Abstract
Anthropogenic carbon dioxide (CO2) emissions are causing long-term changes in ocean carbonate chemistry. This process, ocean acidification (OA), produces reduced seawater pH and overall shifts in carbonate chemistry. Predictive modelling indicates that a seawater pH of approximately 7.8 will be reached in the near future (2100) and worst-case scenarios suggest a pH of below 7.5 could occur under extreme circumstances by the year 2300. Extensive research has been conducted on the effect of OA on calcifying marine invertebrates during early life-history stages. However, transitions between life-history stages, such as the settlement and metamorphosis process, are less well studied. Settlement success determines population recruitment and distribution, and changes in settlement could have large-scale, bottom-up effects on the population dynamics of many key invertebrate species. This study aims to examine how OA affects the settlement selection of the serpulid polychaete, Galeolaria hystrix, in coastal environments by investigating biofilm response to OA and measuring G. hystrix settlement success in response to changes in biofilms caused by OA.
The model species, Galeolaria hystrix Mörch 1863 (Family Serpulidae), is found throughout the low intertidal zone of New Zealand and is common in Otago Harbour. In situ physical and carbonate chemistry parameters were recorded over the course of the study from 23 March to 23 October 2015 (Chapter 2). In their in situ environment G. hystrix experience moderate diel fluctuations in seawater pH, up to 0.116 pH units in a single event. These events are likely associated with primary productivity and tidal variation. Seasonal shifts in carbonate chemistry were observed and seawater pH ranged from pHT 8.03 to pHT 8.24 over the course of the study with an average of pHT 8.13 ± 0.01. Meaningful pH targets were determined for in vitro experiments based on the current site-specific carbonate chemistry conditions in Otago Harbour and standard pH projections normalised by the International Panel for Climate Change (IPCC 2007, 2013). Targets of pHNBS of 7.8 (near-future, 2100) and pHNBS 7.4 (extreme, 2300) were identified as significant thresholds for biofilm development and settlement testing.
A flow-through system was built and monitored at the Portobello Marine Laboratory, Otago Harbour, New Zealand (Chapter 3). Four individual replicate tanks were designed as mesocosms to develop biofilms on glass microscope slides at ambient pH and pHNBS targets, pH 7.8 (near-future, 2100) and pH 7.4 (extreme, 2300). All environmental parameters and mesocosm characteristics were measured and quantified in replicate tanks, including algae community composition. Biofilm sampling technics were verified, and the community composition of in situ biofilms was compared to in vitro biofilms and found to be significantly different. Findings indicated that biofilms developed in vitro did not mimic in situ biofilm variation, but successfully isolated the effect of seawater pH on biofilm community composition.
The larval development, metamorphosis and settlement behaviour of Galeolaria hystrix is described and addresses gaps in the current literature (Chapter 4). Larval morphometrics and behaviour were detailed from fertilisation through to settlement and juvenile growth. Larvae developed as trochophores until approximately 12-15 days post-fertilisation and then underwent differentiation to the setiger larval stage. At this point, important changes in morphology (development of mesodermal bands and lengthening of the larvae) and behavior (slowed swimming and greater affinity for surface exploration) were observed. Metamorphosis was initiated by the delineation of a “neck” at the level of the prototroch and followed by key events such as, head region degeneration, tentacle bud formation, collar evagination and eventually tube secretion. These events occurred more rapidly in larvae that were exposed to substrates and displayed settlement behaviors, particularly temporary attachment. The speed and completion of metamorphosis was closely linked to larval contact with settlement surfaces, which suggested that substrate exploration and temporary attachment may be critical in the settlement process.
Biofilm response to OA was evaluated in three key areas that affect settlement selection in marine invertebrates; 1) microbial biofilm community composition, 2) diatom coverage and 3) initial invertebrate community composition (Chapter 5). Microbial biofilm community composition shifted in response to OA and biofilm age. The most significant shifts in phylum and operational taxonomic unit (OTU) relative abundances were observed in biofilms reared in the pH 7.4 treatments. The OTU richness of biofilms decreased with seawater pH and overall biofilm communities became more heterotrophic with age. Diatom coverage, chlorophyll-a and carotenoid pigment content significantly decreased in biofilms reared in pH 7.4 (extreme, 2300) treatments and were significantly lower on younger biofilms. The total abundance of initial invertebrate colonisers was significantly reduced on biofilms reared in pH 7.4 and was primarily driven by a substantial decrease in Cirripedia spp. (barnacle larvae). Overall, significant shifts in biofilm community composition were observed at all three levels in the pHNBS 7.4 (extreme, 2300) treatments.
Settlement assays of Galeolaria hystrix were conducted on 23-day and 60-day old biofilms reared in the three seawater pH treatments from August – October 2015; ambient, 7.8 (near-future, 2100) and 7.4 (extreme, 2300). All settlement assays were conducted in ambient pH seawater and settlement success was assessed at 24h, 48h and 5d post-larvae introduction and scored on a scale that quantified “loose” (biofilm exploration behaviors) and “close” (settlement success) associations (Chapter 6). Higher prevalence of “loose” associations did not correspond to higher settlement success rates. Overall there were no statistically significant differences in the settlement success of G. hystrix among pH treatments, although, findings indicated that biofilms reared in the pH 7.4 treatments were the least preferred. Settlement success of G. hystrix was significantly higher on older (60-day) biofilms than young (23-day) biofilms at 48h. Juvenile survival (at 5d) was not affected by biofilm age and/or pH treatment.
Overall, results indicate that the settlement success of Galeolaria hystrix may not be significantly altered in the intertidal zone by ocean acidification alone. The dynamic and fluctuating carbonate chemistry and environmental conditions of coastal zones may make G. hystrix settlement somewhat resilient to small-scale shifts in biofilm communities driven by seawater pH. Ocean acidification produces small-scale non-significant shifts in biofilm communities at pHNBS 7.8 (near-future, 2100) but does significantly alter biofilm communities (microbial, diatom and invertebrate) at pHNBS 7.4 (extreme, 2300). However, significant shifts in overall biofilm community composition associated with reduced seawater pH did not result in statistically significant differences in the settlement success of G. hystrix. Interestingly, biofilm age significantly altered overall biofilm community structure and younger age resulted in a significant decrease in settlement success of Galeolaria hystrix.