Verh. Internat. Verein. Limnol. 27 64-73 StUttgart, April 2000

Effects of natural light on nitrogen dynamics in diverse aquaticenVIronments

Wayne S. Gardner, Peter J. Lavrentyev,HarveyA. Bootsma,Joann F. Cavaletto, FedericoTroncone and James B. Corner

Introduction

Solar radiation is a major force affecting the ecosys-tem dynamics of aquatic and terrestrial ecosystems.As the source of energy for photosynthesis, it allowsplants to assimilate nutrients into energy-richorganic material at the base of food webs and pro-vides energy for biogeochemical processes. Theimportance of the quality and quantity of light hasbeen stUdied extensively in terms of its relationshipto photosynthetic ptocesses (e.g. KIRK 1994). Theeffect of light on nitrogen cycling in aquatic ecosys-tems has been less studied and has not been defined

clearly, except for several stUdies of nutrient uptakeas related to primary production processes (e.g.COCHLANet al. 1991 and references cited therein).

Light provides energy for photosynthesis andincreases inorganic nutrient uptake, but excess lightcan also inhibit phytoplankton groWth and nutrientuptake. These effects depend not only on the inten-sity of the light but also on its spectral characteristicsand the physiology of component organisms.

The effects of light on heterotrophic nutrientregeneration processes are less apparent than arethose for phytoplankton uptake processes becauseheterotrophic organisms do not usually depend onlight for energy. For this reason, metabolic studies ofheterotrophic aquatic organisms are usually con-ducted under dark conditions. However, light couldaffect biological nutrient regeneration mechanismsand rates in a variety of indirect ways. To develop anunderstanding of these effects, we must define themechanisms of food web interactions and photo-chemical stimuli that may affect them. Additionally,ultraviolet light may affect nitrogen regeneration byphotochemically converting dissolved organic nitro-gen (or complexed inorganic forms) to ammoniumor nitrate in surface waters (BuSHAWet al. 1996).

Research on microbial ecology over the past fewdecades has expanded our concepts of energy andnutrient transformations in aquatic ecosystems

(POMEROY1974, AZAM et al. 1983, SHERRet al.1988). The current paradigm is that a large portionof the energy and nutrients fixed by primary produc-ers in aquatic ecosystems passes through small organ-isms that constitUte the "Microbial Food Web(MFW)". Details of this changing paradigm are stillbeing defined for aquatic ecosystems. For example,small phytoplankton and bacteria are grazed by pro-tists or microzooplankton, which are eaten by crusta-cean zooplankton or other metazoan invertebratesbefore the energy becomes available for highertrophic levels such as fish. With more trophic levelsinvolved in energy and nutrient transformations, therelative quantity of fixed organic matter that is meta-bolically released as inorganic nutrients and carbondioxide is increased (e.g. SUZUKIet al. 1996). Ifmaterials pass through the MFW before being incor-porated into larger units of biomass, such as zoop-lankton, fixed nutrients will be recycled to a greaterextent than if zooplankton or macrobenthos con-sume the phytoplankton directly. The communitycomposition and trophic dynamics of autotrophicand heterotrophic organisms (e.g. fuCA et al. 1995,MILLERet al. 1995) and their responses to availablelight (LIPSCHULTZetal. 1985) must therefore bestUdied to understand the effects of light on nutrienttransformations and cycling rates in aquatic ecosys-tems.

Direct and indirect mechanisms for sunlight toaffect nitrogen cycling rates and transformations inaquatic ecosystems could include the following.

Direct effects

Biotic

. Provide energy for photosynthesis, nutrientuptake, and/or N2 fixation. Increase algal metabolism and DON release· UV cell damage and inhibition of nitrogenuptake· Light inhibition of nitrification

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W. S. Gardner et al., Effects of natural light on nitrogen dynamics 65

Abiotic

· Photochemical degradation of dissolved organicnitrogen (DON)· Photochemical degradation of humic materialwith release of inorganic N

Indirect effects

· Increased supply rates of dissolved organic sub-strates through photosynthesis· Photosynthetic production of O2 that affectsnutrient cycling processes· Increased grazing due to increased production ofprimary producers· Phytoplankton or zooplankton migration due tophotoreception (biological pump)· Light-induced metabolic activity or behavioralchanges (e.g. respiration or grazing rate changes)

