Beta diversity of a Gulf of California rocky-shore fish community

Synopsis Reef fish community composition in three segments of a peninsular rocky shoreline in the Gulf of California was estimated over four periods by visual observation. ‘Point’ and ‘bay’ segments had regular and distinct species compositions over most periods while a ‘middle’ segment was least distinct but consistently had the greatest number of species. Compositional change along the peninsula was least regular during the coldest sea temperature period (April). Mean species turnover between segments was highest between point and bay. Within segments, the point had greater compositional predictable composition (lowest species turnover). When species with regular frequency of encounter were classified into ‘point’, ‘middle’, ‘bay’, and ‘no trend’ groups it was found that comparatively more ‘point’ species had pelagic eggs and comparatively more ‘bay’ species had demersal eggs. Beta diversity of rocky-shore fishes along the physical gradient of the Punta Doble peninsula reflects a transition between exposed and protected rocky shoreline communities. The correlated physical environmental characteristics associated with exposed and protected habitats are discussed in relation to diversity regulation and dispersal strategies in reef fishes.     

Population persistence in the face of advection

Many populations live in ‘advective’ media, such as rivers, where flow is biased in one direction. In these environments, populations face the possibility of extinction by being washed out of the system, even if the net reproductive rate (R) is greater than one. We propose a formal condition for population persistence in advective systems: a population can persist at any location in a homogeneous habitat if and only if it can invade upstream. This leads to a remarkably simple recipe for calculating the minimal value for the net reproductive rate for population persistence. We apply this criterion to discrete-time models of a semelparous population where dispersal is characterized by a mechanistically derived kernel. We demonstrate that persistence depends strongly on the form of the kernel’s ‘tail’, a result consistent with previous literature on the speed of spread of invasions. We apply our theory to models of stream invertebrates with a biphasic life cycle, and relate our results to the ‘colonization cycle’ hypothesis where bias in downstream drift is offset by upstream bias in adult dispersal. In the absence of bias in adult dispersal, variability in the duration of the larval stage and in oviposition sites have a large effect of the persistence condition. The minimization calculations required in our approach are very straightforward, indicating the feasibility of future applications to life history theory.     

Ontogeny of behaviour in larvae of marine demersal fishes

The development of behaviours that are relevant to larval dispersal of marine, demersal fishes is poorly understood. This review focuses on recent work that attempts to quantify the development of swimming, orientation, vertical distribution and sensory abilities. These behaviours are developed enough to influence dispersal outcomes during most of the pelagic larval stage. Larvae swim in the ocean at speeds similar to the currents found in many locations and at 3–15 body lengths per second (BL s⁻¹), although, based on laboratory measurements, species from cold environments swim slower than those from warm environments. At least in warm-water species, larvae swim in an inertial hydrodynamic environment for most of their pelagic period. Unfed swimming endurance is >10 km from about 8–10 mm, and reaches more than 50 km before settlement in several species. Larval fishes are efficient swimmers. In most species, a large majority of larvae have orientated swimming in the ocean, but the precision of orientation does not improve with growth. Swimming direction of the larvae frequently changes ontogenetically. Vertical distribution changes ontogenetically in most species, and both ontogenetic ascents and descents are found. Development of schooling is poorly understood, but it may influence speed, orientation and vertical distribution. Sensory abilities (hearing, olfaction, vision) form early, are well developed and are able to detect cues relevant to orientation for most of the pelagic larval stage. All this indicates that the passive portion of the pelagic larval duration will be short, at least in most warm-water species, and that behaviour must be taken into account when considering dispersal, and in particular in dispersal models. Although quantitative information on the ontogeny of some behaviours is available for a relatively small number of species, more research in this field is required, especially on species from colder waters.     

What can population genetics tell us about dispersal and biogeographic history of coral-reef fishes?

