Andrew Swales

The vulnerability of Indo-Pacific mangrove forests to sea-level rise.

Sea-level rise can threaten the long-term sustainability of coastal communities and valuable ecosystems such as coral reefs, salt marshes and mangroves. Mangrove forests have the capacity to keep pace with sea-level rise and to avoid inundation through vertical accretion of sediments, which allows them to maintain wetland soil elevations suitable for plant growth. The Indo-Pacific region holds most of the world’s mangrove forests, but sediment delivery in this region is declining, owing to anthropogenic activities such as damming of rivers. This decline is of particular concern because the Indo-Pacific region is expected to have variable, but high, rates of future sea-level rise. Here we analyse recent trends in mangrove surface elevation changes across the Indo-Pacific region using data from a network of surface elevation table instruments. We find that sediment availability can enable mangrove forests to maintain rates of soil-surface elevation gain that match or exceed that of sea-level rise, but for 69 per cent of our study sites the current rate of sea-level rise exceeded the soil surface elevation gain. We also present a model based on our field data, which suggests that mangrove forests at sites with low tidal range and low sediment supply could be submerged as early as 2070.

Reconstruction of urban stormwater contamination of an estuary using catchment history and sediment profile dating

Cores were collected from the length of Pakuranga estuary, a small urban estuary in Auckland, New Zealand, to determine sedimentation and contaminant history, and in particular the impact of urbanization. Catchment sediment loads for the most recent history (1953–1995), including urbanization since 1960, were reconstructed using the landcover history and soil erosion modeling. Pollen and14C dating and pre-urban landcover history were used to reconstruct early estuary sedimentation (i.e., post-3000 yr BP to 1960). Heavy metal concentrations, particle size,137Cs, pollen, and catchment sediment loads were all needed to disentangle the complex estuarine response to urbanization.137Cs profiles did not reflect the historical fallout pattern, but deposition of137Cs-labelled eroded catchment soil, coinciding with peaks in urban construction. Temporal variations in stormwater137Cs concentrations are likely due to varying contributions from137Cs-rich topsoil and137Cs-poor subsoils. A similar pattern was observed in heavy-metal concentrations and attributed to street runoff rather than topsoil being diluted by metal-poor subsoils. Dating of the sediment profiles showed that during urbanization sedimentation rates in the tidal creek and estuary were higher than sedimentation rates associated with past agricultural landuse and the original forest landcover. Urbanization has brought about substantial environmental changes in the upper estuary through continued infilling of shallow, intertidal areas, contamination by heavy metals to levels of ecological concern, sediment textural changes, and rapid mangrove colonization of formerly bare intertidal sediments.

Silt and sand transport in a deep tidal channel of a large estuary (Manukau Harbour, New Zealand)

Abstract Time series of suspended-silt concentration (measured by an optical backscatter sensor) and suspended-sand concentration (measured by an acoustic backscatter sensor) in a deep channel (14 m MSL) of a large New Zealand estuary (Manukau Harbour) are analysed. Suspended-sand concentration varied in phase with the current speed and bed shear stress on a ∼6.2-h cycle, but suspended-silt concentration varied on a semidiurnal (∼12.4-h) cycle such that the channel was clear of silt at high tide and maximum silt concentration occurred at low tide. Threshold for initiation of sand motion was found to be related to local skin friction, sand concentration profiles were consistent with settling flux balanced by gradient diffusion with a two-layer sediment diffusivity dependent on local friction velocity, suspended-sand reference concentration was explainable in terms of local skin friction, and changes in the channel-bed sediment were correlated with the suspended-sand load. Therefore, sand suspension in the channel is a “local” process and the channel bed is the source of the suspended-sand load. Suspended-sand flux was found to be a highly nonlinear function of the tidal current speed in the channel. In that case, tidal-current velocity asymmetry is the principal determinant of direction of net sand transport and the ebb/flood-dominance concept is valid. The relative phase of the M 2 and M 4 constituents of the tidal-current velocity, which quantifies tidal-current asymmetry, was calculated from a tide model of the estuary and results were mapped. The harbour was found to be broadly ebb dominant and therefore self-flushing of sand. Silt suspension in the channel was not related to the local boundary-layer dynamics, but was explainable in terms of the movement up and down the channel of a horizontal gradient in silt concentration, which implies the existence of a turbid water mass that is perched on the surrounding intertidal flats at high tide and that drains into the channel during ebb tide. Silt concentration in the channel increased during a short storm, which corresponded to the passage through the measurement site of a “turbid fringe” that was formed on the surrounding intertidal flats during the previous high tide under the action of breaking waves. Wave activity on the surrounding intertidal flats is therefore the principal determinant of suspended-silt load in the channel and an approach that treats waves and sediment pathways between intertidal flats and channels is needed for modelling silt transport.

