IC16_NZ

Project description
Sediment stability (i.e. potential for sediment entrainment or deposition) can be calculated using the physical properties of the sediment (grain size, shape density) and water (velocity, force and density) via the Shields parameter. When combined with wave and atmospheric information this kind of calculation is useful to sediment transport models. However, these calculations often exclude biological processes and cohesion properties that can have a considerable impact on sediments (see Black et al. 2002 for a review of cohesive sediments).
Benthic macrofauna (MF) such as bivalves, gastropod, or crustaceans, can act as bioturbators, building burrows and tunnels, destabilizing sediments. In contrast, microphytobenthos (MPB), microscopic photosynthetic algae, stabilize sediments (Lucas et al. 2003). The MPB excrete extracellular polymeric substances (EPS), a mucus like substance largely made up of proteins and carbohydrates that bind sediment grains together (Underwood and Smith 1998). MPB are thought to be limited by light, nutrients, and herbivorous grazers (i.e. MF feeding on MPB, thus limiting the population) (Andersen and Pejrup 2002). Therefore, the net effect of the benthos on sediment stability relies on a combination of factors including species-specific behaviours, population densities, and species interactions (Andersen and Pejrup 2002, Widdows and Brinsley 2002, Hewitt et al. 2008).
The goal of this PhD project is to examine the relationships between MF, MPB, and sediment stability on intertidal flats. The thesis consists of three studies, each a separate thesis chapter/manuscript: (1) manipulation MPB populations, and influences on sediment stability; (2) identifying how the relationships between MF, MPB and sediment stability vary across a sedimentary gradient (sand to mud); and (3) monitoring changes in benthic community compositions and sediment stability following defaunation caused by decomposing macrophytes.
Sampling for MF, MPB, sediment properties and erodibility is standardized across the three studies. Erodability is measured using the EROMES (erosion measurement system) (Schünemann and Kühl, 1991). The EROMES sediment cores are subject to a regular increase (0.1 N m-2) in bed shear stress every 2 minutes until 2 N m-2. During the erosion sequence an optical backscatter (obs) records suspended sediment within the water column and water samples are taken and filtered at intervals to calibrate the obs. After each erosion run, the cores are sieved through a 500 µm mesh, where MF are removed, preserved, and identified to the species level. Hydraulic conductivity cores are processed using the constant head method (Klute and Dirkson 1986). The syringe cores are used to determine: (1) MPB biomass using a fluorometer to determine chlorophyll-a and phaeophytin (Parsons et al. 1984), (2) EPS bulk and colloidal carbohydrate content using the Dubois assay (Underwood and Smith 1998), (3) grain size and mud content using a MALVERN, and (4) organic content.

Objective (1) manipulation of MPB populations and influences on sediment stability
The aim of this chapter is to determine in situ how key factors known to limit MPB populations: (1) shade and (2) grazer densities affects sediment stability. Shade was used to inhibit MPB photosynthesis while the density of the grazer Macamona liliana, was used to regulate the MPB biomass. This study was a collaboration with scientists in the benthic ecology and physical oceanography groups at the NZ National Institute of Water and Atmosphere (NIWA). The field experiment was established 25-28 October 2011 in Manukau Harbour, NZ. One hundred and ninety six plots were excavated to 10 cm depth and sieved to remove MF. Adult Macamona liliana found within these plots were stored and then later re-planted into the plots at known densities of: high (200 individuals m-2), med. (50 individuals m-2), and low (0 individuals m-2). This was followed by a shaded (cloth blocking 70% light, hobbo light logger) or non-shaded (control) treatment placed on top of each of the plots. Sampling for sediment stability was conducted 6-9 February 2012. All of the field and laboratory work for this study was completed by 30 May 2012. A large part of the time spent in Germany was dedicated to sorting through this data, and beginning to understand different types of statistical analysis that might be useful. I am now in the process of applying these different statistical techniques to my data.

Objective (2) identify relationships between MF, MPB and sediment stability across a sedimentary gradient
IC9 (Gerhard Bartzke) recorded how sediment stability and initial sediment entrainment dramatically changed when silt was added to sandy sediments. Another PhD student at the University of Waikato identified variations in benthic productivity and taxonomic richness over a gradient of mud content (Pratt et al., in review). Builidng on the results of IC9 and using the same field sites as Pratt et al.,this correlative study looks at how sediment stability and benthic community composition change across a sand- mud gradient . Whangamata and Kawhia Harbours were sampled in April 2012; Whitford was sampled in Nov 2013. The MPB and sediment laboratory work has been completed for all three estuaries, with a percent mud gradient achieved in all estuaries (across estuary total: 0-80 % mud). I am currently sorting the MF samples; data entry, and statistical analysis remain.

Objective (3) monitor changes in benthic community compositions and sediment stability following an experimental Ulva bloom
This study is a collaboration with Clarisse Niemand (NZ IC 7) and is set to begin March 2013. Ulva will be used as a natural disturbance event to create anoxic sediment conditions and defaunation. For me, the resulting interactions of benthic community compositions and sediment stability post-disturbance are of particular interest, while Clarisse will be looking at changes in productivity. Thus far we have collected 28 kg of Ulva, this will be placed out for 30 days and our monitoring will begin on April 1 2013.
This thesis is cross-disciplinary in nature and has linkages to several other INTECOAST projects. The work was developed in part from the results of IC9 (Gerhard Bartzke) who examined sediment stability from different silt-sand mixtures, whose data will compliment my mud gradient study; IC6 (Ruth Gutperlet) who looked at post-settlement shellfish dispersal, which is an important recovery process in disturbed systems and will be considered in the Ulva disturbance recovery study; and IC9 (Franziska Staudt) will build upon Gerhard Bartzke’s work, incorporating microbial information, and I hope that my data will contribute to her study.