Estuaries and Coasts DOI 10.1007/s12237-014-9850-1
Seagrass Viviparous Propagules as a Potential Long-Distance Dispersal Mechanism Alexandra C. G. Thomson & Paul H. York & Timothy M. Smith & Craig D. H. Sherman & David J. Booth & Michael J. Keough & D. Jeff Ross & Peter I. Macreadie
Received: 15 August 2013 / Revised: 10 June 2014 / Accepted: 13 June 2014 # Coastal and Estuarine Research Federation 2014
Abstract Resilience of seagrass meadows relies on the ability of seagrass to successfully recolonise denuded areas or disperse to new areas. While seed germination and rhizome extension have been explored as modes of recovery and expansion, the contribution of seagrass viviparous propagules to meadow population dynamics has received little attention. Here, we investigated the potential of seagrass viviparous propagules to act as dispersal vectors. We performed a series of density surveys, and in situ and mesocosm-based experiments in Port Communicated by John C. Callaway A. C. G. Thomson : D. J. Booth : P. I. Macreadie School of the Environment, University of Technology Sydney, Broadway, NSW 2007, Australia A. C. G. Thomson (*) : P. I. Macreadie Plant Functional Biology and Climate Change Cluster (C3), University of Technology Sydney, Broadway, NSW 2007, Australia e-mail:
[email protected] P. H. York : T. M. Smith : C. D. H. Sherman Centre for Integrative Ecology, Deakin University, 75 Pigdons Road, Waurn Ponds, VIC 3216, Australia P. H. York : M. J. Keough
Phillip Bay, VIC, Australia, using Zostera nigricaulis, a species known to produce viviparous propagules. Production of viviparous propagules was higher at sites with high wind and current exposure, compared to more sheltered environments. A number of propagules remained buoyant and healthy for more than 85 days, suggesting the capacity for relatively long-distance dispersal. Transplanted propagules were found to have improved survivorship within seagrass habitats compared to bare sediment over the short term (4 weeks); however, all propagules suffered longer-term (1-m outside the edge of a meadow and contained 0 % seagrass (100 % bare substrate). Within each habitat, eight propagules were stapled (Bastyan and Cambridge 2008) to the sediment inside PVC
Estuaries and Coasts
rings (225-mm diameter×80-mm deep; n=5), which were used as markers. Stapling of propagules ensured their retention within the particular habitats. Transplanted propagules were left for a period of acclimation (24 h), before assessing EQY using a diving PAM. Propagule EQY and survivorship were monitored weekly and then bi-weekly where conditions allowed. The EQY readings of surrounding established seagrass plants were taken as a comparative control. Where propagules were absent from habitats due to poor weather or sediment scouring, they were not included in survivorship analysis. Weather conditions meant that measurements were not able to be performed at the same time for both sites; site was therefore not a factor in the analysis of results. PAM measurements were discontinued after day 55, as the limited number of remaining propagules would not have provided a reliable insight into the prolonged health of the seagrass propagules. However, survivorship analysis was continued for 14 weeks, when complete propagule mortality/ absence was reached and the experiment was therefore concluded. Establishment of Propagules in Sediment from Seagrass and Bare Habitats A mesocosm experiment was performed to identify sediment conditions that best facilitate propagule establishment. Propagules were transplanted into tanks with sediment taken from bare and seagrass habitats at both Point Richards and Indented Head, sites used for the previous in situ experiment. Sediment from seagrass and bare habitats (n=4) was taken using 100-mm deep (100-mm diameter) metal corer, from each site. Any seagrass ramets, rhizomes or leaves were discarded. Precautions were taken to retain natural stratification of sediment layers, with each sediment replicate placed into two 1,000-ml plastic containers. Propagule rhizome length, number of leaves and nodules, initial EQY (ΔF/Fm’) and fresh weight of propagules (per replicate) were all measured. Four propagules were stapled (Bastyan and Cambridge 2008) to the sediment in each container (eight propagules per replicate) and placed in 8-L flow-through tanks, each with its own fresh seawater supply (each tank representing one replicate) set up outside VMSC. EQY (ΔF/Fm’) was analysed weekly using a diving PAM. In addition, the overall survivorship and number of propagules attached to the sediment were monitored. Attachment (propagules securely rooted into sediment) was used as a proxy for successful propagule establishment within each sediment type. Attachment was determined by checking whether the propagules had grown enough roots/root hairs to successfully
remain attached to the sediment without needing the staple. When this was achieved, staples were removed. The experiment was run over 66 days, and propagules were then extracted from the sediment and remeasured. Sediment Analysis Organic Matter Sediment samples were extracted from the first 10–30 mm of the rhizosphere immediately surrounding the bare, edge and seagrass established habitat plots. A total of 15 samples (five each of seagrass, edge and bare) were taken from each of the two study sites (n=5). A subsample of sediment was taken (≈50 g), and organic matter analysis followed methods by Coles et al. (2001) and Patel et al. (2001). Sediment Grain Size Determination of the sediment grain size composition for each of the locations was performed following methods described by Coles et al. (2001). A total of 15 samples (5 each of seagrass, edge and bare) were taken using 100-mm deep (100-mm diameter) metal corer, from each of the two sites (n=5). Sediment was broken down into >2-mm, >1-mm, >500-μm, >250-μm, >125-μm, >63-μm and
