IC18_NZ

Large intertidal macroalgal blooms such as Ulva sp. (hereafter referred to as Ulva), are a common disturbance to intertidal estuarine benthic fauna worldwide (Everett, 1994). Ulva, commonly referred to as sea lettuce, is a green leafy macroalgae which are natural components of many littoral ecosystems, and attaches to the substratum by a simple holdfast. Parts of, or entire blades may easily become detached and have the ability to continue to grow (Fritsch, 1971). During warmer spring/summer months, and under suitable conditions, Ulva growth is significantly enhanced, and can be as much as 20 – 30% per day (Parker, 1981). This results in large blooms, which can have detrimental impacts on benthic communities. Globally, Ulva blooms have increased in abundance as coastal systems have become increasingly eutrophic (Vermaat & Sand-Jensen, 1987). The overall aim of my thesis is to examine the impact of these macroalgal mats on the biodiversity and ecosystem functioning of estuarine intertidal habitats, and the density dependent effects of two functionally different key species in the processing of Ulva in a benthic intertidal community. This will be achieved through three separate studies, each which will be published as a manuscript.

Study 1: The density dependent effects of two key macrofaunal species on the breakdown of Ulva in intertidal communities.

The objective of this research was to examine the density dependent effects of a deposit- and suspension-feeding bivalve (Macomona liliana and Austrovenus stutchburyi respectively) dominated community on the breakdown, loss and burial of Ulva into the sediment. This was achieved by setting up a laboratory experiment with sediment cores comprising of different community compositions, adding known quantities of Ulva detritus that was labelled with 13C and 15N isotopes, then measuring the amount Ulva remaining and its distribution with depth in the sediments. Initial results have shown clear differences among treatments, with chlorophyll a profiles changing with the with bivalve density and species. The experimental work was completed in December 2011.

Study 2: The recovery of biodiversity and ecosystem function following a disturbance caused by Ulva mats in different inter-tidal habitats.

The objective of this study is to examine the recovery of soft sediment communities in terms of biodiversity and function following a disturbance event. I aim to bridge the gap in the current scientific knowledge of the restoration of the biodiversity and functioning of communities within two distinct community types following transient disturbances. This experiment is due to commence in March 2013 and will be in collaboration with Rachel Harris (NZIC5). My part of the study will examine the impact of large Ulva mats on the biodiversity and productivity of benthic communities, whilst Rachel will focus on sediment stability.

Study 3: The effects of decomposing Ulva detritus on macrofaunal biodiversity and benthic function: a dose-response experiment.

The main objective of this experiment is to examine the effects of different amounts of algal detritus on the biodiversity and ecosystem functioning (i.e. primary productivity and nutrient fluxes) of a sand flat community. This was achieved by conducting a field experiment whereby different amounts of Ulva detritus (zero, 60g/dw, 120g/dw and 240g/dw) were added to experimental plots and changes in biodiversity and function measured over an eight week period. Laboratory and data analysis is currently underway for this study but preliminary results have indicated consistent differences in benthic respiration among treatments.

My work compliments to past and current INTERCOAST (IC) projects. Alex Port (NZIC6) is focussing on the factors that regulate the growth of Ulva mats, whilst Ruth Gutperlet (IC6) examined the post-settlement dispersal by juvenile shellfish, which is a key process in ecosystem recovery following disturbances. I am also actively collaborating with Rachel Harris (NZIC5) in a field campaign that will investigate; (1) the impact of macroalgal mats on the biodiversity and functioning of an intertidal sandflat; and (2) the impact of these mats on sediment stability.