Abstract Coral-reef fishes, like many other marine organisms, generally possess a benthic adult stage and pelagic larval stage. What can population genetics studies tell us about the demographic, evolutionary and biogeographic consequences of this life cycle? Ten studies of geographical patterns of intraspecific genetic differentiation in reef fishes have been published. These studies have included 2t > species/species complexes (14 in the family Pomacentridae, the remaining 12 in 9 different families) and have been about equally divided between the tropical Pacific and the tropical western Atlantic. A survey of these studies shows the following: (i) the existence of the pelagic larval stage appears to have led to high levels of gene flow even among populations separated by thousands of kilometres of open ocean; (ii) an apparent pattern of increased gene flow among populations connected by intermediate 'stepping stones’; (iii) very tentative evidence for a relationship between length of pelagic larval life and gene flow; (iv) no clear relationship between egg type (pelagic rs non-pelagic) and gene flow; and (v) suggestive evidence that damselfishes (family Pomacentridae) may have more restricted dispersal (less gene flow) than other reef fishes. The application of current and future molecular tools has the strong potential to clarify some of these relationships, particularly by using relatively neutral genetic markers. Additionally, discoveries of DNA markers having very high rates of mutation may allow tracking of demographically relevant levels of larval dispersal. Molecular tools are becoming especially valuable in uncovering the biogeographic and phylogenetic history of reef fishes. The one molecular study to date has suggested that at least some speciation events may have occurred during the climate changes and sea-level regressions associated with Pleistocene glacial episodes. Molecular tools need to be used to further explore the means by which high species diversity can be generated in the face of the apparently high gene flow observed in most coral-reef fishes.     

Pacific Coral-reef Fishes: The Implications of Behaviour and Ecology of Larvae for Biodiversity and Conservation, and a Reassessment of the Open Population Paradigm

The two-phase life history of most marine fishes and invertebrates has enormous implications for dispersal, population connectivity, and resource management. Pelagic dispersal larvae of marine animals traditionally thought to ensure that populations are widespread, that chances of local extinction are low, and that marine protected areas (MPA) can easily function to replenish both their own populations and those of unprotected areas. Traditionally, dispersal is considered to depend primarily on two variables: pelagic larva duration and far-field currents. These conclusions arise from the open population paradigm and are usually accompanied by a simplifying assumption: larvae are distributed passively by far-field currents. Unfortunately, they ignore the complex reality of circulation and hydrological connectivity of reefs, and do not consider newly-demonstrated behavioural capabilities of coral-reef fish larvae. Far-field circulation varies with depth and often excludes water bodies where propagules are released, and this has important implications for predicting trajectories of even passive larvae. However, larvae are not passive: late-stage larvae of coral-reef fishes can swim faster than currents for long periods, can probably detect reefs at some distance, and can actively find them. This behaviour is flexible, which greatly complicates modelling of larval fish trajectories. Populations at ecological (as opposed to evolutionary) scales are probably less open and more subdivided than previously assumed. All this means that dispersal predictions based solely on far-field water circulation are probably wrong. An emerging view of larval-fish dispersal is articulated that takes these new data and perspectives into account. This emerging view shows that re-evaluation of traditional views in several areas is required, including the contribution of larval-fish biology and dispersal to biodiversity patterns, the way reef fishes are managed, and the way in which MPA are thought to operate. At evolutionary and zoogeographic scales, reef-fish populations are best considered to be open.     

Variation in the life history pattern of Tetranychus urticae (Acari: Tetranychidae) after selection for dispersal

The spider mite Tetranychus urticae shows variation in its dispersal capacity (i.e., the leaf quality at which a female decides to disperse). We were able to artificially select mites that had either a high or a low dispersal capacity, indicating that this trait was genetically controlled. We then compared correlated responses to this selection. Mites with a genetically high dispersal capacity (‘HD’ strains) had a higher diapause incidence and a lower performance compared to mites with a low dispersal capacity (‘LD’ strains). A possible effect of random genetic drift during the selection was negligible. Our results suggest that differential dispersal capacity is associated with contrasting life history patterns as a result of natural selection. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modelling larval dispersal and behaviour of coral reef fishes

Coral reef fish spend their first few weeks developing in the open ocean, where eggs and larvae appear merciless to tides and currents, before attempting to leave the pelagic zone and settle on a suitable reef. This pelagic dispersal phase is the process that determines population connectivity and allows replenishment of harvested populations across multiple coral reef habitats. Until recently this pelagic larval dispersal phase has been poorly understood and has often been referred to as the ‘black-box’ in the life-history of coral reef fishes. In this perspective article we highlight three areas where mathematical and computational approaches have been used to aid our understanding of this important ecological process. We discuss models that provide insights into the evolution of the pelagic larval phase in coral reef fish, an unresolved question which lends itself well to a modelling approach due to the difficulty in obtaining empirical data on this life history strategy. We describe how studies of fish hearing and physical sound propagation models can be used to predict the detection distance of reefs for settling larval fish, and the potential impact of anthropogenic noise. We explain how random walk models can be used to explore individual- and group-level behaviour in larval fish during the dispersal and settlement stage of their life-history. Finally, we discuss the mutual benefits that mathematical and computational approaches have brought to and gained from the field of larval behaviour and dispersal of reef fishes.     

