Natural and anthropogenic regime variance of a seagrass ecosystem : a late Anthropocene palaeo-reconstruction

Seagrass meadows, such as found in the Little Swanport estuary, Tasmania, Australia, provide a high ecosystem service that typically responds to natural destructive events and nitrogen loading at decadal to inter-decadal scales. Consequently, determining and understanding the effects of anthropogenic impacts, such as the estuary's shellfish aquaculture, requires an ecological time series of sufficient length to develop a long-term predictive theory behind changes to pattern and process and distinguish anthropogenic from natural effects. The aims of this study were to produce the appropriate long-term ecological time series for the Little Swanport estuary by combining sediment core and long term datasets (83 years to 139 years) in a palaeo-reconstruction that includes natural and anthropogenic variability, and to develop a general predictive theory behind pattern and process. The reconstructed time series was designed to include the elements of top down and bottom up control on the reconstructed seagrass‚Äö-micro-algal assemblage: planktivorous fish predation, copepod feeding, calcareous sessile epifauna, seagrass-mediated nitrogen fixation, and external nutrient supply as the concentration of potential inorganic nitrogen (CPN). Ecosystem variance in the upper region of the estuary differed from the lower estuary. Within the upper estuary, weight of evidence suggested variability was consistent with decadal periods of seagrass meadow destruction and recovery as two seagrass transient regime states, followed by stable natural and impacted seagrass regime states. The transient regimes appeared to be limited by nitrogen, the natural stable regimes appeared to be limited by light and the impacted regime had a near complete reliance on light mediated nitrogen fixation. The switch between nitrogen and light limiting resources was consistent with a change from a strong to weak top down control by a planktivorous fish‚Äö-copepod‚Äö-sestonic‚Äö-calcareous epifaunal trophic cascade. Comparisons with both temporal and spatial natural patterns and processes indicated the dependence of nitrogen fixation was the result of the lower estuary shellfish aquaculture 'soaking up' the supply of inorganic nitrogen from coastal waters during an extended period of drought. Within the lower estuary there was no evidence of direct effects of floods, tsunamis or shellfish aquaculture on the long-term pattern and process. Nevertheless, there was an uninterrupted repeatable natural ecosystem periodicity (i.e. a neutral model) of 57.9 years for the seagrass and micro-algal assemblage and a planktivorous fish‚Äö-copepod‚Äö-sestonic‚Äö-calcareous epifaunal trophic cascade with a CPN at twice the biome frequency. The seagrass and micro-algal assemblage was inversely correlated but the trophic cascade was 13 years out of phase, driven by a CPN periodicity at twice the biome frequency in phase with the peaks and troughs of the seagrass and micro-algal assemblage. As a consequence, there was a change from a positive to a negative correlation of the CPN with seagrass abundance that was consistent with a change from seagrass nitrogen to light limitation. This switch was coincident during times of a CPN minimum together with strong and weak top down control respectively. By linking changes in top down control with the evolution of a seagrass meadow's configuration a general theory of seagrass meadow dynamics was developed, constructed, in part, from data and generic postulates taken from other seagrass ecosystems systems. The theory unified two desperate approaches to seagrass meadow variance, namely light and nutrient limitation, within the framework of landscape ecology and consequently highlighted a number concerns for managers of seagrass ecosystems: (1) the loss of a seagrass meadow by flood damage may produce a seagrass transient regime and not a micro-algal regime, in which recovery of the transient is assisted by a moderate increase and not a reduction in nutrient supply; (3) changes to the extent of the landscape patch configuration can determine the light and nutrient limitation status of the seagrass meadow, consequently, rehabilitation of the seagrass meadow after loss or damage, by replanting or through the control of nutrient supply, may need to take into account the landscape patch configuration of the plantation or the damaged landscape in relation to the dynamics of nutrient supply; (4) the effects on the seagrass meadow from over-harvesting of planktivorous fish fishing or the introduction of piscivorous fish will depend on the light or nutrient limitation status of the seagrass meadows.