Coastal Ecosystem Processes

Daniel M. Alongi

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000095173
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Coastal Ecosystem Processes, written by the renowned marine scientist Daniel Alongi, describes how pelagic and benthic food webs, from beaches and tidal flats to the continental edge, process energy and matter. This volume focuses on recent advances and new developments on how food webs are closely intertwined with the geology, chemistry, and physics of coastal seas. Dr. Alongi presents a process-functional approach as a way of understanding how the energetics of coastal ecosystems rely not only on exchanges within and between food chains, but how such functions are influenced by terrigenous and atmospheric processes.
There is a need for documentation and an awareness of just how necessary, yet delicate, is the interplay of biological and physical forces between coastal ocean, land, and the atmosphere. Marine scientists today need to make informed management decisions about sustainable development and conservation of these fragile ecosystems. Coastal Ecosystem Processes provides present and future marine scientists the latest coastal ecosystem information to make the right decisions concerning the ecology of our oceans.

Chapter 1

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INTRODUCTION

“And the microscopic organisms … those billions of animalicules, of which there are millions in a drop of water and eight hundred thousand of which are required to make one milligram — their role is no less important. They absorb marine salt, assimilate the solid elements in the water, and by making corals and madrepores, they build calcareous continents. Then the drop of water, deprived of its mineral element, becomes lighter, comes up to the surface again, absorbs the salts left by evaporation, becomes heavier, descends again, and brings to the animalicules new elements to absorb … a double current, ascending and descending — a continuous movement and continuous life!”

Twenty Thousand Leagues Under the Sea (1870)

Jules Verne’s description of life in the sea is antiquated but remarkably prescient in appreciating that the cycles of ocean life, energy, and matter are intertwined and greatly dependent on the activities of microbes. This should not be surprising if one considers that life began in primordial seas 3.6 billion years ago with the evolution of anaerobic prokaryotes ancestral to modern bacteria, Archaea, and Eukarya. It is surprising, however, that the participation of modern microbes in marine food webs and nutrient processes was not fully appreciated until only about 20 years ago, when improved methods and technology led to the discovery of their great abundance, growth, and productivity in seawater and sediments.¹

The solar energy absorbed and fixed by various phototrophs is stored as biomass and eventually dissipated by a variety of small and large heterotrophs. Algae, cyanobacteria, and green and purple photosynthetic bacteria are among the most important primary producers in the sea, but where the seabed is shallow and the water clear enough to receive significant amounts of light, macroalgae (e.g., kelps) and vascular plants, such as seagrass, marsh grass, and mangrove, also contribute to the fixed carbon pool.

While some of this fixed carbon is directly consumed by herbivores, most carbon enters the detritus food web in the form of dissolved and nonliving particulate matter. The energetics of the pelagic food web is dominated by the microbial loop² — a complex network of autotrophic and heterotrophic bacteria, cyanobacteria, protozoa, and microzooplankton — within which a substantial share of fixed carbon and energy is incorporated into bacteria and subsequently dissipated as it is transferred from one consumer to another.

The laws of thermodynamics constrain the rates and efficiencies of energy transfer and transformation, but the requirements of living organisms impose further losses due to respiration (heat loss), egestion of unassimilated matter, and mortality.³ The conversion of energy from one form to another (and from one trophic level to the next) cannot be 100% efficient (the second law of thermodynamics), and actual rates of energy transfer in aquatic and terrestrial food webs are in fact considerably less than the theoretical maximum limit — usually less than 20%, and more often, only 10 to 15%. The input and output pathways of energy and inorganic nutrients are different within ecosystems, with energy transfer being less efficient than for the cycling of inorganic nutrients. The main reason for the low efficiency of energy transfer is heat loss (respiration) — often greater than 50% of assimilated energy.³ Nutrients, on the other hand, particularly those that are scarce and essential (e.g., nitrogen), are cycled much more efficiently, as most organisms have evolved physiological mechanisms for conservation of such elements. The distinction between living and nonliving components becomes somewhat blurred — as for trophic levels — in the energetics of ecosystems.

Despite the fact that most nonliving matter ends up on the sea floor, our understanding of the fate of this detritus within sediments is poor in comparison to our knowledge of planktonic processes. There are several reasons why this is so. The most obvious is that it is much more difficult to sample and isolate organisms and organic matter from inorganic sediment particles. There is also an enormous number of adsorption and absorption reactions between dissolved and particulate nutrients and clay, silt, and sand particles; many of the latter are often coated with organic compounds of varying reactivity. Also, much of the organic matter in sediments (for example, humic and fulvic acids) is refractory to immediate breakdown by microbes and other decomposers. Such organic compounds are often linked with more labile carbohydrates and proteins.⁴ Isolating bacteria, ciliates, flagellates, fungi, nematodes, and other small organisms from sediments is difficult for the same reasons, as they are often intertwined with or stuck to organic coatings, including their own mucus. Recent techniques have somewhat circumvented these problems, but sedimentary biota are still more difficult to study than pelagic organisms.

