New Zealand’s agricultural sector faces the challenge of maintaining productivity while minimizing impacts on freshwaters. This study evaluates the cost-effectiveness of various green infrastructure systems designed to reduce diffuse agricultural sediment and nutrient loads. Utilizing a quantitative economic and contaminant reduction modeling approach, we analyze the impacts of five interceptive mitigation systems: riparian grass filter strips, constructed wetlands, woodchip bioreactors, filamentous algal nutrient scrubbers, and detainment bunds. Our approach incorporates Monte Carlo simulations to address uncertainties in costs and performance, integrating hydrological flow paths and contaminant transport dynamics. Mitigation systems are assessed individually and in combination, using a greedy cyclical coordinate descent algorithm to find the optimal combination and scale of a system for a particular landscape. Applying the model to a typical flat pastoral dairy farming landscape, no single system can effectively address all contaminants. However, strategic combinations can align with specific freshwater management goals. In our illustrative catchment, the mean cost to remove the full anthropogenic load is NZD 1195/ha for total nitrogen, NZD 168 for total phosphorus, and NZD 134 for suspended solids, but results will vary considerably for other landscapes. This study underscores the importance of tailored deployment of green infrastructure to enhance water quality and support sustainable agricultural practices.

Understanding eutrophication effects on stream macrophytes is key to meaningful environmental management and governance. However, the response of macrophyte communities to eutrophication is complex and often the effect of eutrophication is obscured by other factors related to habitat variability including flow regime and light availability. In this study macrophyte community composition and abundance was quantified along a eutrophication gradient in 30 lowland streams. Slow flowing, medium sized streams with minimal shading were selected, to focus on examining water quality effects. The internal nutrient status of the plants and the bioavailable nutrient fraction of the sediment was measured and a national database of modelled stream descriptors was used to obtain information about land use and water quality variables. Statistical relationships between macrophyte community composition and clogginess and nutrient, underwater light, and carbon availability and flow disturbance were examined along the eutrophication gradient. Macrophytes with different growth strategies were related to different aspects of eutrophication and non-native taxa were found to dominate eutrophic streams. There was no relationship between nutrient concentrations and nutrient content of the macrophytes but sediment P was correlated with P concentrations in streams. Our results showed that 1) non-native species were common and clogginess was high in unshaded lowland streams with low water clarity and high nutrient and bicarbonate availability; 2) emergent species were associated with high nitrate availability and sediment phosphorus and low disturbance; 3) submerged species prevailed where water column total phosphorus and bicarbonate availability were high; and 4) nutrient concentrations that limit the development of clogginess are expected to be low where bicarbonate is high and flow or other physical disturbance is low. Our findings improve understanding of how stream macrophyte communities respond to changes in water quality related to eutrophication and anthropogenic changes in catchments and will contribute to informed management of macrophytes in streams.

Motivated by stream ecosystem degradation by eutrophication, we mimicked slow flowing lowland stream conditions with a novel experimental setup to further our understanding of aquatic plant responses to increases in nitrate and light. We conducted a mesocosm growth experiment of two species from the genus Potamogeton: P. crispus (alien) and P. ochreatus (native), grown at four nitrate and four light levels. We hypothesised that (i) internal nutrient status of the plants would scale with water column nutrient concentration, and that (ii) plant performance would reflect the nutrient status of the plant. Furthermore, we hypothesised that (iii) a low irradiance level would negate the effects of an increased nitrate level. In relation to (ii) we hypothesised that (iv) the traits of the alien species would enable it to outperform the native species where both the availability of light and nutrient resources was high. Internal tissue N content was broadly similar in the two higher (>250 μg NO3− L−1) and the two lower nutrient treatments (<20 μg NO3− L−1) in both species and plants were therefore collapsed into high and low N-groups. High-N individuals had higher growth rates than low-N ones regardless of species or light treatment and plants had reduced growth rates at the lowest light treatment, however this response was less evident for P. crispus. The highest growth rate was found at the high-N individuals of P. crispus at the highest light treatment, and correspondingly, in this treatment this species exhibited an increase in branching degree and lateral spread from the low-N plants. As P. crispus spreads by fragmentation, our results show it to be a highly effective competitor in anthropogenically impacted areas compared to its native counterpart. Our study exemplifies how light can influence eutrophication responses of plants and how both need to be accounted for in management decisions.

