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IC17_NZ

Estuary-shelf nutrient exchange controls and interannual patterns in estuarine Ulva dynamics

The presence of unusually large amounts of benthic macroalgae in coastal and estuarine habitats, usually occurring seasonally, is often referred to as a "green tide" (Fletcher, 1996). Such events are drawing increasing attention both from the scientific community as well as the general public. Especially the accumulation and subsequent decomposition of macroalgae on the shoreline is a particularly conspicuous symptom. Green tides are now widespread, occurring regularly in several dozen countries around the world (Schramm and Nienhuis, 1996), and in extreme cases can lead to densities of accumulated biomass of up to 400 kg (wet weight) per square meter (Morand and Briand, 1996).
Ephemeral species of Ulva and other genera of the family Ulvaceae have been introduced to a number of non-native habitats by human activities. Due to undesired ecological or economical impacts of their presence, they are then often referred to as "invasive" (in contrast to the less judgmental "introduced") species (Williams et al., 2007). In a global review of introduced seaweeds, Molnar et al. (2008) found the number of invasive species from the family Ulvaceae to be significantly higher than expected by chance. In contrast to species which grow excessively only in their role as an introduced species (usually due to the absence of predators or competitors in the new habitat), many Ulvaceae show this behaviour in their native habitat and are therefore sometimes also referred to as "proliferating" algae (Morand and Briand, 1996). In New Zealand, at least 19 distinct taxa of Ulva have been identified, including both native and introduced taxa (Heesch et al., 2007).
The proposed research is centered around the documented high interannual variability of Ulva abundance in Tauranga Harbour, an estuary on the east coast of the North Island of New Zealand. Currently, the factors that predominantly control or trigger blooms of Ulva in Tauranga Harbour are unknown. Past and current hypotheses include the sporadic input of nutrients from coastal upwelling events (Park, 2007) as well as terrestrial, catchment-derived inputs (Bay of Plenty Polytechnic, 2010). The aim of the proposed research is to identify the role and relative importance of factors contributing to Ulva abundance in Tauranga Harbour. The methods employed to address this question include: the analysis of observation data of Ulva and related environmental variables, the development and application of an Ulva growth model well-suited to the local conditions and the integration of this growth model with both hydrodynamic and water quality models and/or empirical or model-derived forcing data to examine the relative importance of factors contributing to Ulva abundance in the range of environmental conditions described by the observation data collected by the regional environmental administration, the Bay of Plenty Regional Council (BOPRC).
So far, a database of historical and recent observations of relevant environmental parameters has been compiled, including, but not limited to: solar irradiation, air temperature, water temperature, rainfall, freshwater inputs to Tauranga Harbour as well as corresponding nutrient concentrations. Observed Ulva abundance as well as tissue nutrient concentration time series have been obtained from BOPRC. A mathematical model describing the growth of Ulva has been developed, based on and improving upon previously published models (e.g., Solidoro et al., 1997a; Solidoro et al., 1997b; Martins and Marques, 2002; Aldridge and Trimmer, 2009), and is currently being applied to local environmental / forcing data. Several field methods to measure various parameters of the Ulva population(s) in Tauranga Harbour were tested and refined (e.g., underwater video camera surveys of subtidal abundance and cage-mounted nets catching drifting tissue biomass), and are currently being employed as part of an intensive field campaign during the 2012/2013 season designed to provide missing parameter estimates crucial for both empirical as well as model-based analysis of Ulva population dynamics in Tauranga Harbour.
Collaboration is ongoing with the project NZIC7 ("The impact of macrophyte mats on benthic nutrient dynamics"). The Ulva that this project is studying eventually deposits on the intertidal flats, and breaks down, to be incorporated into the sediments (which is studies in the NZIC7 project). It also compliments Dorothea Kohlmeier’s project IC7, which studies the factors driving growth in estuarine plant species.

Members

Proponents:Prof. Dr. Karin BryanUniversity of Waikato
Prof. Dr. Conrad Pilditch
:Prof. Dr. Kai BischofUniversity of Bremen
PhD Candidate:Alex PortUniversity of Waikato

Publications

N / A

Miscellaneous

Research Stay at the University of Bremen: 05.2012 - 08.2012