David R. Schiel

TEN YEARS OF INDUCED OCEAN WARMING CAUSES COMPREHENSIVE CHANGES IN MARINE BENTHIC COMMUNITIES

One of the most commonly predicted effects of global ocean warming on marine communities is a poleward shift in the distributional boundaries of species with an associated replacement of cold-water species by warm-water species. However, these types of predictions are imprecise and based largely on broad correlations in uncontrolled studies that examine changes in the distribution or abundances of species in relation to seawater temperature. Our study used an 18-year sampling program in intertidal and subtidal habitats and before-after, control-impact analyses. We show that a 3.58C rise in seawater temper- ature, induced by the thermal outfall of a power-generating station, over 10 years along 2 km of rocky coastline in California resulted in significant community-wide changes in 150 species of algae and invertebrates relative to adjacent control areas experiencing natural temperatures. Contrary to predictions based on current biogeographic models, there was no trend toward warmer-water species with southern geographic affinities replacing colder- water species with northern affinities. Instead, the communities were greatly altered in apparently cascading responses to changes in abundance of several key taxa, particularly habitat-forming subtidal kelps and intertidal foliose red algae. Many temperature-sensitive algae decreased greatly in abundance, whereas many invertebrate grazers increased. The responses of these benthic communities to ocean warming were mostly unpredicted and strongly coupled to direct effects of temperature on key taxa and indirect effects operating through ecological interactions.

Patterns of distribution and abundance of large brown algae and invertebrate herbivores in subtidal regions of northern New Zealand

Abstract The distribution and abundance of subtidal laminarian and fucoid algae, echinoids, and four species of herbivorous gastropods were assessed. Line transects and stratified random quadrats were used, encompassing different depths and seven localities ranging over 1000km in northern New Zealand. This sampling was carried out to form a quantitative basis on which to pose hypotheses for later experimental work. Several patterns were apparent, both within and between localities. Fucoid algae were characteristic of depths between 0–6 m, while laminarian algae had peaks of abundance at 3–6 m and from 10–17 m. Intermediate depths were characterized by areas bare of large brown algae, with high densities of echinoids and gastropods. This pattern of a bimodal depth distribution of algae and abundant grazers at intermediate depth was a consistent feature of the northern coastal sites and the semi-exposed offshore islands. Size—frequency distributions for algae at these sites showed that the largest fucoids generally occurred in very shallow water, while the dominant laminarian alga tended to have larger individuals at depths of 10–14 m. In all cases, the biomass distribution with depth of the algal species mirrored their numerical abundance. Differences between localities were associated both with environmental factors and the unique algal floras of some offshore islands. Echinoids and gastropods were rare at the two most exposed localities, which were also at the extremes of the geographic range. At these sites, depths between 6–10 m were occupied by large brown algae. Deep-water fucoid algae occurred at two offshore localities but the predominant species at one of them was peculiar to that island group. Because of the general consistency between localities, and the few exceptions to these patterns, hypotheses are considered which could be tested on a local scale to account for echinoid and algal abundances and depth distributions. Our results indicate that the chief questions concerning the structuring of subtidal algal communities are demographic in nature and that an assessment of the status of individual organisms, rather than biomass or productivity, is the appropriate approach to resolve these questions.

Effects of trampling on a rocky intertidal algal assemblage in southern New Zealand

The effects of human trampling on a mid-intertidal assemblage dominated by the fucalean alga Hormosira banksii were tested experimentally on two rocky intertidal platforms on the east coast of the South Island of New Zealand. The major factors were Trampling intensity, Platforms and Timing of trampling. Seven trampling intensities (0, 10, 25, 50, 100, 150 and 200 passages per area) were used in 2-m long transects (0.6 m2) during single tides in Spring and Autumn on each platform. H. banksii initially had >96% canopy cover in all treatments. As few as 10 tramples reduced this cover by up to 25% after a single tide. Progressively greater reductions occurred at higher trampling intensities, with >90% of the H. banksii canopy removed at 200 tramples. There was recruitment into the Spring transects during the following summer months, and by five months after trampling all treatments had at least 50% cover of H. banksii. Treatments initiated in Autumn had similar reductions in H. banksii cover as Spring treatments, but the recovery was delayed by seven months until recruitment occurred during the following summer. Fifty percent recovery of the canopy took at least a year in most Autumn treatments. After 21 months the Spring treatments had recovered to control levels (>97% cover), while after 16 months the Autumn treatments were still recovering. Understory encrusting and turfing coralline algae were reduced in cover after trampling, with the greatest effects at the higher trampling intensities. The reduction of corallines was mostly due to burn-off after the H. banksii canopy was removed. By the end of the experiment, corallines had returned to control levels in both the Spring and Autumn treatments. Bare primary space increased significantly with up to 60% bare rock at the higher trampling intensities one year after trampling. By the end of the experiment, bare space was reduced to <10% in the Spring treatments but averaged as high as 24% in the higher trampling intensities of the Autumn treatments. There was an interaction between demographic processes, particularly recruitment, and coralline algae in the recovery of H. banksii. Up to 70% of cover at the 200-trample intensity was from new recruits, while most recovery in the 10 and 25 trampling intensities was from re-growth of damaged fronds. However, at one platform, composed of soft siltstone, recruitment of H. banksii occurred almost exclusively on the damaged encrusting and turfing coralline algae. Overall, this experiment showed that trampling intensity had variable effects and that the interaction of season, location, indirect effects of reduction in coralline algae, and facilitative processes in recruitment of H. banksii all contribute to recovery after disturbance.

