Roger B. Hanson

Experimental degradation of detritus made from the salt marsh plants Spartina alterniflora Loisel., Salicornia virginica L., and Juncus roemerianus Scheele

The pattern of decomposition and potential food value of detritus derived from the C4 photosynthetic pathway plant Spartina alterniflora Loisel. and the C3 pathway plants Salicornia virginica L. and Juncus roemerianus Scheele were compared. Detrital particles < 125 μm in size made from standing dead plant material were incubated aerobically and anaerobically in phosphate-enriched estuarine water with and without added ammonium chloride. The most rapid decreases in ash-free dry weight (AFDW) and increases in microbial biomass (estimated by increases in ATP and in particulate nitrogen as % of AFDW) were under aerobic conditions with added nitrogen. Material in the aerobic treatment without added nitrogen degraded only slightly faster than in the anaerobic treatments. The dissolved organic carbon (DOC) in the cultures was < 5% of the total detrital carbon; the DOC content decreased during the first week, most rapidly in the aerobic cultures, and then began to increase slowly. Rates of nitrogen fixation in the cultures were low, and were depressed by the presence of O2 and NH4+, and by a higher nitrogen content of the detritus. Salicornia detritus initially degraded the fastest but showed the lowest conversion efficiency of plant material to microbial biomass (19.4%). Spartina detritus had the highest conversion efficiency (64.3%), while Juncus, which decomposed most slowly, had a conversion efficiency of 55.6%. From 87 to 100% of the increase in nitrogen content of the plant detritus is ascribed to proteinnitrogen. Our results suggest that 1. 1) optimum in situ sites for rapid degradation of plant detritus in estuaries are the sediment-air and sediment-water interfaces; and 2. 2) detritus made from the C4 plant Spartina yields a richer food source for estuarine consumers than detritus made from the C3 plants Salicornia and Juncus.

Uptake and metabolism of two amino acids by anaerobic microorganisms in four diverse salt-marsh soils

The anaerobic microbial uptake of alanine and aspartic acid was determined in 4 diverse salt-marsh soils (tall and short Spartina alterniflora, creek bank, and mud flat). Uptake in soil slurries was determined by the radioisotopic tracer technique at one substrate concentration (<250 pmoles cm-3). Dissolved free alanine and aspartic acid concentrations in the interstitial nutrient pool ranged from approximately 1 to 500 pmoles cm-3. In the short S. alterniflora soil, maximum microbial uptake of alanine was found at a depth of 10 cm (8.32 pmoles cm-3 h-1); in the tall S. alterniflora soil maximum uptake was at 20 cm (23.4 pmoles cm-3 h-1). The utilization of aspartic acid appeared constant over the depth interval investigated (0 to 60 cm). The turnover times of alanine and aspartic acid in the tall and short S. alterniflora soils ranged from 5 to 25 h and 40 to 100 h, respectively. The percent of the labeled alanine and aspartic acid taken up that was mineralized by tall and short s. alterniflora microbenthos ranged from 20 to 50% and 5 to 20%, respectively.

Effect of detritus supply on trophic relationships within experimental benthic food webs. II. Microbial responses, fate and composition of decomposing detritus

Abstract The dynamics of bacterial, protozoan and fungal populations were examined in a long-term (40–55 wk) microcosm experiment designed to assess the effects of detritus supply on meiofauna-polychaete (Capitella capitata (Type I) Fabricius) interactions. Bacterial and protozoan numbers and bacterial growth rates were inversely correlated with population fluctuations of the polychaete at low (50 mg N · m−2 · day −1) detritus supply, but did not correlate with fluctuating polychaete densities at two higher (100 and 150 mg N · m−2 · day−1) ration levels of detritus. Bacterial and protozoan numbers and bacterial growth rates did not correlate with standing amounts of detritus or with fungi or meiofauna at any of the detritus rations. Fungi were associated primarily with aggregates of detritus particles and fecal pellets produced by C. capitata. Labile (fiber-free) organic matter did not correlate with microbes or meiofauna, but was inversely correlated with population fluctuations of the polychaete C. capitata at all three ration levels of detritus. Polychaete fecal pellets accounted for most of the refractory matter in the tanks with C. capitata and did not accumulate in the sediments, suggesting that fecal pellets were continually being produced, broken apart and decomposed. Our experiments suggest that contradictions in previous studies on the effects of macroconsumers on microbes, especially bacteria, can be explained as a failure to consider the effects of detritus supply on microbial growth rates.

