Thermal Stratification Is Well Developed Soon After Ice-Thaw and Persists Until Fall Overturn

ISOTOPE TRACER METHODS FOR INVESTIGATIONS OP NITROGEN DEFICIENCY CASTLN I E LAKE, CALIFORNIA

R.P. AXLE C.Rd Ran . GOLDMAN Divisio Environmentaf no l Studies, Institut Ecologyf eo , University of California, Davis, California 95616, USA

Castle Lake is a subalpine lake located in northern California. Thermal stratification is well developed soon after ice-thaw and persists until fall overturn. The epilimnion during the major portion of the growing season (mid-June to mid-September) is characterized by relatively constant temperature and depth (19Ï3 C, 5i2m respectively), high transparency (M = 0.21 m~ ), low phytoplankton productivity (»"3 mg G m hr~ at midday), levelanw dlo dissolve f so d inorganiphosphorud an ) ~ c1 sN nitroge g » 5 n(< T^ * (<1 |ig P 1~ )• Nitrogen-fixing algae are not present in the water column and nutrient inputs derived from precipitatio surfacd nan e infloe war generally negligible by early July.

Experimental procedures have been developed to determine the nutritional conditio naturaf no l population predico t algaf so d ean t their growth response to increased levels of nutrients. Previous studies con- ducte Castlt da e Lake suggested that phytoplankton growth coul limitee db d by low levels of dissolved inorganic nitrogen during the summer growing season H .C0~ , NH/i» «oT iwer~ axi/N0 e ^ use determino dt ratee eth s of inorganic carbo nitroged nan n naturauptake th y eb l phytoplankton communit theid an y r respons nitrogeo et n enrichment watel Al .r samples were collected fromid-epilimnioe mth centrae th f no l lake basin.

The bioassay techniques, using isotope-labelled substrates, were: 1. Long-term CO« uptake in the light: Water samples were removo t filtere ) a e 0 Zooplankton(8 d , spiked wit hH CO" thed ,an n —1 distribute flasksdl m int 0 o50 . Nutrienr o ~ t1 additionN g u 0 (5 s wer) ~ el" theo M ng madji triplicatn 5 ei flaske th d s ean resuspende n di th sampline th lak t ea g depth. Subsamples were removed daily, filtered onto 0.4 5membranp e filtersaccumulatee th d ,an d radioactivity counted.

. Short-ter2 ? lightuptake C0 th mthesn n :eI i e experiments, water samples were preincubated with nutrients, in situ, for 16-24 hours

18 in large glass bottles. Each treatmen s thetwa n subsampled int5 o3— smaller bottle) (12ml 5 s which were inoculate incubated an d~ C0 witd hH i_n situ for 4 hours at midday. Subsequent sample processing was identical tlonge o th tha —f o tter m experiments.

. Short-ter3 darkuptake * th CO :n me i Thi s procedur reportes ewa d indicaton a te ob nitrogef ro n deficienc algan i y Morri. (l97ly eb al ^ s)e and Yentsch et al. (1977)« Enhanced uptake of inorganic carbon in the dark aresponssa ammoniuo et m enrichmen assumes wa ) reflecto " dt (80-40 1~ N tg 0ji increasn a carboxylation ei n reactions associated wit increasen ha d ratf eo ammonium assimilation. Ammonium and H CO, were added simultaneously and the water samples allowed to incubate for 6—8 hours at the sampling temperature. Additional treatments with the specific nitrification inhibitor N-serve were used to control for contributions of nitrifying bacteria to dark-CO- uptake (see Gersberg et al., 1980 ). t£ «•*• OTOTM

