book reading note 1

Chapter 5.4-5.5 of book 《Dynamics of Marine Ecosystems: Biological-Physical Interactions in the Oceans》

5.4Comparasion with the Humboldt Current System

A complex set of flow between the coast and 100km offshore(Alheit and Bernal 1993)

1. Furthest: sluggish wide flow towards the equator: Chile-Peru Oceanic Current
2. Towards the coast: Peruvian Oceanic Counter-Current
3. Closer: equatorward-flowing Humboldt Current, the prime driving force for coastal upwelling

The area of upwelling migrates north and south with the seasons, but overall extends from about 38° S in central Chile, through northern Chile and the whole of Peru to parts of Ecuador just north of the equator.

Early work on the upwelling system off Peru, recent details of Chilean upwelling system have been added.

• Off central Peru, upwelling is year-round but reaches a maximum in winter.
• Off northern Chile, upwelling peaks during spring
• Off central Chile, peaks during late spring and summer

JOINT II multidisciplinary cruises studied the Peruvian upwelling system at 15°S in 1976-7.

Three important differences between the upwelling site off Peru and that off northwest Africa.

1. The shelf off Peru is narrower(20km A 50) and drops off more steeply(200m versus 110m).

2. Deep water off Peru has more nutrient(20-25 vs 5-10) .

3. Wind stress is less and more constant(0.79+-0.4 vs 1.55+-1)

   Net result(实际效果):

   • When Ekman transport is active, offshore transport occurs mainly in the top 20m.
   • Shoreward transport is in an intermediate layer over the shelf, at the depth of about 30-80m(in Africa is whole)
   • Off Peru, the water close to the bottom is relatively still, and there is marked accumulation of organic matter to give chemically reducing sediment.

Two further differences:

• since the wind stress is less, the wind-induced mixing does not penetrate so deeply. During periods of strong upwelling, phytoplankton is still retained within the euphotic zone→t primary production is maintained at a relatively constant level, whether the wind stress is high or low。
• poleward counter- current, flowing beneath the equatorward coastal jet, is situated at intermediate depth over the continental shelf off Peru, whereas off northwest Africa it is located on the shelf slope

A detailed analysis of the primary production cycle in a segment at 15° S(MacIsaac et al.1985):

Because the upwelling is relatively constant it is possible to trace a distinct plume of cold water moving out from the coast and to recognize a number of zones along the axis of the plume.

• Zone I: the area of intense upwelling within about 7 km of the coast, where nutrients are abundant but phytoplankton biomass is relatively low.
• Zone II: I the water column is stabilized by solar warming and the phytoplankton cells are found to increase their rates of nutrient uptake, photosynthesis, and synthesis of macro- molecules, a process known as “shift-up.”
• Zone III: characterized by the rapid depletion of nutrients by the “shifted-up” phytoplankton, so that there is a rapid accumulation of biomass and all processes occur at maximal rates.
• Zone IV nutrient depletion occurs, so that the cells experience nutrient limitation.

Using drogues to track water masses, MacIsaac et al. (1985) estimated that phytoplankton cells moved from zone I to zone IV in 8–10 days, during which time they traveled 30–60 km away from the coast. The chlorophyll maximum occurred about 18 km offshore.

5.4.1 Interannual variability in the Peruvian upwelling system

Normal State: the source of upwelled water off Peru in April–May 1977 was at a depth of 30–60 m. The water was at a temperature of 15.5–16.5 °C, and contained 20–25 µg nitrate.

Anomalous State: the trade winds weaken or reverse, thermocline off the coast of Peru sinks to a depth of about 100 m; Ekman transport along this coast continues, but the water that is upwelled is now much warmer and not rich in nutrients.

As a result, there is a sharp reduction in the biomass and productivity of the phytoplankton.

Reason: El Niño – Southern Oscillation (ENSO). t the Peruvian upwelling system seems to be uniquely vulnerable to such drastic changes, so that its interannual variability in productivity is very great.

1982-3: serious in the periodicity of 100 years or more (Rasmusson and Wallace 1983)

the height of the 1982–3 anomaly, in May 1983, the upwelling waters were at 29 °C instead of the usual 16–18 °C, and mean primary productivity was only 10mgCm−3 d−1(Barber et al.1985) Two months later conditions had returned to normal and mean primary productivity was 219 mg C m−3 d−l

5.4.2 Total primary production in the Peruvian upwelling system

Many attempts have been made to calculate the total primary productivity of the Peruvian upwelling system.

