Nitrogen uptake rates in phytoplankton and ice algae in the Barents Sea SVEIN KRISTIANSEN and TOVE FARBROT Kristiansen. S. & Farbrot. T. 1991: Nitrogen uptake rates in phytoplankton and ice algae in the Barents Sea. Pp. 187-192 in Sakshaug, E.. Hopkins, C.C.E. & Britsland, N. A . (eds.): Proceedings of the Pro Mare Symposium on Polar Marine Ecology, Trondheim, 12-16 May 1990. Polar Research l O ( 1 ) . Uptake rates of NH:. NO: and dissolved organic nitrogen (urea) were measured in phytoplankton and in ice algae in the Barents Sea using a "N-technique. NO7 was the most important nitrogen source for the ice algae (f-ratio = 0.92). The in situ irradiances in the subsurface chlorophyll maximum and in the ice algal communities were low. The in situ NOS uptake rate in the ice algal communities was light-limitcd. The in situ N O j and NH: uptake rates in the subsurface chlorophyll maximum were at times light-limited. I t is hypothesised that NHf may accumulate in low light in the bottom of the euphoric zone and inhibit the in situ NOS uptake rate. -..I I,... Svein Kristiansen and Tove Farbrot, Department of Biology, Marine Botany Section, University of Oslo. P . O . Box 1069. Blindern. N-0316 Oslo 3. Norway (revised February 1991). Introduction Nutrient concentrations in the Barents Sea vary considerably through t h e year. During winter nutrient concentrations a r e high d u e t o vertical mixing of the water column. During spring and summer the water column becomes stratified, phytoplankton grows, and an oligotrophic surface layer develops south of the marginal ice zone. A t the same time a pronounced subsurface chloro- phyll maximum is formed at the nutricline (Kri- stiansen & Lund 1989; Loeng 1989; Rey & Loeng 1985). Several ice algal communities have been identified in the Barents Sea ( H o m e r et al. 1988). The ice algal biomass, especially in the sub-ice assemblage, may be very high, and has been suggested to be a n important part of the eco- system in the Barents Sea (Syvertsen 1991 this volume). Irradiance is low in both the subsurface chlorophyll maximum (Rey & Loeng 1985) and in the ice algal communities (E. E. Syvertsen pers. commun.), and algal cells in some of these communities have been shown t o be shade- adapted (Sakshaug 1989). Kristiansen & Lund (1989) found that NHf concentrations >0.8 vmol I - ' significantly reduced NO; uptake in the low light regime in the subsurface chlorophyll maxi- mum. The uptake rates of NH;, NO, and urea in these low light and often nutrient-rich environ- ments, the subsurface chlorophyll maximum, and the sea ice algal communities will be discussed. Materials and methods The samples from the subsurface chlorophyll maximum were collected during four cruises in the central parts of the Barents Sea in May-June and in August 1984 (Kristiansen & Lund 1989), in July-August 1985, and in May-June 1987. T h e ice algae were sampled during four cruises into the ice by divers using gentle suction from a submersible pump (L0nne 1988) or by scraping algae off the ice (Fig. 1). It has been observed that the algal cells usually float directly beneath the ice, making quantitative sampling difficult. LSpi tsbe rgen 0 20" 26" 32' Fig. I . Stations i n the Barents Sea where ice algal samples were collected. The curves represent the approximate southern limit of the sea ice during each of the four cruises. 1 ( A ) . K / V NORDKAPP in February-March 1987: 2 (a), KIV SENJA and K/V 4 (0). R/V LANCE in May-June 1986. ANDENES in April 1986; 3 (0). K/V ANDENES in May-June 1988: 188 S. Krisriansen & T . Farhrot The samples were diluted to contain less than S(b75 pg Chl I - ' before incubation. The values presented here have been corrected for the dilu- tion procedure. T h e nitrogen uptake rates were measured using the stable isotope "N. T h e methods used are given in Kristiansen & Lund (1989). The uptake rates are calculated in situ absolute uptake rates in mol I - ' hK' unless other- wise stated. Saturating amounts. 