MARINE ECOLOGY PROGRESS SERIES Vol. 75: 239-249, 1991 Published September 11 Mar. Ecol. Prog. Ser. Demographic patterns of the palaemonid prawn Macrobrachium intermedium in southeastern Australia: spatial heterogeneity and the effects of species of seagrass Charles A. Gray* School of Biological Sciences, University of Sydney, 2006, NSW. Australia ABSTRACT. The effects of species of seagrass (Zostera capricorni and Posidonia australis) on spatial and temporal heterogeneity in the demography of estuarine populations of the palaemonid prawn Macrobrachium intermedium across 65 km of the Sydney region, southeastern Australia, were examined. Three estuaries were sampled in 1983 and 1984 to assess the magnitude of intra- and inter- estuary variability in demographic characteristics among populations. Species of seagrass had no effect on the demographic patterns of populations: differences in the magnitude and directions of change in abundances, recruitment, reproductive characteristics, size structures and growth were as great among populations within each species of seagrass as those between the 2 seagrasses Abiotic factors, such as the location of a meadow in relation to depth of water and distance offshore, and the interactions of these factors with recruiting larvae are hypothesised to have greater influence than the species of seagrass in determining the distribution and abundance of these prawns. Spatial and temporal heterogeneity in demography was similar across all spatial scales sampled: among meadows (50 m to 3 km apart) in an estuary and among meadows in all 3 estuaries (10 to 65 km apart). Variability in demographic processes among populations in the Sydney region was most likely due to stochastic factors extrinsic to the seagrasses then~selves.I conclude that the demography of seagrass-dwelling estuarine populations of M. lntermedium is basically the same throughout the Sydney region, but differs to that of populations of M. intermedium in seagrasses occurring at higher latitudes in southeastern Australia. This study emphasises the need to consider spatial and temporal heterogeneity in the demographic processes of a species over various scales to assess the magnitude of flexibility in its life history throughout its distribution. INTRODUCTION different geographic regions and in different habitats and conditions within a geographic region [Beck & Intraspecific variations in the life histories and popu- Cowell 1976, Berglund 1980, 1982, 1984, Alon & Stan- lation dynamics of many marine organisms have been cyk 1982, Emmerson 1985, 1986, Gray 1991 (compan- demonstrated over various spatial scales, environ- ion paper)]. In many of these studies, however, expla- mental gradients and throughout time (Strong 1972, nations for the differences in demography have been Stearns 1976, 1977, 1980, Fletcher 1984, 1987, Etter confounded. For example, studies on Palaemonetes 1989). Studies on the demography of estuarine and pugio (Alon & Stancyk 1982) and Palaernon pacificus nearshore palaemonid prawns have revealed intra- (Emmerson 1986) indicated that certain aspects of their specific variations among populations occurring in demographies (abundances, reproduction, recruitment, growth and size-structures) differed between popula- tions in different estuaries in the same general geo- Present address: Fisheries Research Institute, NSW Agricul- graphic region, but also were influenced by different ture & Fisheries, PO Box 21, Cronulla, 2230, NSW, Australia environmental conditions. Both Alon & Stancyk (1982) Q Inter-Research/Printed in Germany Mar Ecol. Prog. Ser 75: 239-249, 1991 and Emmerson (1986), however, did not quantitively mation on similarities in the demography of this species assess variability within each population or among in one estuary in studies carried out 7 yr apart. Finally, I populations within each estuary. Further, Alon & Stan- compare the magnitude of spatial heterogeneity to cyk (1982) did not consider whether differences in temporal heterogeneity and determine seasonal con- demographies between estuaries were consistent in sistencies in spatial heterogeneity for populations of different years. This lack of within-estuary comparisons Macrobrachium in these estuaries. relies on the assumption that the particular population under study in each estuary is representative of all populations in that estuary, whilst a study which lasts MATERIALS AND METHODS only 1 yr relies on the assumption that patterns are the same in all years. Study area. Sampling was done in Zostera capncorni In the companion paper (Gray 1991), I show that and Posidonia australis seagrass meadows in 3 spatial and temporal variability (across 3 yr) in the estuaries (Pittwater, Botany Bay and Port Hacking) demography of 2 populations