Questions

Rather than consider all of these potential effects indetail, we will examine patterns of ammoniumuptake and regeneration in a variety of aquatic envi-ronments and make inferences about direct and indi-

rect effects of light on nitrogen cycling dynamics inpelagic ecosystems. Specifically, we will focus on thefollowing questions:

1. How does sunlight affect community uptake andregeneration rates of ammonium?

2. Does sunlight enhance DON release by phyto-plankton?

3. Are effects of light on community regenerationrates related to microbial food web composition?

We discuss data from both freshwater and marine

coastal systems because of the limited amount ofdata that are available from freshwater lakes. This

approach is reasonable because salinity itself does notappear to be a major factor affecting biotic/lightinteractions.

Experimental approaches and site descrip-tions

Net concentration changes over time in light-darkexperimentsAssuming that autotrophic processes occur in thepresence of light, but not in the dark, and that het-erotrophic processes occur both in the light and inthe dark, we can gain insight into nutrient cyclingmechanisms and rates by comparing nutrient con-centration changes over time in light vs. dark bottles.We have conducted such experiments, in combina-tion with organic (amino acids) and inorganic(ammonium) nutrient additions, to gain informa-tion about autotrophic and heterotrophic nutrientcycling processes as related to lower food web organ-

Isms.

Light/dark isotope dilution experiments with/5NH +4

Isotope dilution or enrichment experiments with 15Ntracers (e.g. 15NH4'or 15N-Iabeled amino acids) canprovide information on nitrogen recycling ratesunder conditions of natural light and temperature.Isotope dilution experiments with added 15NH4'assume that most regenerated ammonium is derivedfrom '4N-organic material whereas both isotopes ofammonium will be taken up in approximate propor-tion to their concentrations in the enriched solution(BLACKBURN1979, CAPERONet al. 1979). Conduct-ing such experiments under both light/dark and darkconditions allows a direct assessment of the effects of

light on community nitrogen cycling dynamics.However, the process of performing large numbersof such isotope experiments with dissolved 15NH4' isquite challenging using mass analysis techniques.The need to distill or extract the ammonium fromthe water and convert it to N 2.before measurementof isotope ratios can be made by mass or emissionspectrometry, requiring relatively large samples(e.g. > 100 mL) for sufficient sensitivity and consid-erable time for sample processing. These constraintslimit the number of samples that can be processedon ship or stored for later isolation and analysis.

We have developed an alternative direct-injectionhigh performance liquid chromatographic techniqueto fractionate directly the tWo isotopic forms ofammonium in water (GARDNERet al. 1995). Thiscation exchange fractionation is possible because aslightly larger proportion of the 15NH: than of the14NH4+occurs in the ionic form in the equilibriumreaction betWeen ammonium and ammonia at pHvalues near a pK of about 9.0. The atom % of 15N_NH4+ is determined by quantifying the retention-time shift in the sample ammonium peak relative toan internal ammonium standard under controlled

chromatographic conditions. This method can beapplied to relatively small water samples in environ-ments where nutrient cycling rates are sufficient toallow measurement. It provides a convenient way toexamine potential rates and processes under a varietyof experimental conditions. We have used thisapproach to examine nitrogen cycling processesunder light and dark conditions.

Site descriptionsAs one of the Laurentian Great Lakes, Lake Michi-

gan is a meso trophic lake with many of the charac-teristics expected of a large temperate lake (5CAVIA&FAHNENSTIEL1987). Phytoplankton are not normallylimited by nitrogen in this lake, but examining nitro-

66 Plenary lectUres

gen dynamics can provide insight into food webmechanisms relevant to nutrient and energy trans-formations that may be generalized to other aquaticsystems. Our studies in Lake Michigan involvedmeasuring net ammonium and amino acid concen-tration changes in light/dark subsrrate additionexperiments and conducting ISNH4+isotope dilutionstudies by tracer techniques, using mass spectrome-try for isotope ratio determinations. The latter workwas part of a larger stUdy to examine the cycling ofnitrogen through the various pool of nitrogen in thelake (BooTsMA unpublished data).

