Aims of study and wider implications

This thesis will attempt to describe the larval supply of barnacles to the rocky shore and relate that process to the process of larval settlement. Barnacles, like many sessile marine animals, produce dispersive larvae. However unlike many bryozoans, fucoid algae and spirobid polychaetes, barnacles produce larvae with extensive dispersal potential, moving them far from the original location of their parent. For species with large dispersive stages, the task of returning to the shore and identifying a new and suitable place to settle is arguably more complex. The supply of larvae to a location and the relative settlement of those larvae is crucial in understanding the

ecology of sessile benthic communities and yet it has not been described in any detail due to restrictions on the measurement of settlement and, particularly, larval supply. This thesis will describe a means of quantifying the supply and settlement of larvae to a shore simultaneously and at fine scales. Understanding this relationship is crucial to distinguishing constraints in settlement driven by larval supply alone from that of physical and biological constraints on settlement rates as well as providing a means for understanding settlement responses beyond that of substrate determination, as previously described in laboratory and field studies, such as wave action.

The rate of observed recruitment of organisms into an environment depends on three things; (1) the largely passive supply of larvae, (2) the rates of settlement, of which the latter are driven by larval behaviour, and (3) post-settlement mortality up to the point of census by the investigator (Keough and Downes 1982). If larvae respond strongly to cues then variation in settlement may be primarily a result of larval behaviour. If, however, variations in larval supply outweigh variation in larval substrate selection then understanding cue responses of larvae alone is of limited predictive value in quantifying recruitment.

Understanding how selective factors influence critical events in the life cycle of species that make up the constituents of intertidal communities is important to developing models and understanding stock-recruitment relationships. Studies of larval mortality, behaviour and post-settlement mortality have been of great interest in marine population biology throughout the 20thcentury, particularly in order to

elucidate the stock-recruitment relationship of commercially and ecologically important species (Morgan 2001).

Despite this, the predictive powers of stock-recruitment models are notoriously weak. This is largely because the processes determining variation

throughout the many and complex life history stages of marine organisms are not necessarily examined or quantifiable. Yoshioka (1986) documented discrete events throughout the life history ofMembranipora membranacea, a species of bryozoan. He demonstrated that stock size predicted recruit density only when larval abundance was incorporated into a model with biotic and abiotic factors (predation, upwelling and temperature). Larvae of this species are particularly large and do not disperse far; hence the tight correlation between stock size and larval abundance. More dispersive species exhibit a much more complex planktonic dispersal period (Eckman 1996). The variation in benthic zooid densities was seemingly unrelated four to six weeks following recruitment in Yoshioka’s study, suggesting post-recruitment processes had a large effect on stock densities.

The work in this thesis is focused on the transitional point linking variation in abundance of larvae at the benthic (settlement and beyond) and dispersive stages (planktonic period prior to settlement) by demonstrating variation in larval supply and settlement. Integrating aspects of the biology and ecology of larval and post-larval stages are vital if significant advancement of community ecology is to proceed (Eckman 1996; Morgan 2001).

Due to the considerable spatial and temporal variation in community characteristics and recruitment, describing coastal community dynamics requires studies that cover broad temporal and spatial scales. In the present study, passive larval traps similar to those of Yundet al.(1991) were used to quantify rates of larval supply. Further modification were made to traps which had previously been

developed by Todd (2003) to increase the per area inlet capture rates so as to provide better resolution and consistency in measuring larval input. The traps were replicated within sites and combined with replicated artificial settlement substrate. This substrate

was developed to provide high rates of settlement per unit area in order to elucidate the true nature of the supply-settlement relationship. Previous studies (Bertnesset al. 1992) demonstrated a positive relationship between rates of larval supply and

settlement that varied temporally and spatially. Despite this the relationship was quantitatively weak and poorly described. This was because the larval traps were deployed sublittorally and 10 – 45 m from the settlement substratum in the sublittoral and collected fortnightly. The array used in the present study utilises a means of accurately and concisely measuring supply and adjacent settlement of larvae. The shores were surveyed daily and provided a profile of within- and between-year settlement across multiple sites. These data were studied in light of abiotic and biotic factors to understand their relative importance to this stage in the life history of a typical benthic marine organism. Variation in the supply-settlement relationship due to variation in turbulent wave action typical of a rocky intertidal shore was examined to demonstrate the applicability of the experimental array in understanding the relative importance of cue responses in larvae compared to rates of larval input in a natural environment.

The short field work season typical of species that exhibit seasonal life cycles restricts the logistical framework of empirical studies in natural environments. The development of autonomous devices that are able to integrate measures of relevant factors over time, whilst minimising cost, are crucial to field ecologists. Demonstrated here is a method for obtaining supply, settlement and hydrodynamic data on a wave swept intertidal shore that satisfies these requirements.

Chapter 2 - Wave Sensors