Reproductive strategies of coastal marine fishes in the tropics

Synopsis A synthesis of ethnobiological, behavioral and physical oceanographic information leads to the conclusion that temperate zone models of reproductive strategy are inapplicable to many fishes of the coastal tropics. Intense predation appears to exert heavy selection pressure on fishes that spend their adult lives in coral, mangrove or tropical seagrass communities. Many exhibit spawning behaviors and spawn at times and locations that favor the transport of their pelagic eggs and pelagic larvae offshore where predation is reduced. This creates a countervailing selection pressure — the need to return the larvae to shallow water once they are ready to colonize their post-larval habitats. Accordingly, spawning is often concentrated at times of the year when prevailing winds or currents are at their weakest, thereby reducing the transport of larvae long distances from where they originated. Spawning is also concentrated in the vicinity of nearshore gyres which similarly favor the ultimate return of the larvae to their natal area. Among these species, therefore, offshore larval dispersal does not seem to be an adaptation for dispersal of the species, but rather an evolutionary response to intense predation pressure in the adult habitats. Lunar reproductive periodicity is more common among these species than has previously been recognized, and is one of the strategies employed to enhance the offshore flushing of eggs and larvae.This paper forms part of the proceedings of a mini-symposium convened at Cornell University, Ithaca, N. Y. 18–19 May 1976, entitled Patterns of Community Structure in Fishes (G. S. Helfman, ed.).Contribution No. 524, Hawaii Institute of Marine Biology, University of Hawaii, Honolulu.     

A Second Look at Mitochondrial DNA Variability in European Anchovy (Engraulis encrasicolus): Assessing Models of Population Structure and the Black Sea Isolation Hypothesis

Genetic architectures of marine fishes are generally shallow because of the large potential for gene flow in the sea. European anchovy, however, are unusual among small pelagic fishes in showing large differences among sub-basins and in harbouring two mtDNA phylogroups (‘A’ & ‘B’), representing 1.1–1.85 million years of separation. Here the mtDNA RFLP dataset of Magoulas et al. [1996, Mol. Biol. Evol. 13: 178–190] is re-examined to assess population models accounting for this subdivided population structure and to evaluate the zoogeographical origins of the two major phylogroups. Haplotype and nucleotide diversities are highest in the Ionian Sea and lowest in the Aegean and Black seas. However, this gradient is absent when ‘A’ and ‘B’ haplotypes are examined separately. Neither the self-sustaining nor the basin population models adequately describe anchovy population behaviour. Tests for neutrality, mismatch and nested clade analyses are concordant in depicting recent expansions of both phylogroups. Unimodel mismatch distributions and haplotype coalescences dating to the last (Eemian) interglacial (‘B’) and the Weichselian pleniglacial period (‘A’) indicate separate colonizations of the Mediterranean Basin. Phylogroup ‘A’ is unlikely to have arisen through continuous long-term isolation in the Black Sea because of climate extremes from displaced subpolar weather systems during the ice ages. Ancestors of both groups appear to have colonized the Mediterranean from the Atlantic in the late Pleistocene. Hence, zoogeographic models of anchovy in the Mediterranean must also include the eastern (and possibly southern) Atlantic.     

Inbreeding and promiscuity in the endangered grand skink

The inbreeding avoidance hypothesis predicts that organisms that often encounter relatives as potential mates should evolve behaviours to avoid incestuous matings. Avoidance behaviours have practical importance for small populations because deleterious genetic processes may be less imminent than otherwise expected from genetic models that assume random mating. I used genetic techniques to investigate the extent of inbreeding and inbreeding avoidance behaviours in rare lizards from southern New Zealand. Grand skinks, Oligosoma grande, live in small patchily distributed groups, and have low rates of inter-group dispersal (ca. 3–20% disperse). I used data from 15 microsatellite loci to test the hypothesis that adults are likely to encounter kin as potential mates and will inbreed. These data showed that adult skinks usually inhabited rock outcrops with adult relatives of the opposite sex – up to 35% of potential mates were of equivalent relatedness as half-sibs and 17% were equivalent to full sibs. However, skinks did not preferentially breed with less related mates, and 18.2% of matings were between individuals of equivalent relatedness as full-sibs. Instead, skinks mated with partners of all levels of relatedness, and were promiscuous – almost half of adult females and nearly three quarters of adult males reproduced with multiple partners. In addition, inbreeding had no effect on survival of offspring in their first year. Two other putative mechanisms of inbreeding avoidance, sex-biased and natal dispersal, were not pronounced in this species. This study adds to a growing list of species that inbreed despite the risks.     