The Mississippi River plume represents the regionwhere the Mississippi River outflow enters the Gulfof Mexico both through the Southwest Pass of theMississippi River mouth and, in addition, about30% of the outflow of the Mississippi River entersthe Gulf of Mexico via the Atchafalaya River andBay located west of the Southwest Pass. The Missis-sippi River basin drains more than one-third of theUSA mainland and thus delivers a large amount ofnutrients into the Gulf of Mexico (ATWOODet al.1994). The plume is an example of an importantregion where massive quantities of nutrients from atUrbid (low light) freshwater source are suddenlymixed with a low-nutrient environment with highlight penetration. Primary production in the lowerreach of the Mississippi River is low because of exces-sive tUrbidity, but primary and secondary produc-tion and associated processes peak at mid-salinityregions where abundant nutrients are combined withadequate light for high rates of primary production(LOHRENZet al. 1988). This system provides anexcellent example of a region where both nutrientand ecosystem dynamics are affected by the lightenvironment.

Lake Maracaibo is a tropical, hypereutrophic lakein Venezuela on the northern tip of South America.At about 150 km long and 110 km wide, it is thelargest lake in South America. It has a mean depth of25 m and a maximum depth of 34 m (PARRI-PARDI1983). This estUarine lake is connected to the Gulfof Venezuela via the Straits of Maracaibo and Bay EITablazo and is brackish due to the intrusion of

marine waters from the Straits to the hypolimnion ofthe lake. Primary productivity is very high in the lakewith values as high as 3-8 g C m-2 day-I reported forthe northeastern area of the lake (SUTION 1974).Lake Maracaibo receives large loads of nutrientsfrom tributaries, sewage discharges, agricultUralsources and, probably, the atmosphere. Despite largenutrient inputs and very high concentrations ofammonium in bottom waters (e.g. 70 llM NH4+-NL-1in the center of the lake; unpublished data), LakeMaracaibo phyroplankton are considered to be lim-ited by nitrogen (REDFIELD& EARLSTONDOE 1964).

Laguna Madre is a shallow, coastal lagoon alongthe Texas coast extending from Corpus Christi Bayto the Mexican border. Padre Island separates thislagoon from the Gulf of Mexico. The trophic statUsof the lagoon varies spatially and temporally depend-ing on the amount of rainfall. Historically, most ofLaguna Madre has been characterized as havinghealthy seagrasses, but, for the 7 year period of 1990to 1997, the northern region had a continuousbloom of Aureoumbra lagunensis, "Texas BrownTide", and displaced the sea grasses by making thewater very turbid in some regions (BuSKEYet al.1997). For example, Baffin Bay, which branches offthe Laguna Madre south of Corpus Christi, was sub-jected to a particularly dense bloom of the browntide during this period.

Results and discussion

1. How does light affect community uptake ofammonium?

Isotope dilution experiments in plastic bottlesthat eliminated UV light (i.e. polystyrene bot-tles incubated in a temperature-controlled,deck-top water bath in a blue plexiglass tank)showed enhanced ammonium uptake in thepresence of natural light relative ro dark bottles(e.g. Fig. 1 taken from GARDNERet al. 1997).Similar results from the literature are summa-rized by COCHLANet al. (I 991).

However, ultraviolet (280-400 nm) and pho-tosynthetically available radiation (PAR = 400-700 nm) light can inhibit photosynthetic activ-ity (see KIRK1994) and nutrient uptake rates(COCHlANet al. 1991). We investigated theeffects of a variety of light conditions on poten-tial uptake rates in Lake Maracaibo surfacewaters (GARDNERet al. 1998). Light conditionsfor surface samples included unshielded quartztubes, screened polystyrene bottles (45% PARreduction), screened (53% PAR reduction) ortaped (dark) polypropylene syringes incubatedin a deck-rop incubator (open ice chest withoverflowing lake water to maintain tempera-ture) under full sun. Highest uptake rates wereobserved in the screened polypropylenesyringes, followed by the dark syringes (Fig. 2).Light inhibition of uptake occurred in both thequartz and polystyrene botdes even thoughalmost as much light passed through thepolypropylene syringe as the polystyrene bot-

W. S. Gardner et al., Effects of natural light on nitrogen dynamics

"""'

~

1.'-"

2

1.5

1

0.5

o-0.5

Ate h afalay a

8 10 15 24

~~....