ABSTRACT Replanting riparian vegetation can improve ecological health but there are few guidelines that help stakeholders achieve shade targets cost-effectively. A computer model was extended to calculate average shade to direct and diffuse solar radiation in straight channels. It was used to recommend strategies for achieving shade targets, including the 70% guideline shown to prevent nuisance aquatic plant growths and extreme water temperatures. For similar trees on both banks, 70% shade occurs when the ratio of tree height (H) to stream width (W) lies in the range 1.0 < H/W < 2.6 depending on stream orientation and tree shape (canopy overhang and dripline height). With trees on both banks, shade is lower by up to 34% in east–west (EW) than north–south (NS) streams except for tall trees at the equinox. EW streams planted only on the north bank can have a 30% higher shade than NS streams planted on one bank, but low vegetation on the unplanted bank eliminates this difference. For EW streams in the Southern hemisphere, planting tall vegetation on the north bank is cost-effective, with low vegetation on the south bank providing additional benefits. Further work is planned on channel meandering, gaps between trees and canopy transmission.

Seagrass extent has declined globally, including in New Zealand. Following the success of an initial trial to transplant and re-establish intertidal seagrass at a former site in Whangarei Harbour, a second trial was initiated. This trial aimed to restore seagrass at a more distant former site and tested the efficacy of transplant units ranging in size from small cores (5 or 9 x 0.01 m diameter cores placed within a 0.25 m(2) plot) to larger entire plots (0.25 m(2) or 0.5 m(2)). Seagrass was transplanted in winter when plants were dormant. Plant cover of transplanted and donor plots and light climate at both sites were monitored over a period of four years. All but one of the 24 units survived, and the plants began to spread after 12-18 months to eventually develop patches ranging in size from 5 to 68 m(2). Seagrass recolonised all donor plots completely within 10 months. Results show that small cores and larger plots can both be used successfully to transplant and restore populations of Zostera muelleri with no long-lasting damage to donor meadows. However, use of small cores is recommended to minimise extractions and loss of transplanted material, and to simplify logistics.

Seagrass Zostera muelleri has been reported to reproduce mainly asexually in New Zealand. However, a recent study in Tauranga Harbour suggested that flowering might occur more often than previously thought. Here we provide evidence of intertidal seagrass flowering events in six New Zealand estuaries in which it has not before been documented. Further research on seagrass sexual reproduction is desirable as we hypothesise that either flowering events have formerly been missed due to the cryptic nature of the flowers, or that sexual reproduction is emerging as a response to increasing pressures on seagrass ecosystems. In addition, with evidence of seagrass decline occurring globally, and likely also in New Zealand, we recommend collection of seagrass seeds to preserve genetic variability and to attempt restoration efforts through assisted sexual reproduction techniques that have not yet been used in New Zealand.

The filamentous green alga Chaetomorpha ligustica (Cladophoraceae, Cladophorales) was recorded covering seagrass meadows at Pāuatahanui Inlet, New Zealand. Species of the genus Chaetomorpha are difficult to identify by their morphologically, and the identification of C. ligustica was confirmed by sequencing the 28S rRNA large subunit providing a high level of confidence in the species designation. In November 2019, we found C. ligustica intertwined with Ulva spp. forming dense, heavy and entangled structures. Here we report, for the first time, negative impacts of this species upon meadows of the New Zealand seagrass Zostera muelleri. We observed a significant loss of seagrass cover and evidence of anoxia under C. ligustica mats two weeks from the first sighting. Chaetomorpha ligustica can easily be misidentified in the field. This may lead to over- and under-reporting of species, and we recommend the need for more careful identification of macroalgal blooms in the future and further research on growth requirements and origins of strains.