The role of microbes and meiobenthos in marine sediments is now often equated to the role their pelagic counterparts play in the microbial loop. This idea has gained some credence as recent data indicate that sediment bacteria and protozoa are more abundant than believed less than a decade ago. Even to the present, the dual roles of sediment bacteria as (1) food for protozoans, invertebrates, and some vertebrates, and (2) as mineralizers of organic matter have not been fully reconciled as they have for pelagic bacteria. Both roles are closely interdependent but, until very recently, have been studied separately by ecologists and biogeochemists, respectively. The concept held by many benthic ecologists (and bordering as an act of faith) that sediment bacteria are heavily grazed by higher organisms is untenable and unrealistic, as it conflicts with our knowledge of their extensive participation in nutrient recycling and ignores the fact that complex physical and biogeochemical gradients exist in marine sediments, due in part to the activities of bacteria and other benthic organisms. Overemphasis on the grazing role developed from early work on trophic interactions and later work on cophrophagy, detritus aging, microbial gardening, resource limitation, and the applicability of optimal foraging theory to marine deposit-feeders.⁵ Work on the trophic role of bacteria was essentially divorced from biogeochemical work on rates and pathways of organic matter decomposition.

More realistic views of the role of bacteria in sediments have since evolved,^5–7 taking into account the vertical distribution of consumers in relation to sediment redox chemistry and oxygen profiles, and the known decomposition pathways of organic matter. The role of sediment bacteria in benthic food chains and nutrient cycles can best be simplified as aerobic trophic links and anaerobic nutrient sinks. This infers that bacteria are linked trophically in surface aerobic sediments where most consumers reside, but not in deeper, anaerobic sediments, where life consists mostly of a variety of anaerobic bacteria and some protozoa. A notable exception is the oxidized lining of burrows and tubes of larger metazoans. Such structures may extend to considerable depths into the seabed.

Grazing of bacteria is discernible in aerobic environments depending on several factors, including the rate of detritus supply and the abundance of grazers. Significant depletion of bacterial biomass by grazers is observable in marine sediments if bacterial growth rates are less than or equal to rates of ingestion. If bacterial growth is equivalent to or outpaces consumption — as it appears to in organic-rich sediments — bacteria are likely to be regulated by other factors, such as temperature and nutrient supply.⁵ The proportion of bacterial standing crop that is consumed depends not only on how fast they are eaten, but also on how quickly they can multiply. With the exception of suitable substrates (tube and burrow linings, leaf blades, sand grains), only a small fraction of bacteria is grazed, even in aerobic benthic habitats. Most bacteria in sediments must die and lyse naturally, with the next generation of bacteria consuming and mineralizing this material, either into new biomass or dissolved material. This situation is greatly different than for pelagic bacteria, which are more readily captured and consumed in greater proportion. Clearly, the distinction between feeding relationships and the participation of microbes in nutrient recycling is more complicated in sediments than in the water column, reflecting what is, in many respects, a more complex environment.

FIGURE 1.1
Model depicting the major oxidation reactions in the decomposition of organic matter with depth in marine sediments. Organic matter is represented by the formula (CH₂O)₁₀₆ (NH3)16 (H₃PO₄). The oxidation reactions are identified by number: (1) Aerobic respira-tion; (2) denitrification; (3) manganese reduction; (4) iron reduction; (5) sulfate reduction; and (6) methanogenesis. Note that nitrogen, phosphorus, and water are common metabolic products.

The decomposition and recycling of organic matter in sediments is orchestrated by a variety of bacterial types which use different electron acceptors sequentially as the geochemistry of sediment changes with depth. Three major zones of organic matter oxidation (Figure 1.1) are recognized:

• Oxic zone, in which oxygen is the major oxidant and limited by the depth to which O_Z can penetrate by diffusion, advection, and mixing

• Suboxic (or postoxic) zone, in which oxygen is available in bound form in nitrate, n...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Preface
• The Author
• Table of Contents
• Dedication
• Chapter 1. Introduction
• Chapter 2. Beaches and Tidal Flats
• Chapter 3. Mangroves and Salt Marshes
• Chapter 4: Seaweed and Seagrass Ecosystems
• Chapter 5. Coral Reefs
• Chapter 6. The Coastal Ocean I. The Coastal Zone
• Chapter 7. The Coastal Ocean IL The Shelf Proper and Shelf Edge
• Chapter 8. Degradation and Conservation
• References
• Index