Assessing Long-Term Ecological Changes in Wetlands of the Bass Strait Islands, Southeast Australia: Palaeoecological Insights and Management Implications

Abstract

Understanding long-term ecological development of wetlands is critical to effective management. The islands of Bass Strait, southeast Australia, have several biologically diverse natural wetlands, including Ramsar sites, yet little is known about their ecology. Here, through a multi-proxy palaeoecological approach, we seek to understand how wetland floristic composition and hydrology has changed on truwana/Cape Barren Island (CBI), and how changes in fire regimes have affected wetland integrity and ecological dynamics. We use wetland fossil pollen, non-pollen palynomorphs and sediment geochemistry to reconstruct wetland development and compare it to existing records of terrestrial vegetation and fire regimes from the island. Our results suggest periods of moderately saline–brackish conditions and moderate fire activity supported high floristic richness in wetlands, while floristic richness reduced during periods of extremely high or low salinity or high fire activity. Past changes in precipitation regimes primarily drove water-level changes in wetlands; however, changes in wetland and surrounding terrestrial vegetation cover also contributed to wetland water level dynamics. We recommend long-term monitoring of wetland salinity and water-level changes to track potential changes in wetland floristic richness on truwana/CBI. Controlled fires could also be used to manage wetland biodiversity after careful consideration and experimentation to determine appropriate fire levels that maximize wetland floristic richness and biocultural values.

Appendices

Appendix 1

Table 2 AMS radiocarbon dates for Bumpy Lagoon (BL), measured at ‘DirectAMS’, Washington

Fig. 8

Age-depth model (Bacon) for Bumpy Lagoon, showing calibrated ¹⁴C dates (purple), age-depth model (dark shaded grey), 95% confidence intervals of calibrated range (light shaded grey), and single ‘best’ model based on the weighted mean age for each depth

Appendix 2

Table 3 Summary table of NPP types on truwana/Cape Barren Island, Bass Strait, southeast Australia, and their ecological interpretations based on previous ecological and palaeoecological studies

Appendix 3

Notes and comments on sparsely-recorded and indeterminate NPPs in sedimentary records ( Fig. 4 ).

Neorhabdocoela (HdV-353), HdV-3, Nebela and Arcella were sparsely recorded in BRL, but Nebela was only present in the basal lateglacial sample, dated to ~14,000 cal yr BP. HdV-299 was sparsely recorded in top samples of YL. HdV-179, Tiletia (HdV-27) and HdV-20 were sparsely recorded in GL, but HdV-179 was only recorded in one sample, dated to ~5800 cal yr BP (Fig. 4₎.

ANH-1 (Fig. 4_14–15): This NPP type has a serrated-circle shape and is about 14 μm. It consists of 14 appendages arranged like fruit mesocarps, which are invisible in most cases as in Fig. 4₁₅. Considering the shape and near-transparent colour of this type, we suspect it is of an alga origin. It was consistently recorded in BRL from ~14,000 to 9000 cal yr BP, intervals dominated by aquatic macrophytes and chironomid remains. It is also present in tECL in the last ~1000 years, which is dominated by aquatic macrophyte Myriophyllum muelleri and alga type Gloeotrichia. This may suggest an ecological overlap exists between ANH-1 and aquatic/wetland plants, especially Myriophyllum species.

ANH-2 (Fig. 4_16–17): This type is a large echinate cell (about 92 μm) and may be a non-vascular plant spore (e.g., bryophyte). It is sparsely recorded in BRL lateglacial samples (~14,000–13,000 cal yr BP) and abundant in tECL in the last ~1000 years, which are characterized by shallow wetland and herbland assemblages, including Myriophyllum species. This suggests ANH-2 may be affiliated with shallow wetlands or/and aquatic herbland and dry conditions.

ANH-3 (Fig. 4₁₈): This NPP type (~35 μm) may be a dinoflagellate cyst and looks like the cysts of Ataxiodum chaoane and Scippsiella trifida (Mudie et al., 2017).

ANH-4 (Fig. 4₁₉): This NPP type is about 49 μm in size and may be of algal origin. It was sparsely recorded in BRL between ~8000 and 6000 cal yr BP, a transitional phase from a high (lake system) to low hydrology (wetland system) regime, and dominated by Restionaceae, copepod spermatophore and HdV-18.

ANH-5 (Fig. 4₂₀): This NPP type (~30 μm) is likely a fungal spore and may be related to HdV-20 (Fig. 4₄). ANH-7 was sparsely recorded in YL.

ANH-6 (Fig. 4₂₁): This NPP type is large (~120 μm) and may be of fungal or invertebrate origin. It was sparsely recorded in BRL.

ANH-7 (Fig. 4_22–23): This NPP type is a fungal cell (possibly spore) in cluster and is about 33 μm. It is consistently present in tECL record in the last ~4000 years, an interval generally dominated by Myriophyllum species, Restionaceae, sponge spicules and Glomus, suggesting ANH-7 is related to shallow wetlands.

ANH-8 and ANH-9 (Fig. 4_24–25): These NPP types are likely of fungal origin. ANH-10 is about 141 μm long and ANH-11 is about 215 μm long. They are consistently present in BRL between ~12,000 and 8000 cal yr BP, an interval dominated by Isoetes and chironomid remains. The long and narrow shape of these NPPs suggests they may be ascospore/conidia of freshwater fungi (Shearer and Raja, 2010). Specifically, ANH-10 looks like Isthmosporella sp., which mostly inhabits submerged woody remains in lakes (Raja et al., 2020; Shearer and Crane, 1999), and inferred lake system and woodland development (Adeleye et al., 2021a) in BRL during the early Holocene may have contributed to the presence of ANH-10 at the site.

ANH-10 (Fig. 4_26–28): A large (~82 μm) globose NPP type, with wall like an amoeba test; however, the aperture is unclear or not visible. It may also be an aquatic invertebrate egg. The NPP was recorded in BRL and most abundant around 11,000 cal yr BP when Isoetes was most abundant. ANH-12 may share ecological preference with Isoetes species growing at BRL at this time. ~11,000 cal yr BP falls within the period interpreted as the development of a lake system or increasing hydrology at BRL.

Appendix 4

Notes and comments on modern NPPs in Big Reedy Lagoon surface samples (Fig. 5).

Fig. 9

Percentage diagram for major palynomorphs recorded in Big Reedy Lagoon (BRL) surface samples (10 sampling points on the wetland surface). Note: sponge spicules were abundant in all samples

Big Reedy Lagoon (BRL) is presently a peaty swamp, with pockets of standing water. The swamp surface is walkable, especially during summer, and supports thickets of tall and medium-sized shrubs (e.g., Melaleuca, Leptospermum and Kunzea), as well as sedgelands (see Fig. 1b in main text). This suggests the water table at BRL is low in most parts of the year in recent times, hence, NPPs recorded on the surface of the lagoon (Fig. 5) likely reflect the present low water level at the site.