-K. von Brockel (IfMK) & M. Meyerhofer (IfMK) Objectives
In general , tropica l oceani c region s fa r awa y fro m an y continenta l coas t sho w a lo w primar y productivity. Thi s i s du e to a rathe r stabl e mixe d laye r reachin g belo w the euphoti c zon e with lo w nutrient concentrations. Seamounts which reach into the mixe d layer as well as islands within these oceanic region s ar e o n th e othe r han d surrounde d b y a surprisingl y ric h sealife . Especially loca l fisheries take advantage of this phenomenon. So, traditional local fisheries on tunas and other fishes exist in the vicinity of the Rocks of Sao Pedro and Sao Paulo.
The maintenanc e o f hig h standin g stock s o f pelagi c organism s nea r island s an d seamount s suggests tha t thes e area s ar e locations fo r hig h rate s o f energ y transfe r (Boehler t & Geni n 1987 ; Rogers 1994) . Th e energ y whic h drive s th e enhance d biologica l productivit y ma y eithe r b e generated fro m loca l small-scal e hydrographi c processe s o r b e advecte d fro m elsewher e an d concentrated at the islands an d seamounts (Boehler t & Genin 1987) . I t has bee n suggested that i n oligotrophic waters the uplifting of isotherms into the euphotic zone induced by seamount structures results in the formation of Taylor columns (Boehlert & Mundy 1993) . This phenomenon can increas e local nutrient concentration and thus enhance primary production (Genin & Boehlert 1985) . Evidence for increased primary production leading to aggregations of zooplankton and nekton at seamounts is yet not well documented.
One of the objectives during the leg JOOPS II/4 a was the description of the hydrographical situation around th e Rock s o f Sa o Pedr o an d Sa o Paul o an d th e stud y o f thei r influenc e o n distributio n o f nutrients as well as composition and production of the phytoplankton community (se e also the repor t of B. Ueberschar).
Material and Methods Sampling
On the le g JOPS II/ 4 a, betwee n January 28 . and Februar y 4 . 1995 , 27 stations wer e sample d o n eight transects approaching the Rocks. Distances between stations and Rocks ranged from about 55 nautical miles to about hal f a nautical mile. Water samples were taken at each station from five light depths (100 %, 30 %, 1 0 %, 1 % and 0,1 %) calculated from Secchi disc readings with water bottle s (each 101) attached to the ctd-probe. From 27 stations sampled, on 8 'early morning' stations oxygen uptake was measured (Fig. 1).
The exact locations of stations sampled, as well as the sampling procedure are given in detail within the cruise-report of B. Ueberschar .
Nutrients
Samples fo r nutrient s (nitrate , nitrite , phosphat e an d silicate ) wer e take n fro m th e ligh t depths , immediately deep frozen and are to be analysed in Brazil. The data for the characterization of the 'is-land-mass effect' are not yet available.
Biological parameter
Samples for phytoplankton pigment s (HPLC) , chlorophyll, particulate organic carbon (POC) , particu-late organi c nitroge n (PON) , micro-pytoplankto n compositio n an d nanoplankto n compositio n wer e taken at all stations from the light depths mentioned.
On the 'early morning ' stations dissolved oxygen was determined in water samples from light depths on fiv e station s clos e t o th e Rock s an d fro m thre e station s relativel y fa r away , t o detec t possibl e influences o f the Rocks . From the sam e depths wate r wa s incubate d (simulate d i n situ) for oxyge n production. Fo r eac h dept h 2 ligh t an d 1 blac k bottl e (25 0 ml , PE ) wer e incubate d o n dec k fro m about sunris e t o noon . Oxyge n concentration s wer e measure d wit h a n automati c oxyge n stan d (SIS). Oxygen production was determined as the difference between the oxygen concentration of the
70
Fig. 1: Rocks of Sao Pedro and Sao Paulo with the underwater plateau, stations sampled, "early morning"
stations (encircled) and direction of surface and subsurface currents (North Equatorial Counter Current) dark and the light bottles at the end of the incubation period and calculated as oxygen production in
^M/day.