The role of microfauna in detrital utilization by the polychaete, Nereis succinea (Frey and Leuckart)

Abstract Net incorporation rates of detritus derived from carbon-14 labelled seaweed, Gracilaria foliifera (Harvey) Taylor, by the polychaete, Nereis succinea (Frey and Leuckart), were measured in the presence and absence of the ciliate, Aspidisca sp. Net incorporation rates averaged 79 in the absence and 129 μg dry wt detritus·mg −1 dry wt worm·day −1 in the presence of ciliates. Ciliates did not significantly affect oxidation rates. In the presence of Nereis succinea , however, oxidation rates approximately doubled due to respiration and feeding activity. The increase in net incorporation probably resulted from a combination of stimulation of bacteria by grazing, fragmentation of detritus by ciliates, and ingestion of ciliates. The rate of heterotrophic uptake of tritiated glucose was used as a relative measure of microbial activity in the sediment. Glucose flux was greatest when N. succinea and Aspidisca were present together. Our data clarify the regulatory role of the microfaunal community in the transfer of energy from detritus to the macrodetritivore and the role of macrofauna in enhancing microbial activity in the sediment through bioturbation.

Nucleic Acid Synthesis in Oceanic Microplankton from the Drake Passage, Antarctica: Evaluation of Steady-State Growth

Differential uptake of [3H]adenine and [3H]thymidine, and incorporation of tritium label into proteins, RNA and DNA as a function of depth, indicate that surface microplankton incorporated most of the assimilated radioisotopes into nucleic acids. Growth processes for deep-sea microplankton were shifted towards higher rates of [3H]-adenine incorporation into RNA compared to rates of thymidine incorporation into DNA. Deep-sea microplankton also diverted a larger portion (up to 80%) of the assimilated tritium into biosynthetic pathways for amino acid and eventual incorporation into proteins. These results imply that protein synthesis is vital for deep-sea microplankton where populations may be stressed by low levels of available nutrients. The rates at which microplankton incorporated [3H]adenine into RNA and DNA, and [3H]thymidine into DNA, suggest that oceanic microplankton are in a transient state of balanced growth, i.e. between two steady-state growth conditions, irrespective of potential growth rates. Our results support the hypothesis that oceanic microplankton are in various growth states.

Dissolved free amino acids in interstitial waters of Georgia salt marsh soils

The patterns of concentration of dissolved free amino acids (DFAA) are different and more variable in pore water of soils from a Georgia salt marsh than in most surface waters. Elevated concentrations of DFAA (up to 8850 nmoles liter−1) and high relative levels of alanine and glutamic acid suggest that bacteria may produce these amino acids. In addition to the common protein amino acids, an unidentified compound was present and was the dominant free amino acid in some sections. This compound appeared to be specific to pore waters as it was not observed in surface waters or in hydrolysates of sediments, bacteria or plants from the marsk.

Fate and effects of a heavy fuel oil spill on a Georgia salt marsh

Abstract Addition of a heavy oil to a Spartina salt marsh in the autumn resulted in high concentrations of polycyclic aromatic hydrocarbons in sediment and benthic animals. The highest concentrations of phenanthrene, chrysene and fluoranthene in the sediment were 112, 105 and 75 ng/g sediment, respectively. These concentrations rapidly decreased during the 20 week period following the spill. The times for these hydrocarbons to decrease to 50% of their highest values, i.e. half-life, were approximately 100, 70 and 30 days in sediment, mussels and oysters, respectively. Benthic macrofauna species showed three responses to oil addition which included no change, an increase, or a decrease in the population. No changes were noted in populations of fiddler crabs ( Uca pugnax ), oysters ( Crassostrea virginica ), and mussels ( Modiolus demissus ). Mud snails ( Nassarius obsoleta ) increased in density after the spill due to immigration of adult snails from untreated areas to scavenge on animals killed by the oil. Many of the adult periwinkles ( Littorina irrorata ) were killed by the oil. In the spring, juvenile periwinkles recolonised to oiled areas as a result of larvae settling.

Organic nitrogen and caloric content of detritus: II. Microbial biomass and activity

The effects of organic nitrogen, total caloric and/or ‘available’ caloric content on the regulation of microbial dynamics were studied using 23 types of detrital rations. Because most natural benthic systems rarely receive rations above 50 mg N m −2 day −1 , the results of this work predicts that the organic nitrogen content of the detrital ration is the primary factor regulating microbial metabolism. Estimates of microbial biomass (total adenylates, A) and activity (rates of O 2 consumption, CO 2 production and nitrogen fixation) were measured at low (>3 to 100 mg N m −2 day −1 ) and high rations (>100 mg N m −2 day −1 to 216 mg N m −2 day −1 ). At low rations microbial biomass, activity and the mineralization (CO 2 ) of ‘available’ detrital carbon per unit microbial biomass (CO 2 : A ratio) correlated best with the nitrogen content of the detritus while respiration per unit microbial biomass (O 2 :A) correlated best with ‘available’ caloric content. At high rations, microbial biomass and activity correlated best with ‘available’ caloric content. In all cases the metabolic quotient (CO 2 :O 2 ) correlated best with ‘available’ calories and in no case did total caloric content become an important factor in microbial dynamics. This work suggests the rate of organic enrichment to benthic systems is an important variable to microbial dynamics as is the organic nitrogen and/or ‘available’ caloric content of the ration. It also suggests the importance of relying on more than one type of measurement and in determining the ‘availability’ of the detritus to detritivores in nutritional studies.