1. Lack of agreement about upwelling area leads out many different result. Cushing (1969) used 479,000 km2 in his calculation while Ryther (1969) used 60,000 km2
2. Chavez and Barber (1987) argued that the offshore dimension of coastal upwelling is limited by the Rossby deformation scale (Section 5.2.3). From Eqn. 5.11 we see that the formula for this scale is (g′ H0 )1/2/f.Thus the radius is a function of the Coriolis force f (which varies with latitude), the depth H0, and the vertical density gradient.
3. Chavez and Barber (1987) calculated that the width of the Peruvian upwelling system varied from 270 km at 4° S to 60 km at 18° S. Calculating the width at degree intervals they arrived at an area of 182,000 km2, intermediate between the two values quoted above.
4. The mean of the large number of determinations of primary production in 1983–4, after the recovery from the El Niño event, was 2.28 g C m−2 d−1 or 834 g C m−2 y−1 . This value converted into a total production of 1.52 × 1014 gCy−1 .
5. With increased availability of satellite pictures of chloro- phyll distribution, it should be possible to refine the estimate of the average area affected by upwelling, and to determine its variability. For example, Carr (2002) defined the area of an upwelling system as the area over which surface chlorophyll concentrations, as estimated by remote sensing, exceeded 1 mg m−3 on average (see Section 5.9).

5.4.3 Secondary production in the Humboldt Current system

Cushing(1971) the type of fish in HCS is the same in CCS.

The sardines and anchovies usually spawn in the areas of most intense upwelling, close to shore, and Cushing speculated that both the juveniles and the adults make use of the two counter-currents (onshore and poleward) to maintain themselves within the upwelling system.

lack of agreement about whether the anchovies and sardines predominantly consume phytoplankton. It seems that larvae consume mainly microzooplankton, while adults feed mainly on larger phytoplankton.

Barber et al. (1985) showed (Fig. 5.12) that the years of temperature anomalies (El Niño years) were associated with reduced landings of anchoveta Engraulis ringens.

Possible explanations:

1. the adults could have starved for lack of phytoplankton food,
2. the fish could have migrated away from the areas where they are usually caught
3. or the larvae could have failed to survive through lack of both phytoplankton and zooplankton.

Some Evidence for each hypothesis.

h1: . There were reports of anchoveta having moved to cooler, deeper water at about 100 m, but such water has very little phytoplankton and it seems likely that the fish would not survive there long. Barber et al. (1985)

h2: anchoveta seek out the upwelling areas by exhibiting a preference for water of 16–18 °C. Barber et al. (1985).migration south to find cooler waters may well have been one of the strategies for survival (Valdivia 1978).

h3:

• The anchovies spread their spawning over 7–8 months of the year, presumably as an adaptation to the occurrence of unfavorable conditions at particular times and places.
• However, there are two peak periods: the austral winter–spring spawning (July–September) and the summer spawning (February–March) (Valdivia 1978).
• Walsh et al. (1980), investigating the survival of larvae off the northern coast of Peru during winter, found that survival was higher when dinoflagellates in relatively high concentrations were available to the first-feeding larvae. Strong wind events had the effect of dispersing the dinoflagellate concentrations and adversely affected survival. We shall return to this topic in Section 5.5 on the California Current.(把这个加上)
• At the onset of the 1976 El Niño there was a bloom of the dinoflagellate Gymnodinium splendens along 1000 km of the coast from March until the end of May. It was attributable to the stabilization of the water column as the warm water invaded the area. While it provided an excellent feeding environment for the early larvae, it apparently was not a suitable food for the adult fish, which had smaller fat content, a reduced weight at a given length, and reduced length at sexual maturity. The 1977 recruitment of fish spawned in 1976 was extremely poor and the stock along the Peruvian coast fell to the lowest levels ever observed (Barber et al. 1985)

5.4.4 Exploitation of the Humboldt Current fish stock

History of the anchoveta fishery in Peru was reviewed by Glantz (1985)

1. Mining of bird-drooping

• beginning in the 1840s, the major industrial activity along the Peruvian coast had been the mining of the bird- droppings, guano, from the rocky islands.
• Large populations of fish-eating birds are characteristic of upwelling populations worldwide, and prominent white accumulations of droppings at their roosting sites are an inevitable concomitant. As Cushing (1971) remarked, it is no accident that upwelling areas commonly have a Cabo Blanco, Cap Blanc, or Cape Blanc.
• The guano of Peru was mined and exported for fertilizer to many parts of the world.