~2.5

21.5

10.5

o

Southwest Pass

7 15Salinity

Fig. 1. Light and dark NH4+ uptake rates (pM h-') atAtchafalaya and Southwest Pass in the MississippiRiver plume.

a 23

des. However, the light spectra varied consider-ably between the two rypes of plastic containers(Fig. 3 taken from GARDNERet al. 1998). Bothplastic containers shielded much of the ultravi-olet wavelengths, but the polysryrene did notblock as much PAR as the polypropylenesyringe. Thus, light inhibition may have beendue, at least in part, to the visible part of thelight spectrum. This conclusion is reasonablebecause the polypropylene container wallblocked some of the visible light that wouldnormally be absorbed by phyroplankton (KIRK1994).

3

Site

Fig. 2. Potential NH4+uptake rates in Lake Mara-caibo surface waters using differenr incubation ves-sels.

-~

~ 9

:E 8:1.-7Q)1U 6a:: 5Q)

.::t:. 4t1J15.3:J 2co:;= 1c:~ 0oa.. -1

Lake Maracaibo, surface waters

2 54

67

0.9~

~~~u

~

:~~

guocU~~u~u

0.1O~. . . - - - -300 350 400 450 500 550 600 650 700

Wavelength (nm)

Fig. 3. Wavelength spectrum for fraction of lightabsorbed by container walls of polypropylenesyringes and polystyrene culture bottles used for15NH4+isotope dilution experiments (taken fromGARDNERet al. 1998).

2. How does sunlight affect ammonium regenera-tion rates?

Abiotic nutrient regeneration. As mentionedabove, a direct mechanism by which light mayenhance ammonium regeneration rates is pho-tochemical mineralization of DON with pro-duction of ammonium or nitrate (BuSHAWet al.1996, MORAN& ZEPP 1997). This mechanism

could be more important in systems containingN-rich DaM, as may be expected in hyper-eutrophic lakes such as Lake Maracaibo, butmay be less important in systems containingDaM with a high C:N ratio. We conductedpreliminary experiments in Lake Maracaibo onfiltered (Rainin 0.2 rm nylon syringe filter)water incubated in quarrz vessels. Measurableproduction and isotope dilution of ammoniumwas observed on sunny days compared to negli-gible production on one cloudy day (GARDNERet al. 1998). If we assume that the productionwas due to photochemical regeneration, theresults indicate that up to 16-30% of the totalammonium regeneration could be contributedby photochemical breakdown of DON in sur-face waters. This value could be conservative

because filtering the water before incubationsmay prevent normal production of DON bybiotic mechanisms. On the other hand, such

measurements likely overestimate the acrualimportance of photochemical regenerationbecause the sample was maintained arrificially

68 Plenary lectures

at the water surface where ultraviolet effects

would be maximized. In subsequent studies inthe Laguna Madre, regeneration rate differencescould not be distinguished betWeen light anddark quartz bottles of filtered samples (unpub-lished data).

Biotic nitrogen regeneration. Intuitively, onewould think that a heterotrophic process, suchas biological ammonium regeneration in natu-ral waters, would not be affected by the pres-ence or absence of natural light. Based on this. . .assumptIon, ammOnIum regeneratIon rates canbe estimated by observing the accumulation ofammonium in dark bottles. We have used this

approach in Lake Michigan to examine ammo-nium regeneration rates and to gain informa-tion about substrate limitation in the lake (e.g.GARDNERet al. 1987, 1989).

Several food web components can interact tocause a nutrient regeneration response to light.For example, WHEELERet al. (1989) observedthat ammonium regeneration in the PacificOcean occurred exclusively at night, a resultthat was attributed to food web characteristics.

Conversely, we have observed a positive effectof light on ammonium regeneration rates, asdetermined by isotope dilution, in a variety ofaquatic environments when results from light/dark incubation vessels are compared to thosefrom dark vessels under otherwise identical