4.0 pmol I - ' N H f or urea-N or 8.0 pmol NO;. of the isotopes (95-99 atom%' ' i N ) were used in the experiments. The half-saturation coefficients used in the cal- culations of the in situ rates were l . 3 ymol I - ' for N H f , 1.8 pmol I - ' for N O , and 0.2 umol I - ' for urea-N. These half-saturation coefficients are typical for NOT-rich Barents Sea water con- taining > 1 pmol I - ' ( p e r s . observation): in NO; -poor Barents Sea Water ( < 1 pmol I - I). the half-saturation coefficients for the three nutrients were 0 . 1 4 . 2 p m o l l - I . T h e growth rate. 11. in doubl. h - I , was calculated as: p = p * ( P N * l n 2 ) - ' where I-, is summed in situ absolute uptake rate of NO; + N H ; +urea in pmol I - ' h - ' and PN is particulate nitrogen in umol I - ' . Scalar irradiance beneath the ice was measured using a 4-rr collector (Biospherical Instruments Inc.. model QSL-140). and the irradiance in the incu- bator was adjusted accordingly with stainless steel nets. At one station, in March 1987. quantitative ice algal samples were incubated in in situ incu- bation chambers. T h e chambers were acrylic cyl- inders closed in o n e e n d . 11 cm in diameter and 950 ml capacity. Tightly f i t acrylic lids were used to cap off the chambers after sampling. The iso- tope was added through a septum mounted in each lid. The chambers were filled. capped off. and isotopes were added. all under the ice. and the chambers were left to incubate under the ice. Results and discussion The N O T concentration was typically about 1 3 p m o l l - ' in the whole water column during winter and below the pycnocline during summer (Table I ) . I n the oligotrophic surface layer the NO, concentration was usually not detectable. The concentrations of N H ; and urea were usually 4 . 3 pnol I - ' . At stationswith a pronouncedsub- surface chlorophyll maximum. however. a sub- surface N H ; maximum was often found associated with the chlorophyll peak (Table 2 ) . During the productive season. high NH: con- centrations were found also in the ice/water interface (Table 2 ) . In winter the NH: con- centration was < O . I pmol I - ' . The sampled ice algal biomass was very high and variable (Table 3). though not unexpectedly considering the sampling methods used. The mean ratio between particulate carbon and nitro- gen ( C : N) in the ice algae and in the water column in the marginal ice zone ( M I Z , as defined in Vincent 1988) were both close t o 7 which is typical for multispecies communities of phytoplankton (Redfield et al. 1963). T h e mean ratio between chlorophyll a and particulate organic C (Chl/C) was 2.6 times and 13.8 times higher in the ice algal communities than in the water column in M I Z and in the oligotrophic surface layer south of M I Z respectively (Tables 3 and 6 ) . This indi- cates that the ice algal communities were shade- adapted (Sakshaug 1989). T h e C and N values have not been corrected for non-phytoplankton carbon. and the Chl/C ratios are minimum esti- mates. A mixture of one-year and multi-year ice was found at the two northernmost stations (Fig. 1 ) . and one-year ice dominated at the other sta- tions ( pers. observations; Syvertsen 1986, 1987, 1988; Vinje 1987). T h e sub-ice assemblage was the dominating ice algal community both in one- year and in multi-year ice. Dominating species were Nitzschia frigida and Thalassiosira spp. in one-year ice and Melosira arcdca in multi-year ice (Syvertsen 1986. 1987. 