Short-term variability during an anchor station study in the southern Benguela upwelling system: Phytoplankton dynamics

Pitcher, GC; Walker, David; Mitchell-Innes, B. A.; Moloney, Coleen L.

doi:10.1016/0079-6611(91)90020-m

Cited by 100 publications

(55 citation statements)

References 27 publications

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“…Variation in female SGR across the shelf during different phases of upwelling mirrored changes in chl a, which increased from the onset of upwelling to reach a maximum under quiescent conditions and then declined during downwelling (Mitchell-Innes et al 1999). This is a consequence of phytoplankton bloom development and subsequent successional changes in the assemblage as upwelled water matures (Brown & Field 1986, Pitcher et al 1991. At the onset of upwelling, SGR is slow inshore because phytoplankton is virtually absent from newly-upwelled water (Brown & Field 1986, Pitcher 1990.…”

Section: Event-scale Variationmentioning

confidence: 99%

Seasonal and event-scale variation in growth of Calanus agulhensis (Copepoda) in the Benguela upwelling system and implications for spawning of sardine Sardinops sagax

Richardson¹,

Verheye²,

Mitchell-Innes³

et al. 2003

Mar. Ecol. Prog. Ser.

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We investigated the growth rate and egg production of Calanus agulhensis, the dominant copepod on the western Agulhas Bank region of the southern Benguela upwelling system (South Africa), and assessed the implications for the spawning of sardine Sardinops sagax. Daily weight specific growth rate (SGR) of copepod developmental stages (N6 to female) was measured monthly between September and March 1993/94 and 1994/95. Seasonally, SGRs of small stages (N6 to C2) remained relatively constant. By contrast, SGRs of large stages (C3 to female) mirrored changes in chl a related to seasonal warming and wind patterns, with a moderate peak in September/October and a larger peak in January to March. Superimposed on this seasonal cycle were fluctuations in response to winds on the event scale. At the onset of upwelling, mean female SGR across the shelf was slow. During sustained upwelling, female SGR increased, with a peak associated with enhanced chl a in the upwelling front and slower rates inshore in newly upwelled water and offshore in oligotrophic water. Female SGR was fastest during prolonged quiescence when high chl a levels extended over most of the shelf. During downwelling, female SGRs decreased again, with fastest rates inshore. Overall, SGRs were not related to temperature, but were related to chl a, with stronger relationships for larger stages. This suggested food limitation of larger stages, which may be a consequence of their preference for larger phytoplankton cells that only occur at high concentrations in restricted locations in time and space in the southern Benguela upwelling system. The spawning of sardine varied seasonally and coincided with the maximum chl a concentration and production of C. agulhensis eggs (daily egg production × female density), both of which are a good source of food for fish larvae. Thus the timing of sardine spawning maximizes the food available to their larvae.

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Section: Event-scale Variationmentioning

confidence: 99%

Seasonal and event-scale variation in growth of Calanus agulhensis (Copepoda) in the Benguela upwelling system and implications for spawning of sardine Sardinops sagax

Richardson¹,

Verheye²,

Mitchell-Innes³

et al. 2003

Mar. Ecol. Prog. Ser.

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“…As a consequence, the physico-chemical environment at the anchor station can be seen as representative of the wider bay environment. This was evidenced by the fact that the phytoplankton communities did not display significant heterogeneity in composition that could be linked to patchiness (Pitcher et al 1991). It should be noted, however, that the surface and deepwater layers typically moved at 90°to each other .…”

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confidence: 99%

“…Full details on the environment at the time of sampling can be found in , Mitchell-Innes & Walker (1991), Pitcher et al (1991) and Verheye (1991), from which the following has been culled. Water that was upwelled at Cape Columbine was advected into the study area some 2 to 3 d later on 3 occasions, namely March 19/20, April 10/11 and April 14/15 .…”

Section: Environmentmentioning

confidence: 99%

“…At the onset of upwelling, assemblages are dominated by diatoms, which are then replaced by dinoflagellates and microflagellates, as the water column stabilises (Margalef 1962, Pitcher et al 1991. Specific changes in assemblage composition are accompanied by an initial peak in biomass and production, followed by gradual declines in both as the thermocline deepens and new nitrogen is stripped from the euphotic zone (Mitchell-Innes & Walker 1991).…”

