As biogeographers have long been aware, the most significant environmental gradient and discontinuity in the ocean is horizontal, between shallow and deeper layers, rather than in the vertical plane at the frontal systems discussed earlier. This gradient lies at the seasonal or tropical pycnocline and is globally associated with the change from epipelagic
44 Chapter 3: Fronts and Pycnoclines: Ecological Discontinuities
to deeper ecosystems, so it is the most significant feature in the three-dimensional ecological geography of the oceans. It may be useful to remind ourselves of some of its main characteristics: most importantly, what determines its depth and the strength of its density gradient and whether or not it occurs within the lighted zone.
The epipelagic zone, whose ecological geography is the subject of this book, is a thin layer of light, lighted, and wind-mixed water lying atop the cooler mass of the interior of the ocean; the change in physical and chemical properties between the two depth zones is more frequently abrupt than gradual. When abrupt, the ecological changes that occur across a few tens of meters are greater than across most vertical fronts that intersect the sea surface. This phenomenon is most striking in the eastern parts of the tropical oceans: here, at around 35–40 m below the surface, the temperature drops from 28C to about 16C.
Although the organisms of the epipelagic zone differ fundamentally between the tropical and temperate regions, the deeper-living biota are much more similar. Consequently, as has been noted by many authors, a diel migrant copepod or euphausiid moving across this boundary in the tropics at dawn and dusk makes an environmental adjustment as great as traveling several thousand kilometers equatorward or poleward.
Though the depth at which the pycnocline occurs is determined principally by baro- clinicity associated with the ocean circulation, local processes also intervene, at least seasonally: turbulence induced by wind stress at the sea surface, shear-stress turbulence due to inertial oscillations of the mixed layer water mass that are induced by impulsive changes in wind strength, local heating at the surface by short-wave solar radiation, and the local supply of fresh or brackish water. All these factors will be discussed in the next chapter, so here it is sufficient to note that the depth of the surface wind-mixed layer varies from 25 m in the eastern tropical oceans to 250 m in the center of the subtropical gyres. It may also be noted, parenthetically, that the baroclinic upsloping of nitrate iso- pleths toward the edges of the anticyclonic subtropical gyres is a necessary consequence of the bowl-shaped mixed layer of the gyres and is reflected in the surface chlorophyll field. For this reason, chlorophyll takes consistently higher values around the margins, and lowest values in the center, of the subtropical gyres of each ocean.
At higher latitudes, where seasonality of wind stress and solar heating is stronger, there is a clear discontinuity between the mixed layer of one year and that of the next.
Winter mixing and surface cooling progressively deepen the surface mixed layer down to the depth of the deep permanent pycnocline until, with the return of surface heating and reduced wind stress in spring, a new shallow seasonal mixed layer develops. This progressively deepens and strengthens during the summer, only to be eroded again by wind mixing during the subsequent winter. Peak development of the summer mixed layer is typically several months after midsummer, and greatest penetration of deep convection occurs several months after midwinter. In some regions, a halocline may somewhat obscure the simple model.
You should be aware that seasonal graphs of mixed layer depth, which are based on archived time series of monthly mean values (such as those used in this book), may not capture the establishment of near-surface stratification in spring. This occurs de novo, far above the winter pycnocline, by solar warming and the consequent induction of buoyancy in the near-surface layer. Instead, seasonal graphs may seem to show a progressive shoaling from a very deep to a very shallow mixed layer depth—a process that simply cannot happen, or at least only as a consequence of geostrophic adjustment. This, of course, would violate Dodimead’s first rule for thermoclines: that they can deepen by mixing warmer surface with cold deeper waters but they cannot shoal by unmixing the same! Confusion appears to have arisen between the diel depth of wind mixing, which does of course follow the pattern often used by modelers, and the depth to the seasonal pycnocline. It is this latter, of course, that has the greater biological significance because it carries the seasonally variable nutricline with it. The confusion may also arise by the
The Ubiquitous “Horizontal Front’’ at the Shallow Pycnocline 45
averaging of shoal and deep mixed layer values during spring, thus indicating a false mean value somewhere between the two.
At all latitudes, vertical velocity (Ekman’s WE) is imparted to the water column by the curl of the local wind stress at the sea surface, and this motion shoals or deepens the pycnocline. Cyclonic stress imparts positive values; anticyclonic stress imparts negative values. In this book I shall follow the notation of Isemer and Hasse (1987). Therefore, positive values of WE (upward motion) shall be labeled “Ekman suction” and negative values (downward motion) shall be labeled “Ekman pumping.” You should be aware that in many papers the latter term is loosely used without definition and often in the opposite sense to that used by Isemer and Hasse, or by Tomczak and Godfrey (1994).
