||14||6 ± 3||3 ± 3|
|Out Wetland 1||6||26 ± 10||6 ± 3||8||10 ± 6||5 ± 3||13||14 ± 6||6 ± 3|
|Out Wetland 2||6||24 ±12||5 ± 3||8||13 ± 9||6 ± 5||13||13 ± 9||7 ± 5|
|Downstream||6||21 ± 16||5 ± 4||8||8 ± 5||4 ± 2||12||5 ± 2||3 ± 3|
Sediment Basin. TSS leaving the sediment basin was similar to the water entering the basin, indicating little or no settlement of finer particulates. During the 12 h wet sampling, there was resuspension at the top end due to the higher velocities of incoming water. TSS was consistently higher at the bottom of Wetland 1, indicating resuspension. During dry weather, this was caused by ducks, which use the shallower bottom end.
Wetland 1 and Wetland 2. A comparison between the bottom of Wetland 1 and Wetland 2 shows that TSS is generally reduced during dry weather but increases during wet weather, probably due to resuspension of accumulated sediment in the culverts and particles in Wetland 2. TVS was always higher at the bottom of Wetland 2 than Wetland 1, indicating an export of organic particulates.
Downstream. Water at the last sampling site, 600 m downstream from Wetland 2 consistently had the lowest TSS (below 15 mg/L) in the 24 h wet weather and dry weather samples. However, the 12 h wet weather samples showed little reduction in TSS. High-water velocities probably precluded filtration and settlement.
Nutrients. Water-quality data for nutrients for wet-weather samples (collected up to 12 hours after a storm event) are given in Table 10. The water-quality data for nutrients from dry-weather samples are summarized in Table 11.
Table 10. Environmental indicator frameworks for measuring impacts on water resources.
|Site||NH4-N (mg/L)||NO3-N (mg/L)||TN (mg/L)||PO4-P (mg/L)||TP (mg/L)|
|Upstream||0.10 ± 0.10||0.44 ± 0.33||1.27 ± 0.63||0.06 ± 0.05||0.11 ± 0.05|
|Out Sediment Basin||0.09 ± 0.09||0.38 ± 0.28||1.23 ± 0.58||0.06 ± 0.06||0.08 ± 0.05|
|Out Wetland 1||0.08 ± 0.07||0.28 ± 0.18||1.19 ± 0.58||0.06 ± 0.06||0.13 ± 0.07|
|Out Wetland 2||0.07 ± 0.04||0.22 ± 0.16||0.94 ± 0.51||0.05 ± 0.05||0.11 ± 0.06|
|Downstream||0.06 ± 0.03||0.18 ± 0.13||0.90 ± 0.42||0.05 ± 0.04||0.13 ± 0.08|
Table 11. Summary of nutrients for dry-weather samples (mean ± SD).
|Site||NH4-N (mg/L)||NO3-N (mg/L)||TN (mg/L)||PO4-P (mg/L)||TP (mg/L)|
|Mean ± SD||Mean ± SD||Mean ± SD||Mean ± SD||Mean ± SD|
|Upstream||0.03 ± 0.03||0.53 ± 0.67||0.57 ± 0.31||0.04 ± 0.04||0.08 ± 0.02|
|Out Sediment Basin||0.05 ± 0.05||0.56 ± 0.71||0.70 ± 0.33||0.03 ± 0.03||0.08 ± 0.01|
|Out Wetland 1||0.03 ± 0.03||0.25 ± 0.49||0.63 ± 0.33||0.02 ± 0.02||0.07 ± 0.02|
|Out Wetland 2||0.08 ± 0.09||0.25 ± 0.23||0.97 ± 0.50||0.05 ± 0.08||0.14 ± 0.07|
|Downstream||0.03 ± 0.03||0.09 ± 0.11||0.60 ± 0.28||0.02 ± 0.02||0.05 ± 0.03|
Sediment Basin. The wet weather samples show that soluble nutrients leaving the sediment basin were generally lower than the receiving upstream stormwater. However, during dry weather, some samples yielded an increase in NH4-N and NO3-N, possibly due to ammonification and nitrification of organic matter. At the bottom of Wetland 1 there was a slight decrease in all soluble nutrients, suggesting uptake by plants, algae, and periphyton.
Wetland 1 and Wetland 2. A comparison between the bottom of Wetland 1 and Wetland 2 shows a decrease in the wet but an increase for NH4-N and PO4-P in the dry. The increase appears to be due to particularly high concentrations of nutrients entering Wetland 2 from a piped stormwater outlet.
Downstream. Water at the downstream site consistently had the lowest soluble nutrient concentrations, again indicating removal by aquatic plants, algae, and periphyton.