Section: Introductionmentioning

confidence: 99%

“…The richness of the assemblage was largely accounted for by the meroplanktic species (medusae), but it was the holoplanktic species (in this case, ctenophores) that dominated numerically. This pattern seems to be the norm for coastal and shelf ecosystems, including the southern Benguela (Buecher & Gibbons in press).Upwelling is generally followed by a series of clearly recognisable changes in the physical and chemical environment, which are reflected in (and partly brought about by), successional changes in the composition and processes of the phytoplankton community (Margalef 1962, Mitchell-Innes & Walker 1991. The response by the assemblage of medusae and ctenophores to the changing physical environment that was observed during the upwelling cycle was broadly similar to that of the phytoplankton community, and distinct assemblages were associated with recently upwelled, and stratified water (Fig.…”

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Short-term variability in the assemblage of medusae and ctenophores following upwelling events in the southern Benguela ecosystem

Gibbons

Buecher²

2001

Mar. Ecol. Prog. Ser.

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Changes in the composition of the assemblage of medusae and ctenophores were examined daily over a period of 28 d, encompassing 3 upwelling events in a non-advective environment within the southern Benguela ecosystem. The relationships between assemblage composition and the environment were analysed using canonical correspondence analyses. Although distinct assemblages were associated with upwelling itself, as well as with the periods of water column stabilisation and stratification, indicator species for the assemblages could not be identified. This reflected the near ubiquitous nature of most assemblage members, and their probable response to historical (unmeasured) environmental variables. That notwithstanding, it is clear that short-term changes in characteristics of the surface and deeper water environment are reflected by short-term changes in the composition of the gelatinous zooplankton assemblage. This is more reminiscent of phytoplankton than holozooplankton and probably reflects the meroplanktic nature of most assemblage members.KEY WORDS: Cnidaria · Jellyfish · Zooplankton · Communities · South Africa · Canonical correspondence analysis Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 220: [169][170][171][172][173][174][175][176][177] 2001 the overall zooplankton assemblage across the shelf (Hopson 1983, Gibbons & Hutchings 1996. On a larger scale, it can also be argued that succession is reflected in cross-shelf changes in the trophic structure of assemblages (Steele 1974).The lack of a clear pattern in the zooplankton assemblage in response to short-term variability in the physical environment may be due to our tendency to focus on holoplanktic species. This paper sets out to investigate whether a similar lack of concordance is observed amongst those components of zooplankton assemblages that are substantially meroplanktic. Here, we examine changes in the composition of the assemblage of medusae and ctenophores in St Helena Bay, following an upwelling event. It builds on a suite of papers describing the biophysical environment and processes during a 28 d anchor station study , during which it was observed that there were few links between the patterns of phyto-and holozooplankton (Verheye 1991). MATERIALS AND METHODSField sampling. The fixed sampling site was at 32°3 3.2' S, 18°05.3' E in St Helena Bay (South Africa), where the RS 'Benguela' was anchored for 28 d from 19 March to 15 April 1987. St Helena Bay is a semisheltered embayment downstream of the Cape Columbine upwelling centre. Water circulation in the bay is relatively sluggish and is dominated at the surface by a cyclonic gyre (Shannon 1985), which allows large populations of phytoplankton to develop.Zooplankton samples were collected daily at 12:00 h (diurnal) and 20:00 h (nocturnal) (SAST) from 20 March until 15 April using paired Bongo nets. As large medusae were scarce in St Helena Bay at the time of sampling, we used nets with a mouth diameter of 0.57 m (200 µm mesh size). ...

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Indirect effects shape macroalgal epifaunal communities

Ndhlovu

Lathlean

McQuaid

et al. 2021

Ecology and Evolution

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Short-term variability during an anchor station study in the southern Benguela upwelling system: Phytoplankton dynamics

Cited by 100 publications

References 27 publications

Seasonal and event-scale variation in growth of Calanus agulhensis (Copepoda) in the Benguela upwelling system and implications for spawning of sardine Sardinops sagax

Seasonal and event-scale variation in growth of Calanus agulhensis (Copepoda) in the Benguela upwelling system and implications for spawning of sardine Sardinops sagax

Short-term variability in the assemblage of medusae and ctenophores following upwelling events in the southern Benguela ecosystem

Indirect effects shape macroalgal epifaunal communities

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