The metho d of oxyge n uptak e for productivit y determination s instea d of th e 14C-metho d ha d to b e chosen ou t o f technica l reasons . N o handlin g o f radioactiv e materia l wa s allowe d o n boar d R V
"Victor Hensen" . Becaus e th e oxygen-metho d i s no t tha t sensitive , result s obtaine d have t o b e regarded cautiously, especially within this oligotrophic regio n where the uppermos t wate r layers ar e normally oxygen-saturated (about 10 0 %) or even oversaturated (>100 %).
Results
General surroundings
The Rock s o f Sa o Pedr o an d Sa o Paul o ar e situate d a t abou t 55 ° N an d 29 ° 19 ' W . Th e rock s themselves rise only about 15 m above sea surface. The plateau, they are situated on, rises sharply from severa l thousan d meter s (betwee n 200 0 an d 400 0 m ) t o les s tha n 20 0 m water depth . The several rock s cove r a n are a o f abou t 15 0 m x 30-4 0 m . Th e underwate r platea u stretche s fro m southeast to northwest with an extension of about 5 x 30 nautical miles (Fig. 1).
The Rock s ar e situate d withi n th e Nort h Equatoria l Counte r Curren t wit h a n easterl y directio n an d rather hig h curren t speed s o f u p t o 10 0 cm/se c withi n it s cor e i n about 8 0 m depth. Own curren t measurements wer e take n on Februar y 3 . on four station s (stat . 10 , 11, 19, 20) aroun d the Rock s with a current mete r attached to the hydrographical wire in six depths (5, 20, 40, 60, 80 and 100 m).
They showe d a norther n surfac e current (toward s 350° with a speed of 25 cm/sec), most probabl y induced b y th e prevailin g easterl y winds . Belo w th e surfac e curren t th e Nort h Equatoria l Counte r Current was found extending from 20 to 10 0 m depth. Its direction was about 105 ° (between 90 and 130°) with an average speed of 27 cm/sec (with a maximum of 38 cm/sec in 100 m).
Water column (Temperature, salinity and Secchi disc)
So far, only ra w and uncalibrated data from the CTD ar e available. I t is assumed that they giv e the general picture, although absolute values might be different.
Average sea surface temperatures an d salinities with values of 27.5°C (min . 26.7 and max. 28.1°C) and 35.6 ppt (min . 35.4 and max. 35.8 ppt) respectivel y were rathe r uniform on all stations sampled.
That is , no upwellin g feature s coul d b e observe d i n the se a surfac e aroun d an d further awa y fro m the Rocks.
Temperature and salinity profiles down to 15 0 m for the eight 'early morning' stations are presente d in Fig . 2. These profile s ar e a typical selectio n fro m al l stations . They sho w a ver y unifor m salinit y
station 004 ttupcnlwc A ulmil .
TfCJ«JS|w-l
station 005
•cnpciilaK * uluil y T [ T | « J S | p p . l
station 010 ropinlue A uliuity T |*C | «J S (pat]
station 011 Itmpcitfutt it niuiii )
T[*C!«dS|fflj
•
temperature ' |
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-U - c | » J s | p i > i i M
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IN m
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temperature
/ salinity J
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station 024 r |-c] Ud s ippii
station 030 kmpiitlijn Jc lalinky
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temperature /
j salinit y
Fig. 2: Temperature and salinity profiles for the 'early morning' stations 72
throughout the water colum n with a very sligh t increase with depth. Temperature profile s sho w the mixed layer reaching down to between 60 and 90 m. The thermocline was either very pronounciate d with a high gradient (stat. 04,10 and 30) or stretched over a greater width with a small gradient (stat.
05, 11 , 19b, 20 and 24). Close t o the Rocks al l stations ha d a thermocline wit h a small gradien t stretching ove r abou t 4 0 to 60 m, whereas furthe r awa y fro m th e Rocks al l types o f temperatur e profiles could b e observed. Unfortunatel y nutrien t value s are not yet available. They are needed to see whether these different thermoclines are connected with a recent mixing of water due to currents influenced by the topography around the Rocks and the underwater plateau. During such processes nutrients coul d b e brough t int o th e lowe r par t o f th e euphotic zon e an d thus enhanc e primar y production. On the other hand, these observed differences between the thermoclines could be due to different curren t system s encountered . A s such the y coul d b e olde r features , showin g n o recen t mixing.