Bridgewater Creek Wetland “Pond System.”Suspended Solids. Water-quality data for TSS and TVS are given in Table 12. The 12 h wet samples show that TSS in stormwater in the main Bridgewater Creek inlet is very high compared to Golden Pond stormwater. Between Pond 1(the Sediment Basin) and Pond 6, there is only a 20 percent reduction in TSS concentration. Only two samples of stormwater entering Pond 1 after 24 hours were collected, and these are very low –perhaps indicating clear water following flushing. The 24 h wet samples show a 56 percent reduction in TSS in Pond 1 compared to the 12 h samples. Nevertheless, the Pond 6 outlet concentrations still exceeded water quality objectives (15 mg/L). TSS in dry-weather samples were highly variable in the stormwater entering Pond 1, but almost 50 percent reduction occurred in the sediment basin to produce good water clarity (9.6 ± 5.6 mg/L TSS). However, between Pond 1 and Pond 6, TSS increased to produce an average outlet concentration of 15.6 ± 7.8 mg/L TSS (within the same magnitude as the bottom of Wetlands 1 and 2 at Golden Pond) again indicating re-suspension of sediment. Of particular note is the high (75 percent) organic proportion (TVS) in Pond 1 due to phytoplankton growth (Kasper and Jenkins 2004).
Table 12. Environmental indicator frameworks for measuring impacts on water resources.
|Site||12-hour wet weather|| ||24-hour wet weather||Dry weather|| |
|Creek Inlet||5||57.2 ± 17.8||25.0 ± 11.3||2||5.9 ± 4.8||1.9 ± 1.2||9||18.0 ± 16.9||4.6 ± 3.2|
|Piped Inlet||3||19.6 ± 8.4||8.1 ± 3.4||2||5.5 ± 4.8||1.7 ± 1.4||9||17.4 ± 15.8||4.6 ± 2.6|
|Pond 1 Out||7||41.4 ± 38.3||9.4 ± 5.1||6||18.5 ± 10.4||6.3 ± 3.0||75||9.6 ± 5.6||7.2 ± 4.7|
|Pond 6 Out||9||33.9 ± 33.4||8.3 ± 6.6||7||24.4 ± 9.8||9.5 ± 4.8||103||15.6 ± 7.8||6.6 ± 4.2|
Nutrients. Water-quality data for nutrients for the wet-weather samples are given in Table 13, and for dry weather in Table 14. The 12 h WW samples show that NH4-N and NO3-N in the stormwater runoff are comparable to the stormwater flowing into the sediment basin at Golden Pond. However, PO4-P was higher. TN and TP were also higher, indicating a greater particulate load. There was no reduction of soluble nutrients in Pond 1 but some reduction in TN and TP, suggesting settlement of particulates. Between Pond 1 and Pond 6, there was a large reduction in NO3-N (78 percent) and PO4-P (87 percent) concentrations, suggesting biological removal. TN and TP were also reduced, possibly due to further settlement of particulates. However, there was no reduction in NH4-N concentrations. These trends were similar to the Golden Pond Wetlands.
Table 13. Environmental indicator frameworks for measuring impacts on water resources.
|Site||NH4-N||NO3-N (mg/L)||PO4-P (mg/L)||TN (mg/L)||TP (mg/L)|
|Creek Inlet||0.14 ± 0.12||0.43 ± 0.17||0.14 ± 0.16||4.68 ± 2.70||0.70 ± 0.38|
|Piped Inlet||0.12 ± 0.10||0.42 ± 0.32||0.10 ± 0.09||3.62 ± 3.36||0.31 ± 0.22|
|Pond 1 Out||0.13 ± 0.11||0.46 ± 0.37||0.15 ± 0.21||2.69 ± 3.12||0.21 ± 0.19|
|Pond 6 Out||0.13 ± 0.22||0.10 ± 0.08||0.02 ± 0.02||0.84 ± 0.21||0.12 ± 0.08|
Table 14. Nutrients for Bridgewater Creek Pond System, dry-weather samples (x ± SD).
|Site||NH4-N (mg/L)||NO3-N (mg/L)||PO4-P (mg/L)||TN (mg/L)||TP (mg/L)|
|Creek Inlet||0.08 ± 0.09||0.57 ± 0.60||1.84 ± 1.01||0.19 ± 0.18||0.26 ± 0.11|
|Piped Inlet||0.06 ± 0.05||1.10 ± 0.40||1.97 ± 0.28||0.16 ± 0.10||0.24 ± 0.09|
|Pond 1 Out||0.10 ± 0.09||0.12 ± 0.24||1.28 ± 0.45||0.08 ± 0.06||0.22 ± 0.10|
|Pond 6 Out||0.11 ± 0.11||0.10 ± 0.14||1.04 ± 0.36||0.02 ± 0.01||0.17 ± 0.07|
The dry-weather samples showed similar base flow nutrient concentrations to the Golden Pond catchment for nitrogen, but were higher for phosphorus. NO3 was particularly high from the piped inlet, but was similar to NO3 from the piped systems at Golden Pond. NO3 and PO4 were both reduced in Pond 1. PO4 was further reduced from 0.08 mg/L PO4-P in Pond 1 to 0.02 mg/L in Pond 6, but there was only a small reduction in NO3. NH4 increased suggesting ammonification of organic matter.