Sediment processes and mangrove-habitat expansion on a rapidly-prograding Muddy Coast, New Zealand

Mangrove-habitat expansion has occurred rapidly over the last 50 years in the 800 km 2 Firth-of-Thames estuary (New Zealand). Mangrove forest now extends 1-km seaward of the 1952 shoreline. The geomorphic development of this muddy coast was reconstructed using dated cores ( 210 Pb, 137 Cs, 7 Be), historical- aerial photographs and field observations to explore the interaction between sediment processes and mangrove ecology. Catchment deforestation (1850s- 1920s) delivered millions of m 3 of mud to the Firth, with the intertidal flats accreting at 20 mm yr -1 before mangrove colonization began (mid-1950s) and sedimentation rates increased to ≤ 100 mm yr -1 . 210 Pb data show that the mangrove forest is a major long-term sink for mud. Seedling recruitment on the mudflat is controlled by wave-driven erosion. Mangrove-habitat expansion has occurred episodically and likely coincides with calm weather. The fate of this mangrove ecosystem will depend on vertical accretion at a rate equal to or exceeding sea level rise.

Influence of wave and sediment dynamics on cordgrass (Spartina anglica) growth and sediment accumulation on an exposed intertidal flat

Isolated patches ofSpartina anglica (cordgrass) at two sites on a wave-exposed mid-intertidal flat of the 340 km2 Manukau Harbor (Auckland, New Zealand) have developed very differently since being planted in the mid-1970s. Although the two sites are only 0.5 km apart and at the same intertidal elevation,Spartina patches at the easternmost site (site 1) have as much as an order of magnitude higher biomass and accumulated sediment volume thanSpartina patches at site 2. A field experiment was conducted to characterize waves and associated sediment dynamics at each site, which might explain whySpartina patches at the two sites have developed so differently over the past 25 yr or so. Suspended sediments were measured and wave characteristics were inferred from subsurface pressure data measured for 5 wk at bothSpartina sites and at an intermediate location. Bed-orbital speeds and frictional wave-energy dissipation were consistently lower at the easternmost site with the largerSpartina patches. The west-to-east reduction in wave energy is due to the spatial arrangement of theSpartina sites relative to the predominant wind fetches. The wave-energy gradient is maintained by tidal-cycle variations in fetch and bed friction and results in a west-to-east reduction in sand suspension. Silt, which is largely resuspended under southwest winds, is redeposited in the low wave-energy conditions in and around the larger site 1Spartina patches. Shell accumulation bySpartina patches at site 1 occurs infrequently, during southwest winds >10 m s−1 and water depths >0.7 m, when waves are least attenuated by bed friction. Large between-site differences in the growth of and sediment accumulation by theSpartina patch are consistent with the observed wave-energy gradient. The resulting spatial patterns of silt, sand, and shell resuspension and deposition directly influence the rate of sediment accumulation bySpartina patches and the composition of accumulated sediment on this wave-exposed intertidal flat.

Managing Mangrove Habitat Expansion in New Zealand

While mangroves are indigenous to northern New Zealand and an integral part of functioning estuaries, rapid expansion of mangrove forests has occurred in recent decades, resulting in widespread support for estuarine restoration projects focusing on mangrove removals. Mangrove expansion is primarily associated with changes in land-use that increase terrestrial sediment erosion and deposition into coastal and estuarine environments. Objectives for mangrove removal in northern New Zealand often include a desire by local residents to restore open estuary sandflat conditions in areas that have been colonised by mangroves since the 1950s, and reinstate the navigational, recreational and amenity value of these areas. However, the likelihood of successful restoration is rarely considered in consent decisions, and minimal information is available on long-term trends in ecosystem health from areas where mangroves were cleared. Here, we discuss methods of mangrove removal, and recovery trajectories at numerous mangrove removal sites to identify physical and biological attributes of sites that are associated with limited (or fast) recovery, and minimal adverse impacts. We also discuss cost-effective management strategies to manage further spread of mangroves in New Zealand. Within a challenging and politically vibrant topic, we are informing the ‘mangrove debate’ with science to create better outcomes for estuarine health.