Seagrass meadows are valuable coastal habitats that require sufficient light to flourish. A laboratory experiment was performed to measure the response of the seagrass Zostera muelleri to different levels of irradiance. Plants from a New Zealand estuary were grown submersed in artificial seawater for six weeks at five different daily irradiance levels ranging from 0.3 to 7.3 mol m(-)(2) d(-1). Plants were extinguished within three weeks at <= 1.4 mol m(-2) d(-1), showed some growth but then declined at 2.9 mol m(-2) d(-1) while biomass, quantified as total leaf area, doubled at 7.3 mol m(-)(2) d(-1). The results indicate that Z. muelleri is likely to require at least 7.3 mol m(-)(2 )d(-1) of daily irradiance under average summer water temperatures of 20 degrees C to maintain growth and above-ground biomass accrual. The irradiance requirements for this and other seagrass species that grow in the subtidal zone are anticipated to increase as ocean water temperatures rise, highlighting the critical importance of actions to maintain and improve coastal water clarity.

Seagrass meadows are important estuarine habitats, and in recent decades, have suffered global declines. Fine sediment pollution is recognised as a major cause of decline, usually attributed to the combined effects of reduction of photosynthetically available radiation (PAR) and burial. However, intruded fine sediment affects the seagrass rhizosphere and this interacts with reduced irradiance to affect seagrass performance. We undertook a 2 x 2 factorial mesocosm experiment, to examine the interaction between substrate muddiness and irradiance on seagrass growth and survival over a six-week period. The seagrass Zostera muelleri was grown on two substrates from the same estuary: (1) an inner estuary substrate with high mud content (42 %) from a location where seagrass formerly grew; and (2) an outer estuary substrate with moderate mud content (20 %) from a location at which seagrass persists. Two irradiance levels were used: (1) low (6.3 mol quanta m(-2) d(-1)) and (2) very low (2.3 mol quanta m(-2) d(-1)) both above a published compensation irradiance (E-c) of 1.9 mol quanta m(-2) Belowground biomass and rhizome growth were significantly reduced by substrate muddiness but not detectably affected by irradiance. Shoot growth, was reduced by both reduced irradiance and increased muddiness, with a significant interaction. We conclude that muddification of substrates imposes an increased irradiance requirement for Z. muelleri to cope with adverse rhizosphere conditions, which should be taken into account when planning seagrass conservation and restoration interventions.

Quantifying flow pathways within a larger catchment can help improve diffuse pollution management strategies across subcatchments. But, spatial quantification of flow pathway contributions to catchment stream flow is very limited, since it is challenging to physically separate water from different paths and very expensive to measure, especially for larger areas. To overcome this problem, a novel, combined data and modelling approach was employed to partition stream flow in the Piako catchment, New Zealand, which is a predominantly agricultural catchment with medium to high groundwater recharge potential. The approach comprised a digital filtering technique to separate baseflow from total stream flow, machine learning to predict a baseflow index (BFI) for all streams with Strahler 1st order and higher, and hydrological modelling to partition the flow into five flow components: surface runoff, interflow, tile drainage, shallow groundwater, and deep groundwater. The baseflow index scores corroborated the spatial distributions of the flow pathways modelled in 1st order catchments. Average depth to groundwater data matched well with BFI and Hydrological Predictions for the Environment (HYPE) modeled flow pathway partitioning results, with deeper water tables in areas of the catchment predicted to have greater baseflow or shallow and deep groundwater contributions to stream flow. Since direct quantification of flow pathways at catchment-scale is scarce, it is recommended to use soft data and expert knowledge to inform model parameterization and to constrain the model results. The approach developed here is applicable as a screening method in ungauged catchments.