Water Quality and Retention Time
At Golden Pond, discharge rates calculated for stormwater leaving the sediment basin and entering Wetland 1 ranged from 3 to 5.7 m3/s for extreme (greater than 20 y ARI) storm events, and from 0.15 – 0.8 m3/s for high-intensity rain squalls. At discharge rates greater than 0.45 m3/s, short-circuiting occurs through the middle due to the positioning of a single V-notch weir, the lack of dense emergent macrophytes, and the linear nature of the flow path through the wetland. Between Wetland 1 and Wetland 2, the water flows over a wide concrete sill, and the narrow outlet (1 m width) ensures that the water backs up, thereby increasing the retention time. It has been estimated that the average retention time for both wetlands during non-extreme storm events would be between three and five hours, and between five and 32 hours for less intense rainfall events. By contrast, the retention times for the pond system at Bridgewater Creek during wet weather range from 36 hours for a major storm event to six days for less intense rainfall. These longer retention times would account for the higher removal efficiency of NO3 and PO4 in the pond system compared to the wetland system at Golden Pond. Halcrow suggested that runoff should be retained for a minimum of three to five hours, and preferably 10 to 15 hours for good treatment efficiency (Shutes et al. 1997).
During dry weather, flows entering Wetland 1 at Golden Pond ranged from 0.0015 m3/s to 0.0003 m3/s. Thus, minimum retention time would be eight days in Wetland 1 and 16–20 days for both wetlands. At Bridgewater Creek during dry weather, retention times in the pond system were greater than 20 days.
Overall Discussion on Water Quality
In a comparative study of vegetated and non-vegetated stormwater basins, Bartone and Uchrin (1999) found negative removal efficiencies for TKN, NO3, TP, and PO4 in the vegetated basin during four storm events with export loads exceeding input loads. They attributed this to the stormwater flushing out stored water and associated organic matter and nutrients. This flushing-out effect when a detention system contains a permanent pond has been modelled by Somes et al. (2000). TSS was exported on two of the four events. Export also occurred in the non-vegetated basins, but to a lesser extent.
Bavor et al. (2001) found that reductions in bacterial concentrations in stormwater were significantly higher in a wetland system compared to a pond system, due to the more effective settling of fine particles (less than 2 μm) with attached microorganisms. They also found that most of the nitrogen and phosphorus associated with sediments is associated with the less than 2 μm size fraction, and is therefore more likely to be effectively removed in wetlands.
As previously discussed, during dry weather, TSS (and TVS) increases in the wetland system due to resuspension of organic partiles. In the pond system, TSS decreases in Pond 1 due to settlement, but increases again in Ponds 2 to 6. Kasper and Jenkins (2004) have shown that an increase in TSS 9 and TVS occurs after about 11 days following a storm event, possibly due to a combination of “biological growth” and resuspension. Resuspension appears to be largely caused by water birds which congregate in the shallows of Ponds 2, 3, and 4 to be fed by the local residents. Kasper and Jenkins (2004) have also identified wind as playing a significant role in the resuspension and movement of suspended solids during inter-event periods.
Phytoplankton biomass was highly variable in the pond system. The highest chlorophyll values were in Ponds 1 and 2. This would explain the removal of NO3 and PO4 in Ponds 1 and 2. By contrast, nutrient removal by periphyton would be small in these ponds due to the lack of macrophyte stems and leaves for biofilm attachment.
In the wetland system, phytoplankton biomass was low, but a large surface area for periphyton attachment was provided by the stems of water lilies, roots and stems of Ludwigia peploides, and the submerged pond weeds. The periphyton, submerged pond weeds, and the adventitious roots of Ludwigia and other aquatic creepers would all remove soluble nutrients from the water column. The dense Ceratophyllum in Wetland 2 probably accounts for most of the removal of NO3 and PO4 coming from the piped stormwater outlet. However, Wetland 2 and Ponds 2 to 6 were not effective in reducing NH4 concentrations. In fact, the increase in NH4 suggests amination and ammonification of organic matter.