The pycnocline, whether seasonal or permanent, lies at a depth generally predictable for each region and has a gradient that is predictable from its depth: generally, the shoaler the pycnocline, the sharper the gradient. From its depth (and some knowledge of regional oceanography) the vertical gradients in other properties of ecological significance that are associated with it may also be predicted, especially nutrients, light, and the biomass of both phytoplankton and zooplankton. The interactions between nutrients, light, turbulence, and the biota in the mixed layer and pycnocline have been a central theme of biological oceanography in recent decades: for a good discussion of all this, see Mann and Lazier (2006) or Banse and English (1994).
The simplest expression of the feature is the typical tropical profile (TTP) of Herbland and Voituriez (1977); this is a description of an oligotrophic profile that is either the end member of plankton succession in midlatitudes from spring to summer, or the permanent condition in tropical seas. Under these conditions, the depths of nutricline, pycnocline, deep chlorophyll maximum (DCM), and productivity maximum do not differ by more than a few meters, and the vertical distribution of zooplankton and micronekton also conforms in a predictable manner (Longhurst and Harrison, 1989). Below the pycnocline, in regions where the TTP is a permanent regional feature of the water column, bacterial oxidation of sinking organic material commonly leads to the development of an oxygen minimum zone and a characteristic anomaly in the usual vertical distribution of plankton profiles (Longhurst, 1967; Saltzman and Wishner, 1997a,b).
It is now generally accepted that in many situations the appropriate model for an oligotrophic profile has a two-layered euphotic zone. Where the mixed layer water is sufficiently clear (few algal cells and little suspended particulates) that light attenuation is dominated by seawater absorption, the 1% isolume often lies within the pycnocline.
In this case, the upper (mixed layer) zone is well lit and nitrate poor, while the deeper (pycnocline) zone is poorly lit and nutrient rich. Here we have a sufficient model for the steady-state DCM in which algae are larger, shade-adapted, and receive a constant vertical flux of fresh nitrate so that new production is high relative to production based on regenerated ammonium. On the contrary, the well-lit and nitrate-poor upper mixed layer has low rates of new production relative to ammonium-based, regenerated production.
Production in the DCM relative to the production in the mixed layer is complex to compute but usually lies within the range 5–50%. Though the shade-adapted cells of the DCM may have access to adequate nitrate, their Pmax (photosynthetic rate per unit of chlorophyll at light saturation) may be 10 times lower than for near-surface cells. The plots of seasonal cycles for individual provinces (see Chapters 9–12) show that whereas an illuminated pycnocline is the normal condition in low latitudes, it is ephemeral in polar seas. These graphs thus illustrate the potential for increase in the absolute rate of primary production in the deep chlorophyll maximum during seasons when the pycnocline lies shoaler than the photic depth.
In short, all this describes an ecosystem vertically ordered about a discontinuity in the density, nutrient, and biotic gradients, as shall be discussed later. Nitrate diminishes to very low values upward across the pycnocline as both chlorophyll biomass and the
46 Chapter 3: Fronts and Pycnoclines: Ecological Discontinuities
rate of primary production increase. Zooplankton herbivores are often concentrated as dense layers at the depths of maximum rate of algal growth, though the total vertical distribution of zooplankton is complicated by the vertical migrations of some species, usually with diel or seasonal frequency, across the pycnocline. In this way, the migrant species utilize to their advantage the contrasting ecological conditions of both euphotic and bathypelagic zones.
Ocean basin-scale baroclinicity defines the topography of pycnocline troughs, ridges, bowls, and domes that are associated with the geostrophic flow while, everywhere, the pycnocline itself and the other gradients associated with it remain in the same depth sequence. Generally, the deeper the pycnocline, the greater the depth interval over which the features of the oligotrophic profile are spread. In Chapters 9–12, I shall discuss many regional variations on this general theme, forced by regional topography and climate, but this introduction is sufficient to demonstrate the ubiquity of this most important ecological boundary in the ocean, excepting only the sea surface, the sea floor, and the shoreline.
All this tells us that this boundary layer is also an ecotone, comparable to the examples discussed previously at vertical frontal zones, and has all the characteristics of one; that is, not only are ecological conditions different above and below it, but there are very special ecological conditions within it. If a characteristic flora could be recognized at the pycnocline, as distinct from occasional shade adaptation of phytoplankton also occurring in the upper euphotic zone, we could regard it as the shade flora of the ocean and comparable with that of the forest floor. In fact, for each major group of photosynthetic cells, there is indeed evidence that an oceanic shade flora must be recognized (Longhurst and Harrison, 1989). In discussing this evidence, let us begin with the smallest cells.