This review collates research into fine sediment as a stressor of seagrass and emphasizes the multiple modes of action of this contaminant. The article is based on a bibliographic database search that identified 201 articles describing sediment impacts on seagrasses. Articles were classified by one of three non-exclusive modes of action: 1) light reduction; 2) smothering (burial), and 3) effects via rhizosphere physico-chemistry. Most citations (104) investigated multi-mode impacts of sediments, but the most frequently investigated single mode was light reduction (57 citations), followed by substrate rhizosphere chemistry (31) then smothering effects (6). Mud with high organic content is particularly problematic and smaller seagrasses are particularly vulnerable. Research gaps include polyphasic approaches, and studies of interactions between smothering, rhizosphere biogeochemistry and light climate. Identifying the thresholds of seagrass health indicators under mud stress should benefit coastal resource management, enabling improved decision-making and implementation of protective actions.

Seagrasses are important marine ecosystems situated throughout the world’s coastlines. They are facing declines around the world due to global and local threats such as rising ocean temperatures, coastal development and pollution from sewage outfalls and agriculture. Efforts have been made to reduce seagrass loss through reducing local and regional stressors, and through active restoration. Seagrass restoration is a rapidly maturing discipline, but improved restoration practices are needed to enhance the success of future programs. Major gaps in knowledge remain, however, prior research efforts have provided valuable insights into factors influencing the outcomes of restoration and there are now several examples of successful large-scale restoration programs. A variety of tools and techniques have recently been developed that will improve the efficiency, cost effectiveness, and scalability of restoration programs. This review describes several restoration successes in Australia and New Zealand, with a focus on emerging techniques for restoration, key considerations for future programs, and highlights the benefits of increased collaboration, Traditional Owner (First Nation) and stakeholder engagement. Combined, these lessons and emerging approaches show that seagrass restoration is possible, and efforts should be directed at upscaling seagrass restoration into the future. This is critical for the future conservation of this important ecosystem and the ecological and coastal communities they support.

Seagrass meadows are vulnerable to fine sediment (mud) pollution, with impacts usually attributed to reduction in submerged light. Here we tested two non-exclusive hypotheses, that mud particles (<63 µm) impact seagrasses through both (1) the light climate and (2) changes in substrate physico-chemistry. We tested these hypotheses in Pāuatahanui Inlet, New Zealand, by comparing seagrass presence, abundance, and health, together with light climate and substrate physico-chemistry at contrasting habitats where (1) seagrass used to thrive but no longer grows (historical seagrass), (2) seagrass still persists (existing seagrass) and (3) seagrass has been present recently, but not currently (potential seagrass). Historical seagrass substrate had significantly higher mud (35% average), bulk density (1.5 g cm−3), porewater ammonium concentration (65 µM), and a more reduced redox profile (negative redox at only 2 cm soil depth) as well as a lower light availability when submerged compared to other habitats, while total daily light exposure differed little between habitats. This suggests that failure of seagrass to recolonize historical seagrass habitat reflects substrate muddiness and consequent unfavorable rhizosphere conditions. Our results provide evidence for the multi-stressor effects of fine sediment on seagrasses, with substrate suitability for seagrass being detrimentally affected even where light exposure seems sufficient.

Globally, freshwater ecosystems are under threat. The main threats come from catchment land-use changes, altered water regimes, eutrophication, invasive species, climate change and combinations of these factors. We need scientific research to respond to these challenges by providing solutions to halt the deterioration and improve the condition of our valuable freshwaters. This requires a good understanding of aquatic ecosystems, and the nature and scale of changes occurring. Macrophytes play a fundamental role in aquatic systems. They are sensitive indicators of ecosystem health, as they are affected by run-off from agricultural, industrial or urban areas. On the other hand, alien macrophytes are increasingly invading aquatic systems all over the world. Improving our knowledge on the ecology and management of both native and alien plants is indispensable to address threats to freshwaters in order to protect and restore aquatic habitats. The International Aquatic Plants Group (IAPG) brings together scientists and practitioners based at universities, research and environmental organisations around the world. The main themes of the 15th symposium 2018 in New Zealand were biodiversity and conservation, management, invasive species, and ecosystem response and restoration. This Virtual Special Issue provides a comprehensive review from the symposium, addressing the ecology of native macrophytes, including those of conservation concern, and highly invasive alien macrophytes, and the implications of management interventions. In this editorial paper, we highlight insights and paradigms on the ecology and management of native and alien macrophytes gathered during the meeting.