Light attenuation - a more effective basis for the management of fine suspended sediment than mass concentration?

Fine sediment continues to be a major diffuse pollution concern with its multiple effects on aquatic ecosystems. Mass concentrations (and loads) of fine sediment are usually measured and modelled, apparently with the assumption that environmental effects of sediment are predictable from mass concentrations. However, some severe impacts of fine sediment may not correlate well with mass concentration, notably those related to light attenuation by suspended particles. Light attenuation per unit mass concentration of suspended particulate matter in waters varies widely with particle size, shape and composition. Data for suspended sediment concentration, turbidity and visual clarity (which is inversely proportional to light beam attenuation) from 77 diverse New Zealand rivers provide valuable insights into the mutual relationships of these quantities. Our analysis of these relationships, both across multiple rivers and within individual rivers, supports the proposition that light attenuation by fine sediment is a more generally meaningful basis for environmental management than sediment mass. Furthermore, optical measurements are considerably more practical, being much cheaper (by about four-fold) to measure than mass concentrations, and amenable to continuous measurement. Mass concentration can be estimated with sufficient precision for many purposes from optical surrogates locally calibrated for particular rivers.

Sediment remobilisation from decomposing cordgrass (Spartina anglica) patches on a wave‐exposed intertidal flat

Abstract Concerns about the potential adverse effects of Spartina invasion on New Zealand estuaries, such as habitat loss and degradation, have led to herbicide use to control its spread. An experiment was conducted in the Manukau Harbour (New Zealand) to determine sediment remobilisation rates (?E) from herbicide‐treated S. anglica patches. Paired treatment and control patches (≤25 m diam.) were monitored at two sites 500 m apart with different wave exposures. Above‐ (AGB) and below‐ground (BGB) biomass decomposition and AE were determined from repeat surveys over c. 2.5 years. Work done by waves (W), which integrates wave‐energy dissipation at the bed over time, was used as a surrogate measure of sediment transport potential. Complete AGB loss occurred in both treated patches within 3 months of initial herbicide application, whereas BGB (a matrix of roots and rhizomes) persisted much longer. An exponential decay model (r 2 = 0.6) fitted to the site‐two treatment data predicted an c. 85% reduction in BGB after 5 years, whereas the BGB changes for the larger and more heterogeneous site‐one treatment were more complex. BGB was a better predictor of average bed elevation than AGB in multiple regression models and, although bed elevation decreased through time and with waves, BGB was positively related to bed elevation at both sites. AE was similar for both treatment patches (c. 0.05 and c. 0.06 m yr‐1, P < 0.001, r2 ≥ 0.97) and complete sediment loss was predicted to occur within 6–10 years. The site‐one control patch accumulated sediment (0.01 m yr‐1 , P < 0.001, r 2 = 0.81) whereas the more wave‐exposed site‐two control eroded (0.01 m yr‐1, P = 0.0017, r 2 = 0.61). The potential for adverse sediment effects resulting from Spartina control at wave‐exposed sites will be minimal if the treated areas are small relative to the size of the estuary.

Management and rehabilitation of aquatic ecosystems: introduction and synthesis

Management and rehabilitation of aquatic ecosystems: introduction and synthesis John M. Quinn, Malcolm O. Green, Marc Schallenberg, Roger G. Young, Chris C. Tanner and Andrew Swales National Institute for Water and Atmospheric Research, Hillcrest, Hamilton, New Zealand; Streamlined Environmental, Hamilton East, Hamilton, New Zealand; Department of Zoology, University of Otago, Dunedin, New Zealand; Cawthron Institute, Nelson, New Zealand