Cyanobacteria and prochlorophytes are most abundant in the mixed layer across the entire North Atlantic from the Gulf Stream to Morocco, whereas peak abundance of small eukaryotes, mostly chlorophytes, prymnesiophytes, and chrysophytes, occurs at the DCM (Li and Wood, 1988; Li, 1995). In the Northwest Pacific, very small eukaryotes, especiallyMicromonas1–3m, also dominate the DCM. Thus, there is some evidence for the existence of a specialized shade flora within the smallest photosynthetic cells.
But for the larger cells, especially dinoflagellates and diatoms, we have much better evidence. Early in the season in temperate or subtropical regimes, during periods of relatively high turbulence and diapycnal mixing, the taxa at the DCM resemble those in the mixed layer above (Venricket al., 1973; Taniguchi and Kawamura, 1972). However, at 26N in the Pacific in summer, after pycnocline and DCM have developed, two distinct diatom assemblages meet at the top of the nutricline: the shallower assemblage is nutrient limited while the deeper is light limited (Venrick, 1988). Each assemblage has the characteristics of a mature, predation-controlled assemblage, and community diversity increases to a maximum near the DCM.
From the more extensive general literature on phytogeography, conclusions may be drawn that seem to support the few available floristic profiles. For example, the widespread existence has been noted of a diverse “shade flora” of four Bacillariophycae, ten Dino- phycae, one Prasinophycae, and three Prymnesiophycae. Some of these shade species are very large organisms, such as the diatom Plantoniella sol (Furuya and Marumo, 1983) and the widespread prasinophyte Halosphaera viridis. In the Kuroshio region another 11 species, in addition toP. sol, are shade species (including the diatomsAsteromphalus sarcophagus, Oolithotus fragilis, Thorosphaera flabellata, and Thalassionema spp.). Thus, the hypothesis that the ordered ecosystem of the euphotic zone is partitioned among two different assemblages of algal cells, one of which constitutes a shade flora, appears to be supported.
The protistan consumers of the many size classes of phytoplankton are themselves, not unexpectedly, likewise distributed in a vertically ordered manner in the upper part of
The Ubiquitous “Horizontal Front’’ at the Shallow Pycnocline 47
the water column. Unpublished pumped profiles that I obtained at oligotrophic stations in the North Atlantic (31and 34N) in 1987 showed that protist taxa (both genera and species) occurred preferentially across restricted depths within the mixed layer and the thermocline, in the same way as the larger zooplankton. Unlike these, however, protist cells have negligible powers of locomotion. The depth-differential distribution of protists is best illustrated by the tintinnids, a group whose taxa are relatively simple to identify:
at Station PURPLE, south of the Convergence, 62 species of 31 genera were tallied at 10-m intervals from 0 to 110 m. The vertical distribution of these showed that the sim- ple hypothesis—that within the mixed layer, all species should be distributed randomly by wind-induced turbulence—must be rejected. Some genera (Salpingia,Tintinnoposis) occurred preferentially within the subsurface chlorophyll maximum, others (Xystonella, Eutintinnus) within the mixed layer. Of three species ofDictyocystis, one occurred pref- erentially above, and two within, the subsurface chlorophyll maximum. The mechanism by which these taxon-specific layers of single-celled consumers are maintained is yet to be explained, but one presumes that depth-differential growth rates must be invoked in some way.
For the larger zooplankton, the pycnocline represents a special depth zone that not only has unique characteristics but also lies close to the separation between the two principal life zones of the ocean: lighted and dark (Longhurst and Harrison, 1989). The increase of zooplankton biomass (five or six orders of magnitude) over the vertical distance (5 or 6 km) from the ocean floor to the sea surface would represent an unprecedented degree of variability if translated into horizontal change within the mixed layer; it would also have an unprecedented predictability (Longhurst, 1985a). In the interior of the ocean, the vertical rate of change of biomass is very small, and the gradient is greatest over a few tens of meters of the pycnocline, where the sparse bathypelagic plankton is separated across a planktocline from the much more abundant epiplankton above. The epiplankton and the deeper acoustic scattering layers of diel migrants are the most prominent features in full-depth profiles of pelagic biota and, like the DCM and the pycnocline, they may be traced across ocean basins.
