Ecology of Australian Temperate Reefs

The Unique South

Scoresby A. Shepherd, Graham Edgar, Scoresby A. Shepherd, Graham Edgar, Scoresby Shepherd

• Publisher: CSIRO PUBLISHING
• Year: 2013
• Print ISBN: 9781486300099
• eBook ISBN: 9781486300112
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Ecology of Australian Temperate Reefs presents the current state of knowledge of the ecology of important elements of southern Australian sub-tidal reef flora and fauna, and the underlying ecological principles.

Preliminary chapters describe the geological origin, oceanography and biogeography of southern Australia, including the transitional temperate regions toward the Abrolhos Islands in the west and to Sydney in the east. The book then explains the origin and evolution of the flora and fauna at geological time scales as Australia separated from Antarctica; the oceanography of the region, including principal currents, and interactions with on-shelf waters; and the ecology of particular species or species groups at different trophic levels, starting with algae, then the ecological principles on which communities are organised. Finally, conservation and management issues are discussed.

Ecology of Australian Temperate Reefs is well illustrated with line drawings, figures and colour photographs showing the many species covered, and will be a much valued reference for biologists, undergraduates, and those interested and concerned with reef life and its natural history.

2014 Whitley Award Commendation for Marine Ecology.

PART 1

THE SOUTHERN OCEAN FROM
ITS BEGINNINGS TILL NOW

‘The surface of the earth binds geography and geology together in an indissoluble union rather like that of man and wife. Geography, like a prudent woman, has taken to herself an ‘elder than herself’, (though)… she does not flaunt the assertion that she is a woman with a past.’

Geologist Charles Lapworth

The first three chapters provide the backdrop – an account of the origins and evolution of the Southern Ocean, as Australia drifted north, and the large-scale patterns that give the ‘unique south’ its present character. The unique features of the southern Australian flora and fauna – its high endemism, and species richness – can only be understood when we are made aware of the geology of the Australian continental plate, as described in Chapter 1. The plate has enjoyed a long period of stability with only minor vertical earth movements, as Australia broke free from Antarctica and drifted northwards. However, sea level has gone up and down with the alternate freezing and melting of polar ice caps, creating gulfs, bays and peninsulas.

The long isolation and stability of the southern Australian region, with the longest stretch of an east–west coast in the Southern Hemisphere, the temperate climate and the topographic complexity of the coastline, have created a vast range of habitats. The ocean currents that have washed its shores for 70 million years, the tidal currents and the variable wave climate have all played a role in further diversifying a bewildering complexity of habitats (Chapter 2). Together these factors have contributed in a variety of ways to the evolution and diversification of a rich marine flora and fauna, as described in Chapter 3.

1 Geological history and climate change in southern Australia

‘The history of any one part of the Earth, like the life of a soldier, consists of long periods of boredom and short periods of terror.’

Geologist DV Agar

OVERVIEW

This chapter examines the past history of the Southern Ocean from its beginnings some 80–100 million years ago (Ma) to the present. As Australia broke free from Antarctica and drifted north, the Southern Ocean widened and then, as the Tasmanian gateway and Drake Passage opened, the Circumpolar Antarctic Current developed, insulating Antarctica, and making way for its glaciation. During the past 65 million years (My), Earth’s climate has gone from super-greenhouse to icehouse conditions as atmospheric carbon dioxide (pCO₂) concentration has declined. Southern Australian waters at first cooled with the global temperature decline, but then gradually warmed to the present temperate temperature regime as Australia drifted towards the tropics. Superimposed on these changes were four global chills and some warming episodes. Milankovitch cycles related to Earth’s orbit, and small changes in insolation, underlay climatic oscillations, greatly magnified by pCO₂. Southern Ocean waters have also been influenced by the episodic Leeuwin Current and ENSO events affecting upwellings. Sea levels have fluctuated with global temperatures and the extent of polar ice sheets. The past history of the Southern Ocean can now be used to predict the consequences of anthropogenic increases in pCO₂. These include: higher sea temperatures, rising sea levels, and changes in southern wind patterns and ocean currents.

INTRODUCTION

The marine flora and fauna of southern Australia are undeniably rich, but understanding why they are rich is not easy. This is a question of history, so we need to examine the geological past, the palaeogeography and climate of the Southern Ocean since its beginnings some 80–100 million years ago (Ma). Another reason why geological history is important is the present concern about climate change. As climatic records are short (~100 years), geological records are invaluable in testing theories and developing models about long-term changes. This chapter gives a simplified thumb-nail sketch of geological events over the past 80 million years (My), and examines some of the key factors that have shaped the environment in which the present flora and fauna of southern temperate reefs have evolved. The Southern Ocean has played a major role, and still does, in the global climate system, so we will need to consider in some detail its evolution and that of its ‘parental’ neighbouring land mass, Antarctica. The chapter concludes with a summary of predicted oceanic and climatic changes in southern coastal regions with global warming.

The modern fauna and flora had its origins in the near-total extinction event at the end of the Permian, 250 Ma, when 95% of species disappeared (Benton and Twitchett 2003), but events in the last 80 My, as Gondwanaland fell apart, have been the most crucial for the modern fauna.

