John S. Clayton

Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands—II. Removal of nitrogen and phosphorus

The effect of influent loading rate on mass removal of nitrogen and phosphorus from dairy parlour wastewaters was compared in four pairs of planted (Schoenoplectus validus) and unplanted gravel-bed wetlands (each 19 m2). The wetlands were operated at nominal retention times of 7, 5.5, 3 and 2 days, with in and outflows sampled fortnightly over a 20 month period. Hydraulic flows were monitored to enable calculation of the mass flows of nutrients, and plant biomass and tissue nutrient levels sampled to evaluate plant nutrient uptake. Influent water quality varied markedly during the trial period (TN, 10–110; NH4-N, 5–70; and TP 8–18 g m−3). As theoretical wastewater retention times increased from 2 to 7 days, mean reduction of TN increased from 12 to 41% and 48 to 75% in the unplanted wetlands and planted wetlands, respectively, and TP removal increased from 12 to 36% and 37 to 74%, respectively. In the planted wetlands, mean annual removal rates of TN (0.15–1.4 g m−2 d−1) and TP (0.13–0.32 g m−2 d−1), increased gradually with mass loading rates. The unplanted wetlands showed a marked decline in TN and TP removal at high loadings. Net storage by plants in the first year of monitoring accounted for between 3 and 20% of the greater N removal and between 3 and 60% of the greater P removal in the planted wetlands.

Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands—I. Removal of oxygen demand, suspended solids and faecal coliforms

The effect of influent loading rate on mass removal of BOD, SS and faecal coliforms (FC) from dairy parlour wastewaters was compared in four pairs of planted (Schoenoplectus validus) and unplanted gravel-bed wetlands (each 19 m2). The wetlands were operated at nominal retention times of 7, 5.5, 3 and 2 days, with in and outflows sampled fortnightly over a 20 month period. Hydraulic flows were monitored to enable calculation of the mass flows of pollutants. Influent water quality varied markedly over the trial period (CBOD5 20–300 g m−3; SS, 60–250 g m−3; FC, 103–106 MPN (100 ml−1). NBOD was an important component of total BOD, being around 1.5 times higher than the influent CBOD5, and 2–10 times higher than the effluent CBOD5. Outflow levels of CBOD5, SS and faecal coliforms rapidly mirrored changes in influent loadings. Mean mass removal of CBOD5 increased from 60–75% to 85–90%, total BOD (CBOD5 + NBOD) from 50 to 80% and FC from 90–95 to >99% with increasing wetland retention time during the first 12 months of monitoring. Mean annual SS removals of 75–85% were recorded irrespective of loading rate. High levels of dissolved humic colour in the wastewaters were little affected by passage through the wetland at short retention times, but were reduced by up to 40% at longer retentions. Mass removals of CBOD5, SS and FC showed monotonic relationships to mass loading rates, with little difference between the performance of planted and unplanted wetlands, except for CBOD5 at high loadings (> 3 g m−2 d−1). The planted wetlands showed significantly improved removal rates for CBOD5 at higher loadings, and 1.3 to 2.6 fold higher mass removals of total BOD.

A Monitoring and Classification System for New Zealand Lakes and Reservoirs

Five variables gave strong indications of change in the trophic state of New Zealand lakes, namely; concentrations of chlorophyll a, total phosphorus and total nitrogen, as well as Secchi depth and...

Competitive performance of Hydrilla verticillata in New Zealand

Abstract In outdoor tanks, Hydrilla verticillata was grown in equal start, and established, cultures with each of four other submerged adventive aquatic macrophytes Egeria densa , Lagarosiphon major , Ceratophyllum demersum and Elodea canadensis . Plant growth and performance were monitored for a year, then all the plants and H. verticillata tubers (subterranean turions) were harvested. There was a significant difference in H. verticillata biomass obtained when grown with each of the competitor species and the highest and lowest biomass values were obtained with C. demersum and E. densa, respectively. Planting time also affected H. verticillata biomass. In equal start experiments with any competitor species biomass was significantly greater than when the competitor species had between 2–8 weeks to establish. Similarly, H. verticillata tuber numbers were greater when H. verticillata was grown with C. demersum compared with other hydrocharitaceans. Although its growth varied significantly, depending on the species with which it was planted, and it encroached on the space occupied by its competitors, H. verticillata appeared to have little impact on the biomass of its potential competitors over the duration of this study. Nevertheless, the capacity of H. verticillata to grow and compete effectively with other adventive aquatic macrophyte species that already cause problems in lakes and waterways was evident and highlights the highly invasive nature and threat that H. verticillata poses to the New Zealand aquatic environment.