conditions. For example, consider the resultsfrom observations in the Mississippi Riverplume (Fig. 4 taken from GARDNERet al. 1997).In every paired comparison in this study,ammonium regeneration rates were higherunder natural light conditions than in other-wise identical dark bottles. As a specific exam-ple of the concentration and isotope dilutiondifferences, consider the changes in isotopedilution of 14NH4' by looking at the changes inconcentration and isotope ratio (acom %'SNH4') and ammonium concentrations overtime. Both ammonium concentration and

atom % ISN decreased much more in the lightthan in the dark (Fig. 5, taken from GARDNERetal. 1997) indicating that both ammoniumuptake and regeneration were enhanced by thepresence of visible light.

Fig. 4. Light and dark NH4' regeneration rates(pM h-1) at Atchafalaya and Southwest Pass in theMississippi River plume.

These results were obtained with relativelyhigh level additions of ISNH4" but, interest-

ingly, even more striking results were observedin Lake Michigan with tracer-level additions of'SNH4' in relatively cold waters (Fig. 6). Thus,the pattern of light/dark differences in regener-ation rates was similar in the Mississippi Riverplume and Lake Michigan, even though envi-ronmental conditions and analytical approachesused to examine the tWo systems were quite dif-ferent.

Why were ammonium regeneration rateshigher in the light than in the dark for identicalsub-samples incubated identically over the sametime intervals? Hypothesized reasons for theselight-dependent differences in ammoniumregeneration rates are: (1) increased release ratesof labile DON compounds (e.g. amino acids)coupled with rapid microbial remineralization,(2) increased growth and grazing loses of smallphytoplankton, or (3) modified activity orbehavior of the grazing organisms, such as pro-tists, responsible for ammonium regenerationin the water column. These and other processesmay be occurring simultaneously and interac-tively.

3. Is light-driven DON releaseby phytoplanktonimportant topelagic nitrogen cycling?Recent evidence suggests that release of DONcompounds, such as dissolved free amino acids,

1 Atchafalaya113Light0.8i . Dark

0.60.40.2

08 10 15 24

:.c:

:;........

Southwest Passc.O 0.8,0.6

+0.4

:iZ 0.2

00 7 15 23

Salinity

w. S. Gardner et al., Effectsof nacurallight on nitrogen dynamics 69

..::--::~'-'

0.035;

0.03 I

0.cr25 ;0.02 i

0.Q15

0.Q1I0.005 '

o

8 ugtt ,

8Da11<I

cit

+...=Z

Nearshore Ofshore

Fig. 6. Light and dark NH4+ regeneration (/lM h-I)Lake Michigan at nearshore and offshore sites.

by phytoplankton may be a major process fornitrogen cycling in aquatic ecosystems (BRONK& GUBERT 1991, COTNER& GARDNER1993,BRONKet al. 1994, FUHRMAN1990). Althoughthis potentially light-driven process appears tobe important, it is difficult to assess in nature

because of the rapid turnover races of highlylabile compounds such as amino acids. Oneapproach that we have used to examine poten-tial release rates for amino acids is co add suffi-cient levels of amino acids co "saturate"

bacterial uptake sites, and then observe differ-ences in net amino acid uptake rates in light vs.'dark bottles. If concentrations of added amino

acids are reduced more slowly in the light thanin the dark, these differences in removal rates

can be interpreted as representing amino acids(primary amines) that are released by metabo-lizing phytoplankton, assuming that light anddark bacterial uptake rates are the same. A lin-ear relationship betWeen these apparent aminoacid release rates and dark ammonium accumu-

lation rates in similar unfortified water samples(GARDNERet al. 1987) is consistent with theidea that amino acid release by phytoplanktoncould be an important process for ammoniumregeneration by microbial food web organisms.Comparison of amino acid turnover rate pat-terns with ammonium regeneration rates acrossa salinity gradient in the Mississippi Riverplume showed almost identical patterns for thetWo processes (Fig. 7 taken from COTNER &GARDNER1993).

To gain more insight into the interactions ofammonium and dissolved free amino acids

