Geoscience Reference
In-Depth Information
CHAPTER 6
Effects of ocean acidii cation
Ulf Riebesell and Philippe D. Tortell
6.1 Introduction
view of marine food webs (Steele 1974) involved a
relatively short trophic system in which large phyto-
plankton (e.g. net plankton such as diatoms) were
grazed by a variety of mesozooplankton (e.g. copep-
ods), which were in turn consumed by second-level
predators, including many economically important
i sh and invertebrate species. This 'classic' marine
food web is typical of high-productivity regions such
as coastal upwelling regimes (Lassiter
et al.
2006 ).
A characteristic feature of these systems is a strong
decoupling between primary production and graz-
ing, which results from the different metabolic rates
of consumers and producers and, in many cases,
ontogenetic and seasonal delays in the emergence of
feeding predators. The uncoupling between phyto-
plankton and their consumers leads to signii cant
export of organic material out of the euphotic zone,
the so-called biological carbon pump (discussed
further below). In a steady-state system, export of
organic material from the euphotic zone is balanced
by external nutrient inputs (e.g. NO
3
-
supply from
vertical mixing; Eppley and Peterson 1979), and the
primary production that is fuelled by these nutrient
supplies is referred to as 'new' or 'export' production
( Dugdale and Goering 1967 ).
In addition to the classical food web, there exists
a parallel 'microbial loop' (Azam
et al.
1983 ; see
Chapter 5) which contributes to total marine pri-
mary productivity and plays an important role in
nutrient recycling. This microbial food web is
driven by small photosynthetic algae, heterotrophic
bacteria, and microzooplankton (which are actu-
ally protists). While many of these small organisms
were unknown or greatly undersampled before the
latter half of the 20th century, it is now clear
that they are amongst the most abundant cells in
Over the past decade there has been rapidly grow-
ing interest in the potential effects of ocean acidii -
cation and perturbations of the carbonate system on
marine organisms. While early studies focused on a
handful of phytoplankton and calcifying inverte-
brates, an increasing number of investigators have
begun to examine the sensitivity to ocean acidii ca-
tion of various planktonic and benthic organisms
across the marine food web. Several excellent review
articles have recently summarized the rapidly
expanding literature on this topic (Fabry
et al.
2008 ;
Doney
et al.
2009 ; Joint
et al.
2011). The focus of this
chapter is on the potential ecosystem-level effects of
ocean acidii cation. Starting with a brief review of
the basic physical, chemical, and biological proc-
esses which structure pelagic marine ecosystems,
the chapter explores how organismal responses to
perturbations of the carbonate system could scale
up in both time and space to affect ecosystem func-
tions and biogeochemical processes. As with many
chapters in this volume, and indeed much of the
ocean acidii cation literature at present, our review
raises more questions than it answers. It is hoped
that these questions will prove useful for articulat-
ing and addressing key areas of future research.
6.2 Planktonic processes and the
marine carbon cycle
6.2.1 Planktonic organisms, pelagic food webs,
and ecosystems
Complexity in marine pelagic food webs results from
the interactions of multiple trophic levels across a
range of temporal and spatial scales. The traditional
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