The impact of invasive submerged weed species on seed banks in lake sediments.

Seed banks within submerged sediments were sampled from 21 New Zealand lakes with varying degrees of invasion by vegetatively reproducing, adventive hydrocharitacean species, to examine the influence of vegetation type on seed bank characteristics. Seed number and seed species richness were significantly lower at sites where the submerged vegetation was dominated by the adventive weeds Elodea canadensis Michaux, Egeria densa Planchon or Hydrilla verticillata (Linn. f.) Royle, compared with a predominantly native vegetation. The greater the degree of exclusion of native seed producing plants by adventive weed beds, the greater the apparent impact upon seed abundance. In contrast, seed numbers beneath the adventive weed beds did not differ significantly from sites in lakes where a large scale decline in submerged vegetation had taken place subsequent to hydrocharitacean invasion. The observed impact of weed invasion on lake sediment seed banks is attributed to reduced in situ seed production over time and may be accentuated by increased sediment accumulation beneath tall growing, high biomass weed beds. As seed density influences the speed and extent of germination and plant establishment, our results suggest a large reduction in the re-vegetation potential of invaded lakes. Seed bank decline may also provide an additional ‘ecological feedback mechanism’ contributing to the maintenance of stable, turbid de-vegetated lakes.

Invasion strategies in clonal aquatic plants: are phenotypic differences caused by phenotypic plasticity or local adaptation?

BACKGROUND AND AIMS The successful spread of invasive plants in new environments is often linked to multiple introductions and a diverse gene pool that facilitates local adaptation to variable environmental conditions. For clonal plants, however, phenotypic plasticity may be equally important. Here the primary adaptive strategy in three non-native, clonally reproducing macrophytes (Egeria densa, Elodea canadensis and Lagarosiphon major) in New Zealand freshwaters were examined and an attempt was made to link observed differences in plant morphology to local variation in habitat conditions. METHODS Field populations with a large phenotypic variety were sampled in a range of lakes and streams with different chemical and physical properties. The phenotypic plasticity of the species before and after cultivation was studied in a common garden growth experiment, and the genetic diversity of these same populations was also quantified. KEY RESULTS For all three species, greater variation in plant characteristics was found before they were grown in standardized conditions. Moreover, field populations displayed remarkably little genetic variation and there was little interaction between habitat conditions and plant morphological characteristics. CONCLUSIONS The results indicate that at the current stage of spread into New Zealand, the primary adaptive strategy of these three invasive macrophytes is phenotypic plasticity. However, while limited, the possibility that genetic diversity between populations may facilitate ecotypic differentiation in the future cannot be excluded. These results thus indicate that invasive clonal aquatic plants adapt to new introduced areas by phenotypic plasticity. Inorganic carbon, nitrogen and phosphorous were important in controlling plant size of E. canadensis and L. major, but no other relationships between plant characteristics and habitat conditions were apparent. This implies that within-species differences in plant size can be explained by local nutrient conditions. All together this strongly suggests that invasive clonal aquatic plants adapt to a wide range of habitats in introduced areas by phenotypic plasticity rather than local adaptation.