Comparative phylogeography of coastal limpets across a marine disjunction in New Zealand.

Cook Strait, which separates the North and South Island of New Zealand, has been a transient, but re‐occurring feature of the New Zealand land mass throughout the Pleistocene, maintaining its current width and depth for the past 5000 years. Historic land fragmentation coupled with the complex hydrography of the Greater Cook Strait region has created both biogeographic and phylogeographic disjunctions between the North and South Island in several marine species. Here we use mitochondrial cytochrome b DNA sequences of three endemic intertidal limpets, Cellana ornata, Cellana radians and Cellana flava to assess intraspecific phylogeographic patterns across Cook Strait and to look for interspecific concordance of ecological and evolutionary processes among closely related taxa. We sequenced 328–359 bp in 85–321 individuals from 8–31 populations spanning the biogeographic range of the three species. Intraspecific phylogeographic analyses show moderate to strong genetic discontinuity among North and South Island populations due to allopatric fragmentation. This pattern was broadly concordant across the three species and the observed divergence among this group of intertidal limpets (0.3–2.0%) is similar to that of previously studied subtidal organisms. For each species, divergence time calculations suggest contemporary North and South Island lineages diverged from their respective most recent common ancestor approximately 200 000 to 300 000 years before present (bp), significantly earlier than previous estimates in other coastal marine taxa that arose from a miscalculation of divergence time.

Macroalgal assemblages in New Zealand: structure, interactions and demography

Quantitative descriptions of macroalgal distributions from low intertidal regions to a sublittoral depth of 20 m are presented for rocky reefs from several localities ranging over 16° latitude in New Zealand. These include the northern and southern main islands as well as the Chatham Islands and the sub-antarctic Auckland Islands. Fucalean algae are dominant in the shallow sublittoral regions throughout NZ. Laminarian species are rare and generally reach their greatest abundances at 7–17 m depth. The colder waters in central and southern NZ tend to have different dominant species from those in northern NZ. Sea urchins form a characteristic zone devoid of kelp at 5–8 m in the north, but tend to occur only in patches on reefs in southern localities. Summaries of experiments in the north show that strong interspecific effects occur among algal species, particularly due to canopy shading, and between sea urchins and algae. Much of the patchiness in algal assemblages, however, is not accounted for by these effects. A knowledge of the demographic processes of individual species is necessary to understand assemblage organization; life histories, phenology and biogeography significantly affect distributions and interactions.

Algal interactions on shallow subtidal reefs in northern New Zealand: a review

Abstract In north‐eastern New Zealand, nearshore subtidal reefs are dominated by large brown algae of the orders Fucales and Laminariales. Species of the genera Carpophyllum, Sargasswn, and Landsburgia are the most conspicuous fucaleans, whereas Ecklonia radiata is the only common laminarian. Three categories of events affect the composition of stands of these algae: first, the competitive effects of adult canopies on recruitment to the substratum below; second, the seasonality of spore release and availability of free space; and third, the demographic characteristics of species and their influence on recruitment. Adult canopies suppress recruitment of all species. Both repro‐duction and recruitment are seasonal in occurrence and most recruits appear within a few metres of adults of their own species. A comparison of representative Fucales and Laminariales shows important differences in how these taxa colonise substrata. Propagules of fucaleans are larger at settlement and fewer in number than those of la...

Phytoplankton biomass associated with mussel farms in Beatrix Bay, New Zealand

The spatial and temporal variability in phytoplankton abundance is a major factor determining the productivity of mussel (Perna canaliculus) farms. During periods of low phytoplankton abundance, food depletion may be a significant factor in the productivity of mussel farms. Measurements of phytoplankton abundance (as chlorophyll a) were made over the entire depth of the water column both inside and outside four mussel farm sites in Beatrix Bay, over a 13-month period. Ambient (outside) concentrations of dissolved inorganic nitrogen (NO3–N and NH4–N) and dissolved reactive phosphorus (DRP) were also measured. The highest ambient chlorophyll a concentrations in the surface waters occurred during autumn–winter, with peak levels of around 5 μg l−1. At this time, all farms had a significant reducing impact on phytoplankton biomass (P<0.05 in all cases). The lowest ambient chlorophyll a concentrations of <0.5 μg l−1 were recorded in the summer, coinciding with low nitrogen concentrations. In November, there was significantly more phytoplankton inside all the farms (P<0.05 in all cases). It was hypothesised that this increase occurred because mussels are net producers of dissolved inorganic nitrogen. In five out of the 7 months when farms were surveyed, the highest chlorophyll concentrations were in the deeper water, associated with a pycnocline. Two management options to increase mussel productivity are presented: (1) Deployment of mussel dropper ropes to deeper waters to take advantage of chlorophyll maxima in summer; (2) Artificially increasing nitrogen inside farms during spring and summer to increase phytoplankton supply.