Post-settlement dispersal: the neglected link in maintenance of soft-sediment biodiversity

Seafloor integrity is threatened by disturbances owing to human activities. The capacity of the system to recover from disturbances, as well as maintain resilience and function, depends on dispersal. In soft-sediment systems, dispersal continues after larval settlement, but there are very few measurements of how far the post-settlers disperse in nature. Spatial scales of post-settlement dispersal are, however, likely to be similar to pelagic larval dispersal because of continued, frequent, small-scale dispersal over longer periods. The consequences of this dispersal may be more important for the maintenance of biodiversity and metacommunity dynamics than is pelagic larval dispersal, because of the greater size and competency of the dispersers. We argue that an increased empirical understanding of post-settlement dispersal processes is key for predicting how benthic communities will respond to local disturbances and shrinking regional species pools, with implications for monitoring, managing and conserving biodiversity.     

Seed and microsite limitations of recruitment and the impacts of post-dispersal seed predation at the within population level

Seed predation may cause important seed losses in plant populations, but its impact on the dynamics of populations will depend on the degree of seed or microsite limitations for recruitment. Seed losses will only affect recruitment if it is seed limited. The nature of recruitment limitation (seeds or microsites) is usually ascribed to whole plant populations but it may vary within populations among microhabitats and habitats. Thus, the potential impact of seed predation will also vary within the population, being highest where recruitment is seed limited. The impact to the whole population will depend on the spatial concordance between the intensity of seed predation and that of seed limitation. Recruitment limitations (with seed addition experiments), seed predation (with seed removal experiments), and the dynamics of seed availability in the soil (with soil samples taken both after seed dispersal and before the following dispersal event) of the shrub Corema album (Empetraceae) were investigated in dunes in NW Spain, at microhabitats ‘open ground’, ‘underneath C. album ♀’, and ‘underneath C. album ♂’ at two habitats, sparse and dense scrub. The nature of recruitment limitation (seeds vs. microsites) varied within the population. It was seed limited in the microhabitat ‘open ground’ and microsite limited under shrub cover. The spatial patterns of seedling recruitment were unrelated to seed availability but strongly affected by germination requirements. The spatial discordance between seed availability and recruitment implies a crucial constraint for processes affecting seed availability (seed predation but also e.g., dispersal) to impact recruitment. They will not affect its spatial pattern but only its quantity as long as they act in those sites selected by seeds to germinate. Seed predation was highest underneath mother plants and lowest in open ground. Thus, its potential impact is low, as it is centred where recruitment is not seed limited. This study shows that the analysis of seed predation in relation to recruitment limitations at smaller spatial scales within the population provides more insight to understand its impact.     

Long‐lived marine species may be resilient to environmental variability through a temporal portfolio effect

Maintenance of genetic variation may provide resilience of populations to natural environmental variability. We used Pacific ocean perch (POP; Sebastes alutus) to test for the maintenance of adaptive variation across overlapping generations. POP are a long‐lived species characterized by widespread larval dispersal in their first year and a longevity of over 100 years. In order to understand how early marine dispersal affects POP survival and population structure, we used restriction site‐associated DNA sequencing (RADseq) to obtain 11,146 single‐nucleotide polymorphisms (SNPs) from 401 young‐of‐the‐year (YOY) POP collected during surveys conducted in 2014 (19 stations) and 2015 (4 stations) in the eastern Gulf of Alaska. Population clustering analysis showed that the POP samples represented four distinct ancestral populations mixed throughout the sampling area. Based on prior work on larval dispersal of POP, these larvae are most likely from distinct parturition locations that are mixing during their pelagic dispersal life stage. Latent factor mixed models revealed that POP larvae face significant selection during their first year at sea, which is specific to the year of their birth. Thus each adult cohort's genetic composition is heavily influenced by the environmental conditions experienced during their first year at sea. Long‐lived species relying on broadcast spawning strategies may therefore be uniquely resilient to environmental variability by maintaining a portfolio of cohort‐specific adaptive genotypes, and age truncation due to overfishing of older cohorts may have detrimental effect on the population viability.     