At some depth within the epiplankton, and most often also within the pycnocline, a maximum of zooplankton abundance (Zmax) usually occurs. Where the water column has stabilized, Zmaxlies somewhat shallower than the DCM, especially at night, and closer to the depth of the productivity maximum (Pt). Where there is a very shallow mixed layer, as in upwelling regions or at the start of a spring bloom, Zmaxoccurs very close to both PM and DCM, which are coincident in these circumstances. The depth difference between Zmaxand DCM is positively correlated with the absolute depth of DCM. Such observations lead one to enquire whether all taxa aggregate at the Zmax, or is it only certain species that are specialized for life in the ecotone we are considering? It is convenient to discuss this question by reference to special investigations of vertical distribution of zooplankton species made at the BIOSTAT station in the eastern tropical Pacific (Longhurst, 1985b), where there was a shoal pycnocline together with all the features of a TTP. Groups of species could be identified that feed similarly and that occur within common depth horizons, though some rearrangement of the vertical pattern occurs at dawn and dusk, because several of these groups are diel migrants (Fig. 3.5).
The following characteristic groups of copepods were identified by species-specific depths of maximum abundance (“preferred depths”) and depth ranges (“layer depths”) of the central 50% of the populations of the 72 most abundant species:
Small herbivores(<20 mm): All species (genera:Calanus, Clausocalanus, Calocalanus, Undinula, Nanocalanus, Acrocalanus, Paracalanus, Ischnocalanus, Acartia, Eucalanus, Oncaea, and Corycaeus) had preferred day depths on the upper shoulder of the DCM and thus close to the Pt. Some of these shifted a few meters upward into the lower mixed layer at night. Oithona lay deeper than the other genera—closer to
48 Chapter 3: Fronts and Pycnoclines: Ecological Discontinuities
D1
SCM
NO3
D2
200 m
1% light Pt
COPEPOD HERBIVORES
Clausocalanus Calocalanus Mecynocera Paracalanus Temoropia Acartia Acrocalanus Pseudocalanus Oithona Nanocalanus Undinula, etc.
COPEPOD OMNIVORES
Spinocalanus Scolecithrix Aetideus Gaetanus Metridia Gaidius
COPEPOD PREDATORS
Copilia Euchaeta Sapphirina Corycaeus Pontella Euaetidius Mormonilla
CHORDATE HERBIVORES
Oikopleura Salpa
Doliolum COPEPOD
OMNIVORES
Scolecithricella Scolecihrix Aetidius Gaetanus Metridia Rhincalanus Gaidius
COPEPOD PREDATORS
Haloptilus Heterorhabdus Augaptilus Arietellus Pontella Euchaeta
OSTRACOD OMNIVORES Conchoecia wind stress at sea surface
MIXED LAYER MIXED LAYER
Fig. 3.5 Some ecological relationships in a “typical tropical situation” in the eastern tropical Pacific at BIOSTAT in day and night LHPR profiles of groups of plankton that have similar feeding strategies. Shown are the depth of maximum numbers, the depth range of 50% of the population, and the depth range that excludes only extreme outliers. The top and bottom of the thermocline is D1 and D2, respectively; the DCM lies within the depth zone of relatively high chlorophyll while P is the depth of maximum production,NO3 is the depth at which nitrate become undetectable, and 1% is the depth of this level of illumination.
the DCM. However, this is the most narrowly specialized group in its selection of preferred depths.
Larger herbivores (2.5–6.5 mm) and most omnivores (1.5–4.6 mm): These occupied a wider layer depth than the small herbivores. Most exploit the lower shoulder of the DCM down to and even below the bottom of the thermocline, with some extend- ing as deep as 250 m. These are species ofEucalanus, Rhincalanus, Scaphocalanus, Lucicutia, Gaetanus, andNeocalanus.
Interzonal migrant omnivores: MostlyPleuromamma, these occurred at night both in the mixed layer and at the DCM; diel migrantEuphausia, small fish, and siphonophores were similarly distributed. By day, all diel migrants occurred well below the pycn- ocline.
Predatory species: Preferred depths included both the mixed layer and depths down to 250 m, although there was much individual specialization. For example, the two smallest (2.5–3.3 mm) species of Euchaeta preferentially occurred with the small herbivorous copepods on the upper shoulder of the DCM.
Obviously, this analysis supports the hypothesis of a specialized fauna of the ecotone associated with the pycnocline and emphasizes how herbivorous copepod taxa cluster in the upper part of the feature. Other taxa were similarly ordered down these profiles.
Ostracods, which are all detrital feeders, occurred preferentially just below the pycnocline.
Noncrustacean herbivores (e.g., Oikopleura) tended to aggregate like herbivorous cope- pods on the upper shoulder of the DCM, though doliolids exploited a wider depth range;
predatory chaetognaths, like predatory copepods, had overlapping depth ranges cover- ing the whole mixed layer. This vertical distribution of feeding groups is recognizable