4. Ammonium uptake in the dark: Fitzgerald (19^9) proposed a bio- assa J3r —yfo deficienc y base enhancen do d maximum NH,— daruptake y th kb n ei nitrogen deficient algae relative to algae grown with an adequate supply of nitrogen. The experiment was conducted by enriching water samples with NH^s (a ) relativ" l N controlo et g u 0 5 s (N-deficient) which contained <5 jig (Nut'-Ur) 1~ , and then preincubating both sets in situ for 24 hours. The wate thes rwa n apportioned into darkene litrd1 g efi bottle0 7 d an s 15 1 14 15 1 ( NHt-N controlse adde <)jig(th I" 0 e 2 o MHt dt th d o ,-Mj-Nan t )l" enriched samples. The incubations were terminated after 6 hours by filtering the particulate material onto precombusted GF/C filters levele Th .* f so ammonium addition were chosen so that N- uptake rates would be maximal (substrate saturated thao s differenceo tn d )an ammoniun si m concentration would occur between treatments during the -^N incubations. The methods useanalyzinr fo dfilterN ^ fulle e gsar th y describe . Axlen al di t re (1980) and Axler (1979).

5. Ammonium and nitrate uptake in the light: These experiments can be considered to be bioassays for determining limitations of nitrogen substratw lo o uptakt e e edu concentratio n (Dugdale, 1967). Serial addition eithef so ~ wer1 r N rang^NHe ^ ^NO madth 2-5f g r eo tn o ~ ji 0i e

litr2 r o e t o1 wate r sample1 s which were then incubate sitn r di ufo 24 hours. We also used differences in the maximum rate of -'NO" uptake functioa s a molybdenuf no ammoniud an m m enrichments, relativ— un o t e amended controls, to examine the role these nutrients play in the assimila- 19 tion of nitrate by phytoplankton. The effect of added nitrate on the short- term rate of nitrate uptake was determined using the radiotracer H. The 10 minute half—life precluded in—lake studies, but allowed rate determina- tions during time minutew intervalfe a s f s(Gersbero g ert a^L., 1978; Axler, 1979).

The results (see Axle Goldmand ran pressn ,i ) indicated thae tth phytoplankton were nitrogen deficient throughou growine tth g seasone Th . N and N studies demonstrated that ammonium uptake was rate-limited by the external concentration of ammonium and that nitrate uptake was regulated availabilite byth nitratf yo molybdenumd ean potentialld ,an e th y yb assimilatio ammoniuf no m (Bioassay—5). Nitrogen deficienc alss ywa o indicated by increased uptake of NH. and H CO" in the dark in response to ammonium additions (Bioassays-3 and -4). Although N-enrichment enhanced C- photosynthesis in short-term experiments during the early portion of the growing season, the photosynthetic response was initially negative during midseason positiva d ,an e responsoccut no rd untiedi day3 d 2- lsha elapsed. This suggests tha laca t stimulationf ko ever ,o n inhibitionf ,o inorganic carbon uptake upon nitrogen enrichment does not necessarily pre- clude W-deficienc therefored yan , interpretatio thesf no e data (Bioassays-1 mus) —2 t d takan e into consideratio duratioe nexperimentth e th f no . Healey (1973) reviewe literaturde mucth f ho e regarding algal nutrient deficiencies and suggested that a short term suppression of inorganic carbon fixation migh expectee tb d upon readditio limitina f no g nutrient. Competi- reductanr o P AT tio r t nfo betwee inorganie nth c carbo nitroged nan n assimilation processes would temporarily favour N—assimilation unti nitrogee lth n defici relieveds i t , after which enhanced growth would occur.