(DFAA) with labile high molecular weight dis-solved organic carbon (HMW DOC; AMON &BENNER1994) under light vs. dark conditions,we conducted experiments involving additionof these materials to water samples containingnatural biota in the Mississippi River plume(GARDNERet al. 1996). The HMW DOC islabile and has relatively high CN ratios (e.g.C:N of about 14-20; personal communication,R. BENNER,University of Texas Marine ScienceInstitute) relative to that for amino acids (CNof about 5). Concentration trends of ammo-

nium and amino acids and changes in atom %15N_NH/ over time in both light and dark bot-tles, with and without the addition of HMWDaM, were observed over time. Thus, poten-tial interactions of light, HMW DaM, nitro-gen (inorganic and organic), and natural biotawere observed in these samples (GARDNERet al.1996).

Atchafalaya15°/00

L15 July 1993 100

LIGHT

6 NH/ Conen -180

5-160r

4-aL "---"'- -140:::

..=!:2c 200

:;::::: 1 ZttI 10....

a -- 0cQ) 0(,) Ec0 7 00 -+ 6 80 «

J: 5DARK

z -I 604

a L Ratio .... 40

2 I -I 201

o I . I . I . . . . . . · 00 2 4 6 8 10 12

Time (hours)

Fig. 5. Light and dark NH4+ concentration and iso-

tope ratio (acorn % 15N) vs. incubation time atAtchafalaya in the Mississippi River plume.

70 Plenary lectures

-.::c:::E..5ob~

:r:.z

~

-5~~..~ocE~ o

o 3010 20

Fig. 7. NH/ regeneration (nM h-') (A) and aminoacid turnover rates (h-') (B) in the Mississippi Riverplume.

Amino acids were removed at comparablerates in the light and dark bur removal rateswere much higher in the presence of addedHMW DOM than in the controls withour the

additions (GARDNERet al. 1996), as expectedfor heterotrophic uptake. The high rate ofamino acid uptake, particularly in the presenceofHMW DOM, suggests that the natural pop-ulations of bacteria were accustomed to assimi-

lating amino acids at high rates even thoughambient concentrations were very low. Anexplanation could be rapid turnover of aminoacids in this enriched mid-plume environment.If labile DOM with a high CN ratio is avail-able to support metabolic energy requirements,the bacteria may be able to use that energy toassimilate available N (ammonium or aminoacids) into biomass. Conversely, ammonium isregenerated more efficiently when amino acids(or other compounds with low C:N ratios) pre-dominate. Bacterial growth rates were high inall bottles with added amino acids but were

stimulated even further by the addition ofHMW DOM. Additionally, they were high inthe light sample with added HMW DOC andammonium, but not in the comparable darksample. These results suggest that, in the pres-ence of light, labile organic nitrogen may becycled in the form of small compounds such asamino acids that can be incorporated intomicrobial biomass or remineralized (GARDNERet al. 1997).

4. Are the effectsof light on community regenera-tion rates related to microbial food web composi-tion?

40

Comparison of light and dark bottle results inLake Maracaibo indicated that ammonium

regeneration rates were usually higher in thelight than in the dark bur that the differenceswere not significant. This lack of significance incomparison of light/dark regeneration ratesmay have been related to the fact that phy-toplankton are nitrogen limited in Lake Mara-caibo despite the hypereurrophic status of thelake. Light/dark differences in phyroplanktonuptake rates tend to be less pronounced innitrogen-starved than in nitrogen-replete phy-toplankton (COCHLANet al. 1991). However,ammonium regeneration rates under approxi-mately normal light conditions were related tothe carbon-based ratio of heterotrophicnanoflagellate (HNAN) (0 bacteria (r2= 0.58,P < 0.01; Fig. 8) that may reflect the degree ofbacterivory. This interesting result suggests thatammonium regeneration rates may be relateddirectly to bacterivory in Lake Maracaibo(GARDNERet al. 1998). However, dark bottles

from the same sites did not show a significantcorrelation betWeen ammonium regenerationand the HNAN:bacterial carbon ratio, possiblydue to different feeding behavior in the dark.

A similar comparison in Laguna Madre alsoshowed a highly significant relationshipbetWeen ammonium regeneration rates and theHNAN:bacterial carbon ratio under natural

light conditions (r= 0.7, P < 0.01), but againthere was no significant relationship betWeenthese variables for identical samples incubated