Spring recycling rates of ammonium in turbid continental shelf waters off the southeastern United States

Abstract Nitrogen ( NH 4 + ) recycling rates were measured over 3 weeks and 1000 km 2 during April 1985 as part of a multidisciplinary study to determine the fate of low-salinity water off the southeastern coast of the United States. During the study period, low-salinity water was not advected offshore as expected but held against the coast by a weak onshore-southward wind stress. Several analytical nitrogen ( 15 N) models were examined to determine the appropriateness of each model to describe the experimental field data. Two frequently applied mathematical models were compared, and it was determined that Laws (1985, Limnology and Oceanography , 30 , 1340–1350) nitrogen cycling model best described our experimental data set. Mean values for NH 4 + regeneration at each station during the study did not vary by more than a factor of 3 from the overall mean value (0.224 ± 0.041μM h −1 , ±1 S.E.). NH 4 + utilization (0.111 ± 0.030 μM h −1 ) showed the same overall variance, i.e. less than a factor of 3. NH 4 + regeneration exceeded utilization by a factor of 1.7:1 ( U : R ratio was 0.58:1). NH 4 + assimilation, on the other hand, showed a significant decrease across the area (0.090-0.021 μM h −1 ) and correlated with particulate N and chlorophyll a . Regeneration rates were unaffected and assimilation rates were significantly affected (mean 75%) when coastal plankton were deprived of full sunlight. Nanoplankton and microplankton (1 μm) were responsible for most of the regeneration and assimilation of NH 4 + . In addition, NH 4 + disappearance from the NH 4 − phase was not balanced by an increase in particulate N, and NH 4 + assimilation contributed only 10–20% of the phytoplankton nitrogen requirement. These results suggest that phytoplankton are not the sole consumer of regenerated NH 4 + , the fate(s) of regenerated NH 4 + remains unknown and the spring phytoplankton off the southeastern United States may have been nitrogen limited.

POSSIBLE EFFECTS OF GULF STREAM INTRUSIONS AND COASTAL RUNOFF ON THE BENTHOS OF THE CONTINENTAL SHELF OF THE GEORGIA BIGHT

: The Georgia Bight has a broad shallow continental shelf. The major nutrient inputs are deepwater Gulf Stream intrusions along the shelf break and nearshore outwelling from Spartina salt marshes and river runoff. The middle portion of the shelf is not appreciably affected by either process. Nine stations off Georgia were sampled to detect possible differences in the benthos of these three regions. All sediments were moderately- to well-sorted sand. A large coarse fraction occurred at the outer stations. Surface sediment carbon and nitrogen were extremely low at all stations (> 0.1% C and > 0.01% N dry wt). Sediment chlorophyll a values ranged from 29 to 142 μg m2- (1 to 6 μg gm-1). Macrobenthic biomass at the inner three stations was significantly lower (3 to 9 grams wet weight m-2) than middle and outer shelf regions (14 to 22) but all were lower than shelf environments in other areas. In contrast, meiofaunal densities (853 10 cm-2) and biomass (543 μg 10 cm-2) of the middle shelf regions were higher than inner and outer stations. Nematodes generally dominated the meiofaunal taxa. ATP biomass (μg ATP cc-1) ranged from 35 to 106 among the shelf stations with no apparent regional patterns. Microbial biomass was relatively constant along the coast but increased north to south along the shelf break. Interstitial glucose concentrations (5 7 to 100 ng cc-1) and glucose uptake (6 to 19 μg-1 hr-1) at the inner stations were similar to those observed at the outer stations. ATP biomass was positively correlated with glucose uptake but negatively correlated with denitrification rates. Nitrogen fixation (3 to 20 pmoles C2H4 cc-1 hr-1) and denitrification (0.1 to 0.5 μg N2O cc-1 hr-1) decreased along the shelf break from north to south. Preliminary results suggest that a combination of an unfavorable sedimentary regime and low nutrient input results in an impoverished benthos but that nitrogen-rich Gulf Stream intrusions may influence benthic processes on the outer regions of the continental shelf of the Georgia Bight. Microbenthos, with high metabolic rates and fast generation times, probably reflect more immediate nutrient effects, whereas the distribution of macrobenthos might reflect integrative effects of intrusion.