1988: E . N. Hegseth pers. commun.). The mean uptake rates in the ice algal com- munities and i n the water column in M I Z are compared in Table 4. T h e summed absolute uptake rate ( N H ; + N O , + urea) in the ice algal communities was high and variable, mainly because the sampled biomass was variable. The Tublr I . T!pical c o n c e n t r a t i o n s of NO; I N H ; . u r e a - N . a n d particulate o r g a n i c n i t r o g e n ( P N ) i n pmol I- ' i n t h e B a r e n t s Sea d u r i n g w i n t c r a n d d u r i n g w m m e r in thc o l i g o t r o p h i c s u r f a c c la!er a n d i n d e e p w a t e r . Summer O l i g o t r o p h i c Deep S u i r i e n t W i n t e r s u r f a c c l a y s r w a t e r Nitrogen uptake rates in phytoplankton and ice algae in the Barents Sea 189 Table 2 . Concentrations of N H ? in the subsurface chlorophyll maximum. In thc ofigotrophic surface layer, in the icc/scawatcr interface and in the water column 0.3 m under the ice during the productive season. Range Means I SE Samplc (pmol I - ' ) ( p o l I - ' ) n Subsurface chloroph. max. 0.11-2.28 0.67 + 0.08 40 Oligotrophic surface layer 0.09-0.39 0.25 2 0.02 23 Ice/seawater interface 0.4G-2.51 1.17 f 0 . 4 2 i 0.3 m under the ice 0.05-0.58 0.33 + 0. I I 5 Table 3. Concentrations o f chlorophyll a ( p g l - ' ) and of particulale organic nitrogen (PN in }tmoll-'). atomic ratlo between particulate organic carbon and nitrogen (C/N) and ratio between chlorophyll a and particulate organic carbon (Chl/C in g/g) in the ice algal samples and in the water column in the marginal ice zone. Mean * standard error and number of samples ( n ) arc given. Sample Chl PN Chl/C C/N ( x 1000) n ~ ~~~ Ice algae 1260 f 558 227 5 65 8.6 ? 0.5 41.4 2 5 . 2 "18 (9) hWater col. 2.0 2 0.6 1.11 ? 0.17 10.0 * 0.6 16.1 + 3.2 20 ~ - 'Chlorophyll a from 9 samples only h(Personal observation) Table 4 . Summed NH; + NO5 + urea-N absolute uptake ratc in nmol 1-I h - l . I-ratio (NO: uptake ratc divided by summed uptake rate) and NH; and urea uptake rate as % of summed uptake ratc in the ice algal samples. in one quantitative ice algal sample incubated in situ. in the water column in the marginal ice zone (MIZ). and at Stations 905. 947 and 961 south of MIZ. Mean 2 standard error and number of samples (n) are given. n.a. = no values available. Sample Summed uptake f % N H I % Urea n Ice algae 500 2 191 0.92 * 0.02 6 2 2 2 + I 15 Ice algae, in situ n.a. 0.Y6 0 3 1 "Water column, MIZ 10 f 3 0.92 * 0.01 5 + 1 3 2 1 15 905, 26 m 4 0.00 80 20 I 947, 30 m 10 0.74 24 2 1 961, 30 m 15 0.00 100 0 1 a( Pcrsonal observation). corresponding mean N growth rate, assuming 12 hours daily uptake, was 0.04 _t 0.01 doubl. d - ' (mean and SE, n -= 15). T h e N growth rates are minimum estimates because nitrogen was taken up at a reduced rate in the dark (Fig. 2 and Table 5), and because they have not been corrected for non-phytoplankton nitrogen. Reported growth rates for ice algae are 3-8 times higher (see Pal- misano & Sullivan 1985). T h e ratio between NO; uptake rate and summed uptake rate (f- ratio) is often used as an index of new production (Eppley & Peterson 1979; Harrison e t al. 1987). The f-ratio was very high, and NH; and urea were negligible as nitrogen sources in the ice algal communities. High NH; concentrations were found in the ice/seawater interface at some of the stations (Table 2) but not, however, in the uptake samples. T h e sampling procedure and dilution with filtered sea water may have diluted away any elevated NH; concentrations in the uptake samples. The relative importance of the three nitrogen sources may therefore be biased, and possibly the true mean f-ratio was lower than 0.92. This seems unlikely, however, because a similar f-ratio was found in o n e in situ incubation (Table 4). A similar f-ratio was also found in the water column within the ice (Table 4). T h e same sampling and dilution procedures were used in similar experiments in Antarctica during the EPOS cruise, leg 1, in 1988. T h e infiltration 190 S. Kristiunsen & T . Furbrot A h H k 0 100 200 Irradiance W o l m2 s - l ) 0 10 20 30 40 50 Irradiance ( p o i m-2 s.1) Fig. 2 . Maximum absolute uptake rates of NH; (0). NO; (m) and urea-N ( A ) as functions of irradiancc. A . Samples from the subsurface chlorophyll maximum at Station 961. 