2. Anchoveta fishing

• Beginning in the 1950s, a lucrative industry to harvest the anchoveta and convert them to fishmeal was developed in Peru.
• The landings increased rapidly to a peak of about 12 million tons in 1970, then dropped to 2–3 million tons for a few years.
• After 1977 the catch hovered around 1 million tons but in 1985 there began a recovery which has persisted into the twenty-first century (Fig. 5.13)
• Alheit and Bernal (1993) sug- gested that there may have been massive migrations from the southern part of the Humboldt system.

Whether these fluctuations have been part of a natural cycle of events that has occurred many times in the past, or whether they are primarily the result of gross overfishing?

regime shift→

The authors pointed out that anchovy can recover from an ENSO event in 1–2 years (Fig. 5.13). They made a partial recovery after the 1972–3 El Niño, and a full recovery after the 1997–8 event, but the decadal-scale period of warm anomalies, which began in 1968, held the anchovy populations at low levels from 1977 to 1985. Alheit and Niquen (2004) therefore concluded that the well-known crash of the anchovy fishery in the 1970s was caused primarily by the decadal- scale regime shift rather than by the 1972–3 ENSO events

Overfishing

Climate Change(ENSO, regime shift)

5.5 The California Current System

Driven by prevailing northerly winds, and upwelling occurs along the Pacific coast of the United States from the Canadian border south to Baja California and beyond.

The situation of upwelling system off Oregon in several respects resembles that off Peru(这个是谁做的，Huyer1976做的是非洲).The poleward undercurrent appears over the shelf as well as the slope, and the shoreward flow is strongest at mid-depths over the shelf. Upwelling events are less strong and of shorter duration off Oregon than they are off northwest Africa.

Bakun(1973) gave the Bakun upwelling index, a 20-year aver- age of monthly mean Ekman transport for different parts of the coast. The range is from 300 m3 s−1(offshore direction) to −212 m−3 s−1 (onshore).It can be seen that the index indicates year-round upwelling off southern California, with stronger upwelling in summer, but off Oregon and Washington in the north there is strong downwelling in winter, and upwelling is confined to the period April–September.

in summer there is a negative temperature anomaly, indicative of upwelling of cold water, all the way along the coast from Oregon in the north to Baja California in the south, with the exception of a warm anomaly off San Diego. The upwelling is obviously most intense between Cape Mendocino and Monterey. Although the temperature anomaly in Fig. 5.16 shows an apparently uniform area of cold water, this area is an artifact of the method of calculation(why?).

The situation at any one time, as seen by satellite, is extremely com- plex, with coastal upwelling systems tending to be centered on topographical features such as capes and canyons, and with plumes of upwelled water extending far out into the California Current. As in other coastal upwelling systems, the strength of upwelling is strongly dependent on wind speed and direction, and changes from day to day.

Fig. 5.17 shows the distribution of temperature and nitrate off Point Sur, California, on June 9, 1980, as inferred from satellite imagery supplemented by shipboard observations (Traganza et al. 1983). This situation is typical of an early phase of an upwelling event at this site. If the event persists for many days, interaction with the California Current may give rise to a cyclonic structure about 100 km in diameter, with high biological production along the associated fronts, or it may extend into a plume up to 250 km long (Traganza et al.1981, 1987). In summer time, patterns of this kind may be found in various stages of development or decline all the way from Oregon to Baja California. As winter approaches, the upwelling is progressively restricted to the southern portion of the region and in spring the region of upwelling spreads north again.

heterogeneous nature of the California Current, with its admixture of advected and upwelled water.

It is now possible to simulate both physical and biological events in the coastal transition zone, that zone characterized by the presence of highly productive jets, squirts, or filaments of highly productive upwelled water. Moisan and Hofmann (1996) modeled the fate of Lagrangian drifters placed in newly upwelled water and allowed to travel with a filament. Coupled physical and biological models were used, the biological parameters being determined from shipborne observations. The models reproduced well the formation of a subsurface chlorophyll maximum and the changing structure of the food web as the drifters moved offshore. 物理海洋学的东西我就不懂了

Digiacomo (2000) and Digiacomo and Holt (2001) used the latest satellite technology to study the mesoscale and sub-mesoscale eddies in the Southern California Bight. All the eddies were less than 50 km in diameter, and 70% were less than 10 km. They were observed to lie between the equatorward-flowing California Current and the shore, and appeared to be caused by topography (especially islands), wind, and current instabilities. There was also evidence of lateral entrainment of highly productive coastal waters. Associated with the eddies were patches of high chlorophyll density, up to 15 km wide and 60 km long. The authors discussed the potential for influencing nutrient flux, plankton productivity, larval transport and recruitment, and dispersal of pollutants. This smaller-scale pattern was observed to interact with the large-scale variations in time and space of the California Current.