in the dark (r= 0.06, P = 0.63; Fig. 9). Anintriguing observation from this statistically sig-nificant comparison under natural light wasthat the tWo lowest values, for both ammonium

regeneration rates and the HNAN:bacteriaratios, were observed at sites with oppositetrophic characteristics. One of these sites wasthe hypereurrophic Baffin Bay, which was expe-riencing a dense algal bloom of Texas BrownTide (>200,000 cells/mL) at the rime of sam-

pling, whereas the other was a heterotrophicallydominated oligotrophic site overlying a sea

SOD "400300

200

100

0

1000 10 20 30 40

3t2

W. S. Gardner et al., Effectsof narurallight on nitrogen dynamics

2.0

r2 .. 0.58 p< 0.01

---i...c::

~=-----

1.5

oh~

1.0

x.z 0.5

0.00.01 0.06

Ratio

Fig. 8. NH4+ regeneration rates (11M h-I) vs.HNAN:bacteria in Lake Maracaibo under ca. naru-

rallight conditions.

grass bed in the lower Laguna Madre (personalcommunication, SUSANZIEGLER,University ofTexas Marine Science Institute). Furthermore,except for the sample from the oligotrophiclower Laguna Madre station that was incubatedfor about 4 h, all of these incubations weredone over intervals of about 1 h. The differ-

ence, in the predictabiliry of ammonium regen-eration rate patterns relative to the HNAN:bac-terial carbon ratios, between light and darkincubations was surprising. The short incuba-tion intervals (generally 1 h) for the LagunaMadre experiments, relative to expected organ-ism turnover times, may argue against thenotion that enhanced release of DON by phy-toplankton, followed by the uptake and recy-cling of the DON by microbial food weborganisms, was the major cause for this short-term light-mediated result. An alternativeexplanation may be a modified behavior of theHNAN in response to light. This possible rela-tionship offers a logical reason to study possiblephotoreception by heterotrophic componentsof microbial food webs in aquatic ecosystems.

Conclusions

1. Sunlight can affect ammonium regenerationrates, as well as uptake rates, in near-surfacewaters.

71

LIGHT

Fig. 9. Light and dark NH4' regeneration (J.1Mh-I)vs. HNAN:bacteria in Laguna Madre.

2. The major mechanism appears to be microbialfood web process changes in response to light.

Research recommendations

1. Consider light as a factor influencing nutrientregeneration rates in aquatic ecosystems.

2. Conduct heterotrophic process experimentsunder narurallight as well as dark conditions.

3. Consider microbial food web-light interactionsin conceptual models of nutrient dynamics.

4. Quantify specific food web mechanisms respon-sible for light-related nutrient cycling phenom-ena (e.g. DON release by phytoplankton,photoreception by heterotrophic organisms, ver-tical migration).

Acknowledgements

We thank MARKMCCARTHYfor technical and edito-

0.5

. I 0.4

---i

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=-----oh 0.2

+::r:r 0.1Z

0.00.05

I I0.11 0.16 0.5

0.10 0.15 0.20 0.25Ratio

I I II

DARK

. r2 = 0.06 P= 0.290.41-

---i 0.3...c:: I . .=-

0.2

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0.00.05 0.10 0.15 0.20 0.25

Ratio

72 Plenary lectUres

rial assistance, ED BUSKEYfor providing ship time onthe RIV Longhornin the LagunaMadreand BaffinBay, and MICHAELARTS for a careful review of themanuscript. The Nancy Lee and Perry Bass ChairEndowment to The University of Texas Marine Sci-ence InstitUte supported manuscript preparation.

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Aurhors' addresses:

W. S. GARDNER,P.J. LAVRENTYEV,University of TexasMarine Science Institute, 750 Channel View Drive,Porr Aransas, TX 78373, USA.

H. A. BooTsMA,SADC/GEF Lake Malawi I Nyasa

73

Biodiversity Conservation Project, P.O. Box 311,Salima, Malawi.

J. F. CAVALETTO,NOM Great Lakes EnvironmentalResearch Laboratory, 2205 Commonwealth Blvd.,Ann Arbor, MI 48105, USA.

F. TRONCONE,Insticuro para eI Control y la Conser-vacion de la Cuenca del Lago Maracaibo (ICLAM),Maracaibo, £Stado Zulia, Venezuela.

J. B. COTNER,Deparrment of Ecology, Evolution andBehavior, University of Minnesota, St. Paul, Minne-sota 55108, USA.