concludes Ii t d tha mann i t y aquatic ecosystems chemical analyses of the nutrient content of the water and the seston can offer considerable insight toward the identification of algal nutrient deficiency (Healey, 1978). However unproductivn ,i e systems suc Castls ha e Lake interpretatioe ,th f no these data is difficult because ambient concentrations of dissolved nutrients 3 are near detection limits (e.g. NO~-N, M^-Nr PO~ -Pbecausd )an e algal bio- mass and nutrient content are not easily differentiated from the detrital component of the suspended particulate matter. In these systems, it is the rat supplf eo biologicallf o y y available nutrients which ultimately controls ecosystem production (Dugdale, 1967 therefored )an necessars i t ,i y to estimate both rates of regeneration and rates of uptake for the nutrient in question in order to evaluate its importance to algal growth. 20 ) In Castle Lake, we have used the isotopes C, • 'St and N in order to evaluat relative phytoplanktoo eth t . eNH d importancan ~ n N0 growt f eo h andetermino dt factore eth s which regulate their availabilit biologicar yfo l assimilation. There still appears to be no failsafe assay for identifying algal growth limitation by nitrogen. Dependence on uniparameter bioassays is certainly much more likel yielo yt d erroneous interpretatione th f so results tha multiparametena r approach (Arneson, 1979) particularn I » ,a neutral or even negative response of CO. uptake to E-enrichment in short- term experiments doe necessarilt sno y preclud nitrogeea n deficiencye Th . results of short-term and long-term experiments are likely to answer different questions, and are subject to different types of valid criticism.

ACKtTOWIEDGEMEHTS

We thank G.G. Anderso J.Rd nan , Postel (Univ Washingtonf .o r )fo granting us access to their mass spectrometer. E. deAmezaga assisted with analysesthC e Carlton. R . PriscPeart. . ,D J d u,an fieldhelpee th n ,di Maly. anG Smit. dM d jh an helpe d prepar manuscripte eth Gersber. R . g provided field support and advice and criticism throughout the study. Financial support was provided by NSF Grants BM74-02246 A01 and DEB?6- 1952 C.Ro 4t . Goldman.

REFERENCES

Arneson . ,1979P « Effect nutrienf so t enrichmen naturae th n o tl phytoplankton of Lake Tahoe, California-Nevada, M.S. thesis. Univ Calif.f .o , Davis. p 1 ,12

Axler, R.P. 1979» Inorganic nitrogen uptake by phytoplankton in Castle Lake, California. Ph.D. thesis. Univ. of Calif., Davis, 150 p. Axler, R.P«, Gersberg, R.M. and Goldman, C.R, 1980. Stimulation of nitrate uptake and phtosynthesis by molybdenum in Castle Lake, California. Fish. CanAq .. .J Sei 707-712, .37 . Axler, R.P Goldmand .an , C.R Pressn .I . Isotope tracer methodr fo s investigation of nitrogen deficiency in Castle Lake, California. Water Research.

21 Dugdale , 196?G . ,R . Nutrient limitatio seae th :n ni Dynamics , identification significanced ,an . Limnol. Oceanogr 685-695, .12 . Fitzgerald, G.P. 1969« Fiel laboratord dan y evaluation biof so - assay nitroger sfo phosphorud nan s with alga aquatid ean c weeds. Limnol. Oceanogr. 14, 206-212. Gersberg, R.M. , Axler, R.P. , Krohn, K. , Peek, N. , and Goldman, C.R. 1978. Nitrate uptake by phytoplankton: measurements utilizing the radioisotope 13N. Verh. Internat. Verein. Limnol 388-392, .20 . Gersberg, R.M,, Axler, R.P. , and Goldman, C.R, 1980. Isotope studie nitrogef so n transformation Castln si e Lake, California. Panel Proc. Series, IAEA, Vienna, STI/PUB/548, 75-85. Healey, F.P, 1973. Inorganic nutrient uptake and deficiency in algae. CRC Critical Reviews in Microbiol., 3i 69-113. Healey, F.P, 1978. Physiological indicator nutrienf so t deficiency in algae. Mitt. Internat. Verein. Limnol., 21, 34-41.

Morris, I«, Yentsch, C.M, and Yentsch, C.S, 1971. '^e physiological state with respect to nitrogen of phytoplankton from low— nutrient subtropical water as measured by the effect of ammonium ion on dark carbon dioxide fixation. Limnol. Oceanogr. 16, 859-868.

Yentsch, C.M. , Yentsch, O.S. and Strube, L.R. 1977« Variations in ammonium enhancement, an indication of nitrogen deficiency in New England coastal phytoplankton populations. J. Mar. Res., 35» 537-555.