Egeria densa and Elodea canadensis are two common invasive submerged macrophytes in streams and rivers worldwide. We conducted a six-week growth experiment with monocultures of E. densa and E. canadensis and mixed communities of the two species under five different shade levels (35%, 63%, 79%, 90% and 95%). Our aim was to test the effect of shade and competition on colonization of vegetative propagules of these species in flowing waters. We found that biomass accrual in both species was greatly reduced at high shade (≥ 79%) and that E. densa individuals accrued more biomass than E. canadensis individuals at all shade levels. At the least shade level (35%), interspecific competition with E. densa stimulated biomass accrual of E. canadensis individuals, compared to those within an E. canadensis monoculture. At higher shade (≥ 63%), this facilitation response disappeared, although E. canadensis plants continued to have longer stem length and a greater degree of branching than E. densa plants at ≤ 79% shade. At the highest shade level, competition with E. canadensis reduced biomass accrual of E. densa. Hence, shade influenced the competitive interactions between these two species. At low shade, the competitiveness of E. canadensis was enhanced, but in the field E. densa appears to be the dominant species in streams, especially those with higher levels of shade and less frequent disturbance by flushing flow events.

Reducing excessive reactive nitrogen (N) in agricultural waterways is a major challenge for freshwater managers and landowners. Effective solutions require the use of multiple and combined N attenuation tools, targeted along small ditches and streams. We present a visual framework to guide novel applications of ‘tool stacking’ that include edge-of-field and waterway-based options targeting N delivery pathways, timing, and impacts in the receiving environment (i.e., changes in concentration or load). Implementing tools at multiple locations and scales using a ‘toolbox’ approach will better leverage key hydrological and biogeochemical processes for N attenuation (e.g., water retention, infiltration and filtering, contact with organic soils and microbes, and denitrification), in addition to enhancing ecological benefits to waterways. Our framework applies primarily to temperate or warmer climates, since cold temperatures and freeze–thaw-related processes limit biologically mediated N attenuation in cold climates. Moreover, we encourage scientists and managers to codevelop N attenuation toolboxes with farmers, since implementation will require tailored fits to local hydrological, social, and productive landscapes. Generating further knowledge around N attenuation tool stacking in different climates and landscape contexts will advance management actions to attenuate agricultural catchment N. Understanding how different tools can be best combined to target key contaminant transport pathways and create activated zones of attenuation along and within small agricultural waterways will be essential.

Submerged macrophytes are an important component of stream ecosystems but invasion by non-native species can threaten the natural structure and function of these systems. Restoration of riparian shading may help to regulate the growth of macrophytes, particularly invasive species, which often proliferate in open streams. In a large, flowing outdoor mesocosm we studied the establishment and colonization success of three non-native macrophyte species (Egeria densa, Elodea canadensis and Ceratophyllum demersum) and one native charophyte (Nitella spp. aff cristata) that commonly occur in stream systems. Plant shoots were grown at five shade levels (35%, 63%, 79%, 90% and 95%) for eight weeks with regular monitoring of growth, morphology and biomass accrual. All species were able to establish at all shading levels. At high shade 90%) we found a reduction in lateral spread and branching and an increase in main stem length for the non-native species, while Nitella did not show any response for these traits. Biomass accumulation was delayed at high shade for all species. At the end of the experiment, the highest biomass accrual was observed at the lowest shade level (35%) for E. densa and E. canadensis, at 63% shade for C. demersum and at 79% shade for Nitella. Our results confirm, and provide additional examples, that both native and non-native submerged macrophytes can establish and colonize streams that are highly shaded but that the growth of non-native species, in particular, can be limited. Our study also highlights that there are differences in shade tolerance among non-native species.