30m. 4 June 1987: and B . Samples from the sub-ice assemblage at Station H K . 26 May 19% T h e arrows indicate the irradiancc at the sampling depths at n o o n sen. unpubl. values). Olson (1980) found a lower mean f-ratio (0.71 ~ n = 2) in ice algal communities in Antarctica but also claims that his f-ratios were minimum estimates. T h e f-ratio was 0 in the sub- surface maximum a t Stations 905 and 961 (Table 4). A t the biomass-rich Station 947, however, the f-ratio was 0.74 at 30 rn depth (Table 4). Similar variation in the f-ratio has been found in the subsurface chlorophyll maximum earlier (Kri- stiansen & Lund 1989). Typical maximum absolute nitrogen uptake rate versus irradiance curves from the subsurface chlorophyll maximum and the sub-ice assemblage are given in Fig. 2. T h e maximum absolute nitro- gen uptake rates in t h e subsurface chlorophyll maximum were light-saturated at the same irradiance as carbon uptake (91 pmol m-'s-l a t Station 905; Rey, pers. commun.). No definite light-saturation level was found for NO; and urea uptake in the sub-ice assemblage. It is reasonable, however. based o n the values in Fig. 2 , t o assume that the maximum absolute NO, and urea uptake rates in the sub-ice assemblage were light-satu- rated at about 100 pmol m-'s-I. T h e maximum NH; uptake rate was light-saturated a t 21 vmol m - ? s- I (Fig. 2b). These light-saturation levels are within the range of published values for C uptake in ice algal communities (Burkholder & Mandelli 1965; Palmisano & Sullivan 1985; Pal- misano et al. 1985; Horner 1985). Maximum nitrogen uptake rates at in situ irradiance are calculated as percent of light-saturated rates in Table 5 . A linear relationship was assumed between maximum nitrogen uptake rates and irradiances <21-41 pmol m-? s-I in making the calculation. In situ irradiance at noon was low both in the subsurface chlorophyll maximum and in the sub-ice assemblage (Table 5). Even though the Chl/C ratios indicated shade-adapted cells in both communities (Table 6), the maximum nitrogen uptake rates at all stations in Table 5 were light-limited. assemblage (Horner et al. 1988) was the dom- inating ice algal community encountered during the EPOS cruise, and t h e mean f-ratio was 0.96 (SD = 0.05. n = 5 ; S . Kristiansen & E. E. Syvert- Tuble.5. lrradiance at samplc dcpth at noon ( I . in btmol m - ' 5 - ' j and as percentage of surfacc irradiance ( % I \ ) , and in situ maximum uptake rate of NO;. NH; and urca a5 pcrccntagc of light-saturated maximum rates in thc water column south of the marginal ICC' zone and in thc ruh-ice assemblagc. n . a . = no values availahlc. Stat I on Depth I c I . r f NO; (t NH; 5f Urea 61 28 79 68 44 Nitrogen uptake rates in phytoplankton and ice algae in the Barents Sea 191 Table 6 . Concentration of chlorophyll a (pg I - ' ) , ratio between chlorophyll a and particulate organic carbon (Chl/C in g/g), and concentrations of NOT. N H t and urea-N in p o l I - ' in the water column south of the marginal ice zone and in the sub-ice assemblage. The ice algal samples were not collected quantitatively. n . a . = n o values available. Chl/C Station Depth Chl ( X 1000) NO, NH: Urea-N "905 905 "96 1 96 1 947 947 1 1 HK 10 26 10 30 10 30 ice algae ice algae 0.56 7.04 0.19 6.72 5.09 3.49 3844 8120 4.1 47.1 1.9 13.8 17.7 12.7 58.7 n.a. 0.0 0.0 0.0 0.0 0.4 2.6 7.3 4.8 0.09 0.53 0.17 0.21 0.17 0.59 0.10 0.03 0.20 0.17 0.03 0.03 0.12 0.04 n.a. 0.03 *Oligotrophic surface layer Except for NH: a t Station 905 ( 2 6 m ) , the nutrient concentrations at Stations 905 and 961 were low, close t o the detection limit (Table 6), and it is not possible t o decide whether the in situ absolute uptake rates of NH: and urea in the subsurface chlorophyll maximum were light-lim- ited or not. T h e f-ratio was 0 a t both stations (Table 4), and the in situ absolute NO, uptake was clearly substrate-limited a t both stations. T h e NH: concentration