Successive macrophyte invasions within the submerged flora of Lake Tarawera, Central North Island, New Zealand

Abstract Lake Tarawera is the lake with the clearest water in New Zealand to be invaded by four particularly successful invasive adventive weeds: Ceratophyllum demersum, Egeria densa, Elodea canadensis, and Lagarosiphon major. E. canadensis was the first to invade the lake followed by L. major which became the dominant tall‐growing species to 6 m depth. Comparisons of lake survey data for macrophytes in Lake Tarawera from 1988 and 1993/94 show that marked changes occurred in the composition and patterns of submerged vegetation as a result of a recent invasion by C. demersum. This species was first recorded in 1988 as infrequent, within a restricted area of the lake. Within 5 years it occurred in 52% of the lake profiles, had a depth range of 0.5–15.5 m, and a median average cover estimated to be between 51 and 75% throughout this depth range. The spread of C. demersum has been at the expense of native vegetation (particularly Characean meadows) and E. canadensis. E. densa was first recorded at the same ti...

Aquatic plants as environmental indicators of ecological condition in New Zealand lakes

Submerged aquatic plants can act as measurable indicators of ecological conditions occurring within a lake, and they need only be monitored once a year or even less. Historically in New Zealand there has been a reliance on water quality sampling for monitoring the health of lakes and these methods can be complex and costly involving multiple site visits and chemical analysis of water samples. As a result, lake monitoring has been irregular, or not done at all. LakeSPI or ‘Lake Submerged Plant Indicators’ is a new management tool that uses aquatic plants to monitor and assess ecological condition in a wide range of lake types. The method generates three indices: a Native Condition Index (extent and diversity of native plants) and an Invasive Condition Index (extent and impact of alien weeds), which are generated from scores allocated to carefully selected vegetation features; and an integrated LakeSPI Index which is largely derived from components of the other two indices and provides an overall indication of lake ecological condition. The LakeSPI method can be used to assess the status of lakes and monitor trends occurring within them, and it is expected that the use of LakeSPI will facilitate regular monitoring and reporting on a much wider range of lakes than has been possible using traditional water quality methods. By utilizing submerged aquatic plants the method focuses on lake littoral margins where there is greatest public interaction and interest.

Analysis of relationships between maximum depth limits of aquatic plants and underwater light in 63 New Zealand lakes

Abstract Data from 63 New Zealand lakes were analysed to determine latitudinal and other patterns in the relationship between water clarity and the maximum recorded depths for submerged aquatic macrophytes (Zc). Vascular plants were encountered at Zc in 25% of the 63 lakes investigated. Characean algae extended to greater depths and occurred at Zc in 71% of the lakes, with a maximum depth of 34.4 m. There was a difference between North and South Island lakes, in values of Zc predicted from water clarity, that was not consistent with their latitudinal displacement. In clear North Island lakes, Zc was predicted to be less than in lakes of similar water clarity in the South Island and to reach an asymptotic maximum depth of 21 m. In contrast, the relationship between water clarity and Zc in South Island lakes was linear, with Zc of more than 30 m in some lakes at c. 2% of subsurface irradiance. Although the availability of light explained depth limits in most South Island lakes, other factors (e.g., grazing impact from freshwater crayfish) appear to be contributing to the shallower Zc in clear North Island lakes.

Genetic diversity in three invasive clonal aquatic species in New Zealand

BackgroundElodea canadensis, Egeria densa and Lagarosiphon major are dioecious clonal species which are invasive in New Zealand and other regions. Unlike many other invasive species, the genetic variation in New Zealand is very limited. Clonal reproduction is often considered an evolutionary dead end, even though a certain amount of genetic divergence may arise due to somatic mutations. The successful growth and establishment of invasive clonal species may be explained not by adaptability but by pre-existing ecological traits that prove advantageous in the new environment. We studied the genetic diversity and population structure in the North Island of New Zealand using AFLPs and related the findings to the number of introductions and the evolution that has occurred in the introduced area.ResultsLow levels of genetic diversity were found in all three species and appeared to be due to highly homogeneous founding gene pools. Elodea canadensis was introduced in 1868, and its populations showed more genetic structure than those of the more recently introduced of E. densa (1946) and L. major (1950). Elodea canadensis and L. major, however, had similar phylogeographic patterns, in spite of the difference in time since introduction.ConclusionsThe presence of a certain level of geographically correlated genetic structure in the absence of sexual reproduction, and in spite of random human dispersal of vegetative propagules, can be reasonably attributed to post-dispersal somatic mutations. Direct evidence of such evolutionary events is, however, still insufficient.