Connectivity,biodiversity conservation and the design of marine reserve networks for coral reefs

Networks of no-take reserves are important for protecting coral reef biodiversity from climate change and other human impacts. Ensuring that reserve populations are connected to each other and non-reserve populations by larval dispersal allows for recovery from disturbance and is a key aspect of resilience. In general, connectivity between reserves should increase as the distance between them decreases. However, enhancing connectivity may often tradeoff against a network’s ability to representatively sample the system’s natural variability. This “representation” objective is typically measured in terms of species richness or diversity of habitats, but has other important elements (e.g., minimizing the risk that multiple reserves will be impacted by catastrophic events). Such representation objectives tend to be better achieved as reserves become more widely spaced. Thus, optimizing the location, size and spacing of reserves requires both an understanding of larval dispersal and explicit consideration of how well the network represents the broader system; indeed the lack of an integrated theory for optimizing tradeoffs between connectivity and representation objectives has inhibited the incorporation of connectivity into reserve selection algorithms. This article addresses these issues by (1) updating general recommendations for the location, size and spacing of reserves based on emerging data on larval dispersal in corals and reef fishes, and on considerations for maintaining genetic diversity; (2) using a spatial analysis of the Great Barrier Reef Marine Park to examine potential tradeoffs between connectivity and representation of biodiversity and (3) describing a framework for incorporating environmental fluctuations into the conceptualization of the tradeoff between connectivity and representation, and that expresses both in a common, demographically meaningful currency, thus making optimization possible.     

Towards an explanation of the developmental strategy in leptocephalous larvae of marine teleost fishes

Synopsis Various observations on the morphology, physiology and biochemistry of leptocephalous larvae of different groups of marine teleost fishes have been brought together in order to arrive at a model which attempts to explain the ‘leptocephalous strategy’ of larval development. The observation that basic similarities are found in the developmental pattern of all groups of fishes with a leptocephalus (Superorder: Elopomorpha) forms the basis for proposing a common strategy within the superorder. Circumstantial evidence suggesting that premetamorphic (Phase I) larvae obtain a significant fraction of their nutritional needs by absorbing dissolved organic matter across surface epithelia has been reviewed. It is suggested that this might occur via a Na⁺-mediated transport system similar to that seen in various marine invertebrates. Breakdown of the gelatinous matrix formed during Phase I is assumed to provide the nutrients required for the metamorphic larvae (Phase II). This strategy is then contrasted with the more ‘typical’ form of larval development in marine teleosts and shown to differ in several basic respects.     

Contrasting multispecies patterns in larval fish production trace inter-annual variability in oceanographic conditions over the N.E. Aegean Sea continental shelf (Eastern Mediterranean)

The response of larval fish communities of the northeastern Aegean Sea (NEA) to interannual environmental changes is analyzed using data from four ichthyoplankton surveys covering the NEA continental shelf during June 1993, 1994, 1995, and 1996. Waters were significantly cooler, less saline and richer in zooplankton in 1993 and 1996 (‘cold’ years) than in 1994 and 1995 (‘warm’ years). A comparison of monthly SST series (1993–1997) between the NEA, the Marmara Sea, and the Western Black Sea revealed high correlations and similar trend components among these areas implying that oceanographic conditions over the NEA (and observed inter-annual differences) were most likely dominated by the properties and relative amount of Black Sea water inflow in the NEA. The relative composition of the larval fish community was significantly related to the ‘cold/warm’ regime and larval diversity was higher during the warm years. Larvae of the small-sized pelagic species, such as anchovy (Engraulis encrasicolus), and most mesopelagic fishes were relatively more abundant during the cold, zooplankton-rich years. Larvae of the middle-sized pelagics (Sardinella aurita, Scomber japonicus, Trachurus mediterraneaus, Auxis rochei) and certain benthopelagic species exhibited an opposite trend, i.e., they were more abundant during the warm years or absent during the cold years. Most of these species are known to be typical summer spawners (e.g., Serranus cabrilla, Lisa saliens, Trachinus draco, and Symphurus nigrescens). Co-variation in larval fish production might be indicative of similar responses among species to changing physical and/or trophic regimes.     