was significantly higher in the subsurface chlorophyll maximum at Station 905 (26 m) than in the oligotrophic surface layer above (Station 905, 10 m ; Table 6). Probably the in situ NH: uptake rate was light-limited in the sub- surface chlorophyll maximum at Station 905. Sta- tion 947 was sampled at 30 m depth, in NOT rich water below 1% light depth (Table 6). However, the phytoplankton biomass was high in the upper 50 m, and n o pronounced subsurface chlorophyll maximum was found a t this station. The maxi- mum absolute uptake versus irradiance curves A A N0;conccntration <0.5 pmol I-t NO j concenlration >2.0 pmol I-! NO; concentration 0.5.2.0 pmol I-' 1 2 3 Ammonium concentration (lniol 1.') Fig. 3. Maximum specific NO: uptake rate as a function of NH: concentration in the subsurface chlorophyll maximum. were similar t o the curves in Fig. 2 A except that the NO, curve was higher than the curves for both NH: and urea. NO, was the major nitrogen source (f-ratio = 0.74, Table 4), and the in situ uptake rates of both N O S a n d N H $ probably were light-limited a t this depth. High concentrations of NH: and of urea have been found in the subsurface chlorophyll maxi- mum (Table 2 ; Kristiansen & Lund 1989). These high NH: and urea concentrations were obviously produced by heterotrophic processes in the water column. Typical values for irradiance in the subsurface chlorophyll maximum at noon were <25 pmol m-2 SKI, and the irradiance varied with global radiation ( F . Rey pers. commun.). It is suggested that light-limitation of the in situ uptake rates of NH: (and urea) was o n e major reason for this accumulation of nutrients. The data indicate that this actually was happening at Stations 905 and 947 (Tables 5 and 6). NH; concentrations >0.&1.0 pmol I - ' reduced the maximum NO, uptake rate drastically (Fig. 3; Kristiansen & Lund 1989). A t some of the NOT-rich stations the maximum NO, uptake rate was close t o zero, and consequently the in situ absolute NO, uptake rate also was low. Indirectly then, light limitation of the in situ absolute NHZ uptake rate may affect the in situ absolute NO, uptake rate. The in situ irradiance during sampling of the ice algae was well below the optimum irradiance for nitrogen uptake (Fig. 2A, Table 5 ) . Irradiances a t the surface were not measured but both samples were collected o n dark snowy days. The concentration of NO, in the sub-ice assem- blage was high (Table 6), and the in situ NOT uptake rate was probably light-limited. The 192 S . Kristiansan & T . Farbrot NH: and urea concentrations were low. and it is not possible t o decide if t h e in situ rates of these two nitrogen sources were light-limited or not. The absolute uptake rates of NH: and urea were very low in the sub-ice assemblage, however, and both were negligible as nitrogen sources for the ice algae (Table 4 ) . Typical under-ice irradiance in the Barents Sea was between 2 and 3 0 p o l m - ? - I s , the highest values measured on sunny days (E. N . Hegseth pers. commun.). T h e NO, concentration in the ice algal communities at the stations in Fig. 1 was 2.6-10.8 Fmol I - ' . and the in situ uptake rate of N O T , the main nitrogen source at these stations, was probably light- limited. Acknowledgements. - We acknowledge the generous assistance of the other cruise participants and the captains and crews of K / V S E N J A . Wc thank E. N. Hegseth and E . E . Syvertsen for providing ice algal samples: F. Rey for chlorophyll. NO; and some of the particulate organic carbon and nitrogen data from the water column; E. N. Hegseth for chlorophyll data and some of the particulate organic carbon and nitrogen data from the ice algal samples. and B. G . Mitchell for light data. The incubations on board R/V LANCE were done by E. E . Syvertsen. We also thank P. A . Wheeler for reviewing an earlier draft of the manuscript. 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