5.5.1 Fish production in the California Current system

most abundant fishes in the California Current system are sardines, anchovies, hake, jack mackerel, and mackerel.

• Sardines Sardinops sagax were heavily exploited from 1916 to 1967.. The peak landings were in 1936–7 and exceeded 700,000 tons. The catch fell drastically in the 1950s and 1960s and in 1967 the California state legislature imposed a moratorium on the sardine fishery.
• The sardines of the California current system are divisible into four stocks. Of these, the largest by far before overfishing was the one that spawned in the Southern California Bight and migrated to the upwelling areas off northern California to exploit the dense zooplankton stocks that are associated with the coastal upwelling.
• biomass of sardines declined, and some postulated that the two were in competition so that the decline of sardine stocks released resources for the anchovies. However, Soutar and Isaacs (1969) studied the 1850-year record of fish scales in the anaerobic sediments off California and concluded that northern anchovy scales were present in large numbers throughout the series, while sardine scales appeared intermittently for periods of 20–150 years, with absences that averaged 80 years in duration. They concluded that the two species were not in competition.
• The anchovies also have several subpopulations. The stock off Oregon spawns at about 44–46° N, mainly in July at the time of the northern upwelling. The central subpopulation spawns principally in the Southern California Bight. Eggs and larvae can be found throughout the year, but the peak abundance is in the spring, while the minimum is in the autumn. The fish remain in the Southern California Bight throughout their lives and in recent years this has been the largest stock. There is a southern stock off Baja California, for which peak larval abundance is from January to March.

It thus appears that the largest stocks of both sardine and anchovy spawn in the Southern California Bight. Upwelling is relatively weak and phytoplankton production is lower than in the California Current proper. Bakun and Parrish (1982) have suggested that strong offshore flow associated with Ekman transport is likely to carry eggs and larvae too far offshore, to positions from which they may never return, and that the choice of the Southern California Bight for spawning area reflects a need to avoid areas of strong upwelling. They also suggested that areas of strong Ekman transport are areas where there are strong winds that may destroy the fine-scale strata of food organisms needed by first-feeding larvae. This idea, attributable to Lasker (1975), will be examined in more detail in Section 5.5.2.

• Schwartzlose et al. (1999) showed that the exploitation of sardines in the California Current reached its peak with landings of 700,000 tons in 1936, but was down to extremely low levels by 1952.
• From 1916 to 1952 the catches of anchovy were negligible (Fig. 5.18).
• From 1952 to 1966 there were small catches of both sardines and anchovies, and in 1967 the sardine fishery was closed.
• After 1967 there was an expansion of anchovy populations, resulting in a catch of 310,000 tons in 1981, but in 1990 there was a switch to dominance of sardines once again, with a catch of 110,000 tons in 1997.
• The various hypotheses to explain the alternation of species have been discussed, but in the light of the findings of Alheit and Niquen (2004) for the Humboldt Current system, we may expect to find some influence of decadal-scale climate changes.

The various hypotheses to explain the alternation of species have been discussed, but in the light of the findings of Alheit and Niquen (2004) for the Humboldt Current system, we may expect to find some influence of decadal-scale climate changes. These will be discussed in Chapter 9

5.5.2 The survival of first-feeding larvae

Anchovy egg are most abundant in the shorthorn California Bight during February, March, and April. 3 day old first-feeding larvae need a very high density of food organisms about 1790 dinoflagellates per liter.

the larvae were stimulated to feed only when the phytoplankton was at a population density of at least 20–30 cells mL−1

The first-feeding larvae rely on high concentrations of phytoplankton such as naked dinoflagellates of a size class close to 40 µm.

To summarize our understanding of factors influencing anchovy and sardine production in the California Current, we see that strong wind stress and associated upwelling, which are the most characteristic features of eastern boundary currents, appear in themselves to be detrimental to the success of the larvae. For good survival the larvae require a well-developed horizontal layer of high phytoplankton density, in which dinoflagellates are the dominant form.

A second factor to be considered in relation to larval survival is the risk that strong offshore transport will carry the larvae away from the favorable coastal environment.

In our present stage of understanding, there is no way to integrate these two mortality factors, beyond pointing out that both sardine and anchovy appear to avoid the regions of strongest upwelling in their choice of spawning location.

The role of environmental controls in determining sardine and anchovy population cycles in the California Current: Analysis of an end-to-end model 这文章可以看看