The Earth’s history has been marked by continuous climatic fluctuations, which find their origin in four kinds of events: periodicities of the Earth’s orbital cycles (Milankovitch cycles), tectonic processes, occasional aberrant climatic extremes, and the rare global catastrophe. Of all these, only the Milankovitch cycles have an astronomic regularity, and establish an underlying cyclic pattern. Their periodicities, ranging from a few to many thousands of years (ky), are of three types, founded on: (a) the eccentricity of the Earth’s orbit round the sun, going from nearly circular to elliptical, with periods of 400 and 100 ky, and with normally slight effects on insolation; (b) the obliquity of the Earth’s axis (period 41 ky), which mainly affects seasonal climatic contrasts from the tropics to high latitudes; and (c) precession; that is, the wobble of the Earth’s axis (periods of 19 and 23 ky), which, modulated by orbital eccentricity, increases seasonal contrast in one hemisphere and decreases it in the other (Zachos et al. 2001).

The major tectonic processes were the break-up of Gondwanaland, and the developing rift between the Antarctic plate and the Australian and South American plates, as the last two plates gradually drifted northwards. The major extreme climatic aberrations were:

• the late Paleocene warm phase (super-greenhouse), in which sea temperatures increased by 5–8°C according to latitude, with globally higher precipitation and humidity and other consequences
• the extreme glaciation of Antarctica ~34 Ma
• the later brief (200 ky) glacial maximum.

The major global catastrophic event was the extensive volcanic eruptions combined with a huge meteor impact in Mexico ~66.4 Ma. These caused both a mass extinction of life on land, and also the collapse of oceanic marine ecosystems. The near complete cessation of oceanic productivity in the sea for ~10 ky gave rise to a ‘Strangelove’ ocean, so called because of its supersaturated state and domination by inorganic CaCO₃. Modelling showed that the event would have caused a two- to three-fold increase in atmospheric CO₂ (see below), and led to ~3°C warming of surface waters (Zachos et al. 1989).

The above events were all accompanied by increased turnover of marine species and the evolution of new species.

THE CAENOZOIC

Birth of the Southern Ocean

Our story starts with the separation of Australia from Antarctica, which began ~100 Ma, and saw the formation of the Southern Ocean. The major tectonic, climatic, oceanographic and biotic events from ~65 Ma to the present, with sources, are summarised chronologically in Table 1.1. About 100 Ma, Australia, New Zealand, Antarctica, South America, Africa and Maria Byrd Land together formed Gondwanaland, with break-up first occurring as the New Zealand plateau drifted north. By the end of the Cretaceous and beginning of the Cainozoic (65 Ma), Antarctica was still linked tenuously to Australia, whose southern coastline then lay at 60–65°S (Fig. 1.1). Numerous shallow marine transgressions penetrated successively from south-west (SW) Australia eastwards along southern Australia in the succeeding 5–10 My, but Tasmania and the South Tasman Rise remained barriers to deep water circulation until the mid-Eocene (~42 Ma). Two events – the opening of Drake Passage between South America and Antarctica and the accelerated widening of the oceanic gap between the south Tasman Rise and Antarctica at ~41 Ma – gave birth to the proto-Circum-Antarctic Current (CAC), also known as the West Wind Drift, and led to a marked fall in southern shallow sea-water temperatures. The opening of these two ocean gateways and the CAC together played a crucial climatic role, because they cut off Antarctica from a warm, south-flowing tropical current along South America. This led to the thermal insulation of Antarctica and saw the growth of the Antarctic ice-sheets 36–34 Ma (see below), which were key elements affecting the scale of climate change then and later, up to the present time (Fig. 1.2).

Table 1.1 Cainozoic palaeo-events, as reviewed by McGowran et al. 2000.

...

Ma	Epoch	Climatic and tectonic events	Oceanographic, biotic events	References
100	Mid-Cretaceous	Global temperatures at a max. Antarctic–Australia rifting has begun		Crame 1999
70–65	Palaeocene	Meteor impact. Supergreenhouse. Shallow Australia–Antarctic opening	Mass extinction event. Marine productivity collapse. Five shallow marine transgressions.	Keller et al. 2004 Zachos et al. 1989 McGowran 1991 Brinkhuis et al. 2006
55	Eocene	Massive methane/CO2 release		Sluijs et al. 2006 Pearson and Palmer 2000
50–49		Global cooling (Chill 1) Proto-Circum-Antarctic Current	Shallow marine gateway Archaic whales appear	Crame 1999 Lear et al. 2000
44–2		Sea-floor spreading rates increase	Intermittent proto-Leeuwin Current
42–1		Flooding of Australian continental margin in three transgressions	Drake Passage opens, and CAC begins	Scher and Martin 2006 McGowran and Li 1998

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Acknowledgements
• Contributors
• Reviewers and others who assisted in various ways
• Preface
• Introduction
• Colour plates
• Part 1 The Southern Ocean from its beginnings till now
• Part 2 The algae
• Part 3 The invertebrates
• Part 4 Marine vertebrates
• Part 5 Marine ecosystems and their conservation
• Index