Understanding how inter-specific variation in functional traits affects native and non-native species responses to stream disturbances, is necessary to inform management strategies, providing tools for biomonitoring, conservation and restoration. This study used a functional trait approach to characterise the responses of macrophyte assemblages to reach-scale disturbances (measured by lack of riparian shading, altered hydromorphology and eutrophication), from 97 wadeable stream sites in an agriculturally impacted region of New Zealand. To determine whether macrophyte assemblages differed due to disturbances, we examined multidimensional assemblage functional structure in relation to eleven functional traits and further related two functional diversity indices (entropy and originality) to disturbances. Macrophyte assemblages showed distinct patterns in response to disturbances, with riparian shading and hydromorphological conditions being the strongest variables shaping macrophyte functional structure. In the multidimensional space, most of the non-native species were associated with disturbed conditions. These species had traits allowing faster colonisation rates (higher number of reproductive organs and larger root-rhizome system) and superior competitive abilities for resources (tall and dense canopy, heterophylly and greater preferences for light and nitrogen). In addition, lack of riparian shading increased the abundance of functionally distinct species (i.e. entropy), and eutrophication resulted in the growth of functionally unique species (i.e. originality). We demonstrated that stream reach-scale habitat disturbances were associated to a dominance of more productive species, equating to a greater abundance of non-native species. This, can result in a displacement of native species, habitat alterations, and changes to higher trophic level assemblages. Our results suggests that reach-scale management efforts such as the conservation and restoration of riparian vegetation that provides substantial shading and hydromorphologically diverse in-stream habitat, would have beneficial direct and indirect effects on ecosystem functioning, and contribute to the mitigation of land-use impacts.

Seagrass (Zostera muelleri Irmisch ex. Aschers) was historically prolific in Whangarei Harbour, New Zealand, but has since declined. Over two years we compared the survival of mid-intertidal zone transplants (18 x 0.25 m(2) plots) from a remnant meadow to a former site using three methods: (1) intact `sods', (2) unanchored `sprigs' and (3) sprigs amongst `mats' of artificial plants. Before and after transplant, we quantified plant cover in all plots, and in transects across the wider intertidal zone. We also measured plant biomass, irradiance and water quality at each site. We found that sods and sprigs were equally effective with plant cover increased from <1 to 63%, but mats were ultimately not successful. Cover across the wider transplant site increased from 10% to 46% (biomass from 58 to 321 g m(-2)). Donor plots regenerated within 9 months. This study shows that Z. muelleri can be rehabilitated by transplant upon reinstatement of suitable growing conditions at former sites.

Sexual reproduction by seagrasses is highly variable within and amongst species and the factors influencing this variability are not well understood. In particular little is known for Zostera Imielleri in New Zealand. After discovery of a flowering shoot in a North Island estuary, we quantified their abundance across the intertidal zone at multiple sites monthly for 2.5 years. We also measured plant cover, leaf size, light intensity and temperature. Seed bank density, plant biomass and porewater nutrients were included in a subsequent investigation of one site. We found that flowering shoots were present annually during spring and summer but not at all sites, and generally at low densities (mean: 1.3 shoots m(-2), maximum: 72 shoots m-2). Plant cover was significantly higher (by 1.5-fold) and leaf size was significantly larger (length by 1.5-fold, width by 1.8-fold) in plots containing flowering shoots than in plots with none. The single site investigation showed that dense patches with flowering shoots had significantly higher biomass (1.7-3.9-fold) than non-flowering areas. Our results correct a previous misconception that flowering in New Zealand Zostera muelleri is rare. They also suggest that allocation of energy into sexual reproduction in this species is plant cover and biomass dependent. Thus anthropogenic or natural factors that limit the development of dense patches or meadows in the intertidal zone may inhibit sexual reproduction and, consequently, genetic diversity and fitness. (C) 2016 Elsevier B.V. All rights reserved.