REPRODUCTIVE ECOLOGY AND BIOGEOGRAPHY OF INDO-WEST PACIFIC ANGELFISHES (PISCES: POMACANTHIDAE)

Analysis of the relationships between duration of the pelagic larval stage (as indicated by otolith microstructure), adult size, and the extent of geographic distribution for Indo-West Pacific angelfishes (Pomacanthidae) indicates that neither adult size nor larval duration significantly predicts extent of distribution, either individually or jointly in a multiple regression. These results are robust for both the family as a whole and the genus best represented in our data (Centropyge). If larval duration and adult size do have an effect, it is only jointly and at the genus level. However, larval duration and adult size do correlate significantly and negatively with one another. The operational factor seems to be egg size, which correlates positively with adult size, and negatively with duration of the pelagic larval stage. Similar correlations are evident in both marine invertebrates and at least some other coral-reef fishes, suggesting they are of widespread significance. The limited ability of either reproductive parameter to predict extent of species distribution indicates, first, that even in a group as morphologically conservative as the Indo-West Pacific pomacanthids, neither a two-fold difference between species in larval duration nor a two order of magnitude difference in female fecundity markedly affects extant distributions; and secondly, that either undescribed biological factors or historical constraints are of paramount importance. Available evidence suggests that dispersal abilities of most coral reef fishes, in fact, may be limited, despite the nearly universal occurrence of a pelagic stage in development.     

On the spatial scale of dispersal in coral reef fishes

Marine biologists have gone through a paradigm shift, from the assumption that marine populations are largely ‘open’ owing to extensive larval dispersal to the realization that marine dispersal is ‘more restricted than previously thought’. Yet, population genetic studies often reveal low levels of genetic structure across large geographic areas. On the other side, more direct approaches such as mark‐recapture provide evidence of localized dispersal. To what extent can direct and indirect studies of marine dispersal be reconciled? One approach consists in applying genetic methods that have been validated with direct estimates of dispersal. Here, we use such an approach—genetic isolation by distance between individuals in continuous populations—to estimate the spatial scale of dispersal in five species of coral reef fish presenting low levels of genetic structure across the Caribbean. Individuals were sampled continuously along a 220‐km transect following the Mesoamerican Barrier Reef, population densities were estimated from surveys covering 17 200 m² of reef, and samples were genotyped at a total of 58 microsatellite loci. A small but positive isolation‐by‐distance slope was observed in the five species, providing mean parent‐offspring dispersal estimates ranging between 7 and 42 km (CI 1–113 km) and suggesting that there might be a correlation between minimum/maximum pelagic larval duration and dispersal in coral reef fishes. Coalescent‐based simulations indicate that these results are robust to a variety of dispersal distributions and sampling designs. We conclude that low levels of genetic structure across large geographic areas are not necessarily indicative of extensive dispersal at ecological timescales.     

Evidence for Cohesive Dispersal in the Sea

As with many marine species, the vast majority of coral-reef fishes have a bipartite life cycle consisting of a dispersive larval stage and a benthic adult stage. While the potentially far-reaching demographic and ecological consequences of marine dispersal are widely appreciated, little is known of the structure of the larval pool and of the dispersive process itself. Utilizing Palindrome Sequence Analysis of otolith micro-chemistry (PaSA;) we show that larvae of Neopomacentrus miryae (Pomacentridae) appear to remain in cohesive cohorts throughout their entire pelagic larval duration (PLD; ∼28 days). Genetically, we found cohort members to be maternally (mtDNA) unrelated. While physical forcing cannot be negated as contributing to initial cohort formation, the small scale of the observed spatial structure suggests that some behavioral modification may be involved from a very early age. This study contributes to our ongoing re-evaluation of the processes that structure marine populations and communities and the spatial scales at which they operate.     

Allochthonous dissolved organic matter as an energy source for pelagic bacteria and the concept of the microbial loop

Substantial evidence exists that allochthonous dissolved organic matter (DOM) can provide an important carbon source for pelagic bacteria. On the other hand, it is implicit in the concept of the ‘microbial loop’ that the degradation of recalcitrant, allochthonous DOM should be retarded in the pelagic environment, as bacteria able to utilize recalcitrant DOM compounds for slow growth would be outcompeted by faster-growing bacteria utilizing more labile DOM compounds. Several possible solutions of this apparent paradox are suggested in this paper, including formation of labile DOM from recalcitrant DOM by e.g. photochemical reactions, and mechanisms enabling the maintenance of a metabolically diverse bacterioplankton. These mechanisms include an explanation analogous to Hutchinson's classical solution to the ‘paradox of plankton’, and differential mortality of different populations within the bacterioplankton enabled by selective grazing, infections by bacteriophages and predatory bacteria, and spatial micropatchiness.