Seasonal variation in the reproductive activity and biochemical composition of the Pacific oyster (Crassostrea gigas) from the Marlborough Sounds, New Zealand

Abstract Reproductive cycles of the Pacific oyster Crassostrea gigas (Thunberg) from the Marlborough Sounds, New Zealand, were followed between June 1998 and January 2000. Histological examination of the gonad confirms an annual cycle with a winter inactive period followed by rapid gonad development and a single short spawning period. The population gonad index correlated with seawater temperature and changes in tissue dry weight, condition index, and biochemical components. In winter, few individuals with early gametogenic stages were present and rapid development of primary oocytes (diam. 11 μm) occurred during spring (September‐November). The developmental rate and the diameter of mature oocytes (37 μm) was similar for the 1998 and 1999 seasons. For a standard 110‐mm‐length oyster, maximal tissue body weight and condition index were recorded in December. Rapid weight loss in January was length dependent and was attributed to spawning. Temperature was the environmental variable which best correlated with the timing of gametogenesis. Food availability (phytoplankton biomass) may have been responsible for inter‐annual variations. The biochemical composition (% glycogen, lipid, protein) of separated gonad and somatic tissues were variable seasonally and annually. Gametogenesis (oocyte diameter) was associated with increased gonad protein and glycogen and a decrease in lipid concentrations. These changes are similar to those in Pacific oyster populations from other parts of the world.

Depth distribution and abundance of benthic organisms and fishes at the subtropical Kermadec Islands

Abstract Subtidal areas of the Kermadec Islands have not previously been described; samples from a bay at Raoul Island are discussed here. Abundances and percentage cover of benthic organisms were assessed from the high intertidal to 20 m depth using quadrat sampling. Fish abundances were assessed subtidally by random transects in shallow (3–6 m deep) and deep (14–20 m) areas of the reef. Hard corals occurred at 2–18 m depths at densities of 5 colonies per m2, but colonies were generally small. Soft coral colonies occurred at similar densities, but were confined to depths of 5–13 m. The mean percentage cover of corals did not exceed 20% at any one depth, and coral reefs did not occur. Fucalean and laminarian algae were not seen. The most abundant algae were foliose and filamentous species, and encrusting red algae. These covered over 80% of the reef at some depths. Grazing invertebrates were not abundant. Echinoids were found primarily in shallow water, reaching a density of 5 per m2 at 5 m depth. The mos...

Algal populations controlled by fish herbivory across a wave exposure gradient on southern temperate shores

Consumers that forage across habitats can affect communities by altering the abundance and distribution of key species. In marine communities, studies of trophic interactions have generally focused on the effects of herbivorous and predatory invertebrates on benthic algae and mussel populations. However, large mobile consumers that move across habitats, such as fishes, can strongly affect community dynamics through consumption of habitat-dominating species, but their effects often vary over environmental gradients. On temperate rocky shores, herbivorous fishes are generally a small part of the fish fauna compared to the tropics, and there is sparse evidence that they play a major direct role in algal community dynamics, particularly of large brown algae that dominate many reefs. In New Zealand, however, a wide-ranging herbivorous fish, Odax pullus, feeds exclusively on macroalgae, including Durvillaea antarctica, a large low-intertidal fucoid reaching 10 m in length and 70 kg in mass. In four experiments we tested the extent of fish herbivory and how it was affected by algal canopy structure across a gradient of wave exposure at multiple sites. Exclusion experiments showed that fish impacts greatly reduced the cover and biomass of Durvillaea and that these effects decreased with increasing wave stress and algal canopy cover, effectively restricting the alga to exposed conditions. Almost all plants were entirely removed by fish where there was a sparse algal canopy in sheltered and semi-exposed sites, but there was significantly less grazing in exposed sites. Recruit Durvillaea beneath canopies were less affected by fish grazing, but they grew slowly. Successful natural recruitment, therefore, occurred almost exclusively on exposed shores outside canopies where many plants escaped severe grazing, and growth to maturity was far greater than elsewhere. Such large and direct impacts on the local and regional distribution of large brown algal populations by mobile vertebrate consumers are rare and were mediated by an environmental gradient and plant density, both of which interact with algal demographics. The study highlights that, even though herbivorous fish diversity may be low, the impacts of particular species may still be high, even in cool temperate waters where fish herbivory is usually considered to be minimal.