of the palaemonid prawn across 65 km of the Sydney region, southeastern Macrobrachium intermedium (hereafter 'Macrobrach- Australia (Fig. 1). Zostera and Posidonia meadows in ium')in Zostera capricorni and Posidonia australis sea- Careel Bay (Pittwater), Quibray Bay (Botany Bay) and grass meadows in one estuary, Port Hacking, (New at Costens Point (Port Hacking) were sampled season- South Wales, Australia) was similar to, and of the same ally (every 3 mo) between January 1983 and April magnitude as, the inter-estuary variability in demo- 1984. Additional Zostera and Posidonia meadows at graphy described by Alon & Stancyk (1982) and Em- Palm Beach (Pittwater) and Towra Point (Botany Bay) merson (1986). Further, the magnitude of year-to-year were sampled in October 1983, January 1984 and April variability in demography of each population in Port 1984. Distances between meadows ranged from 50 m Hacking was similar to the variability between popula- between adjacent Zostera and Posidonia meadows to tions within each year. 1 also found that there were no 3 km between meadows of the same species of sea- consistent year-to-year differences in the demography grass in different areas in an estuary, and up to 65 km of Macrobrachium in these 2 seagrass meadows, between meadows in different estuaries. All meadows hypothesising that species of seagrass had no effect on the demography of Macrobrachium. Further, the gen- eral demographic pattern of Macrobrachium in Port Hacking differed to that found previously by Walker (1979), Howard (1981, 1984) and Howard & Lowe AUSTRALIA (1984) for populations in higher latitudes in southeast- ern Australia. The consistency of patterns of demogra- phy among populations of Macrobrachium in different areas within each of these geographic regions clearly needs to be determined. Further, assessment of varia- bility among populations in similar and different habitats within each geographic region may give some insight into the effects of habitat on the Life history characteristics of this species. In this paper, I investigate the extent of both intra- and inter-estuary variability in the demographic characteristics of populations of Macrobrachium in 2 PORT JACKSON types of seagrass, Zostera capricorni and Posidonia australis, in 3 estuaries across 65 km of the Sydney region, New South Wales. Firstly, I test the hypothesis that the demography of Macrobrachium does not differ in meadows of different species of seagrass. I compare my findings to those of other studies and discuss factors which may be responsible for determining the distribu- tion and abundance of prawns in seagrasses. Secondly, 15101s -Km ' '' I report heterogeneity in demography of Macro- brachium over various spatial scales: among meadows m km in an estuary and Fig. l. The Sydney region, showing sampling areas in the 3 in 3 estuanes (10 to 65 km apart). I also provide infor- estuanes studed: P~ttwater,Botany Bay and Port Hacking Gray: Spatial heterogeneity in prawn demography were located in the lower reaches of the estuaries, and ing high tide. Estuaries, then areas and meadows influenced predominantly by marine conditions; water within each estuary were sampled in a random order. salinity ranged between 22 and 35 ppt at the time of This should have eliminated biases due to differences sampling in all meadows (Fig.2). Seasonal changes in in the day, time of day and state of the tide. Samples water temperature were similar at all meadows, rang- were collected by towing a small beam trawl (Gray & ing between 14.1 and 22.6"C at the time of sampling Bell 1986), 1 m wide and 0.5 m high with 2 mm mesh in (Fig.2). Zostera meadows were generally located in the body and 1 mm mesh in the cod end of the net, at a shallower water depths (1.2 to 1.5 m at high tide) than constant speed behind a small boat over premeasured Posidonia meadows (1.3 to 2.4 m) (Fig.2). 20 m strips of seagrass. Six replicate trawls were done Sampling procedure. All meadows were sampled on on adjacent but non-overlapping strips of seagrass in consecutive days in the middle month of each season, each meadow on each sampling occasion. Six trawls within 3 d of the full moon and within 2 h of the morn- were found to minimize variance among replicate trawls with respect to the time taken to collect samples and to the sizes of meadows (Gray 1985). Prior to Temperature and salinity trawling, the depth, temperature and salinity of water Pittwater 40 l at each meadow were recorded. All Macrobrachium from each sample were pre- served in 10 % formalin, transported to the laboratory, counted, sexed and measured (carapace length; CL) as described in Gray (1991). Juveniles were defined as prawns < 3.1 mm CL. RESULTS Spatial and seasonal heterogeneity in abundances For populations