Secchi disc readings , ranging from 21 to 32 m, were typical for oligotrophy oceans. No differences could be observed close to the Rocks compared to further away, or between luffsid e and leeside of the Rocks in regard to the current system.
Biological Parameter (POC, PON, Chi a. and Pigments)
The distributio n o f particulat e organi c carbo n (POC) , particulat e organi c nitroge n (PON ) and clorophyll a. is presented i n Fig. 3 for the eight ^arly morningA stations. They are typical example s for all stations visited. In general POC and PON values are somewhat higher in surface and subsur-face waters. Her e too, no relation could b e found betwee n distanc e o f station fro m the Rocks and distribution a s well a s total value s o f POC and PON. That is , no indication for an influence o f the Rocks o n the amount an d distribution o f the particulate organi c materia l suspende d i n the water could be found.
.Toctnfll . roninm , O J . [ „ I : *lnM * KSKIMAI t o i . t n t t -KlCtftl .KXMlf' l ...CM,!,* ! ^.MCIMII „ K N | » « | .I-.I.[,I- I
Fig. 3: Distribution of particulate organic carbon (POC), particulate organic nitrogen (PON) and chlorophyll s on the *eariy momingA stations
Concentrations o f chlorophyll a normally increased rapidl y with depth, showing a very pronounciate d chlorophyll maximum at the end of the euphotic zone (in about 1 0 % light depth). Low surface values and maxi-mal values were around 0.15 and 0.6 g ug Chla/I. But there are also stations with a deeper chlorophyll maximum (in about 0.1 % light depth) or with a rather uniform distribution of chlorophyll a within the water column. No relation could be found between distance of station from the Rocks and distribution or concentrations of chlorophyll g. That is, no indication for an influence of the Rocks on amount and distribution of chlorophyll a. could be found.
Determination of chlorophyll a concentrations within different size fractions (>20 jim, 20-2 urn, 2-0.8
|im and <0.8 urn) for one station close to the Rocks (stat 11) and one station about 30 nautical miles on the luffside of the Rocks (stat 30) revealed some differences (Fig. 4).
On both stations chlorophyll a concentrations are mainly due to phytoplankton i n the size class smal-ler than 2 u,m. Far away from the Rocks this fraction represent s more than 90 % of the chlorophyll a in both depths. Whereas close to the Rocks the importance of the size fraction between 20 and 2 fm is remarkable with about 27 % and 1 8 % in 5 and 60 m depth respectively . This difference can also be observed in the relationship of different phytoplankton pigments to chlorophyll a (Fig. 5).
The markerpigments zeaxanthin and chlorophyll b indicate the presence of cyanobacteria (Synecho-coccus type) an d prochlorophytes, bot h typical for oligotrophic regions with regenerate d production.
These tw o pigment s ar e dominant a t bot h stations an d i n all depths. On the other han d peridinin, 19-butanoyl-oxy-fucoxanthin an d 19-hexanoyl-oxy-fucoxanthi n gai n importanc e a t th e statio n nea r the rocks . These pigment s demonstrat e a highe r contributio n o f dinoflagellates , chrysophyte s an d prymnesiophytes, indicating a greate r availabilit y o f ne w nutrients . This corroborate s th e result s of the size fractionation, showing a higher contribution of bigge r phytoplankton at the near rock station.
27% 30%
64%
Station 11 , 5 m 52%
Station 11 , 60 m
87%
21%
Station 30, 5 m
73%
Station 30, 55m
>20/jm Q 2 to 20pm §| ] 0, 8 to 2pm Q 0to0,8p n
Fig. 4: Distribution of chlorophyll a in different size fractions for two depths (5 and 60 m) of station 11 (close to the Rocks) and for two depths (Sand 55 m) of station 30 (distance to the Rocks about 30 nautical miles)
74