Spring–summer cycling of DOC, DON and inorganic N in a highly seasonal system encompassing the Northeast Water Polynya, 1993

Dissolved organic nitrogen (DON), dissolved organic carbon (DOC) and inorganic nutrient concentrations were determined in samples from an area encompassing the Northeast Water Polynya from June to August 1993. In June, still ice-covered polynya area surface waters (PySW) had significantly higher (p<0.05) DOC concentrations (110 μM, n=68) than surface water outside the polynya area (96 μM, n=6). Melting ice and ice algae are suggested as DOC sources. DOC concentrations found in this study are consistent with other studies showing higher DOC concentrations in the Arctic than in other ocean areas. As the productive season progressed, DOC concentrations in Polynya surface water (PySW) decreased (p<0.05) from 110 to 105 μM, while DON concentrations increased (p<0.05) from 5.6 to 6.1 μM, causing a significant decrease (p<0.05) in the C : N ratios of DOM from spring (C : N ratio 20) to summer (C : N ratio 17). We found a significant (p<0.05) decrease in the DOM C : N ratio in all water masses within the polynya area as the productive season progressed. DON was the largest fraction of total dissolved nitrogen (TDN) in PySW and surface waters outside the polynya area. TDN was calculated as the sum of DON, nitrate, nitrite and ammonium concentrations. DON increased (p<0.05) from 62% to 73% of TDN in PySW from spring to summer, a result of increasing DON concentrations and decreasing inorganic nitrogen concentrations over the productive season. The seasonal accumulation of DON and the corresponding decrease in nitrate concentrations in waters with primary production indicate that it is important to take the DON pool into account when estimating export production from nitrate concentration decreases in surface waters. PySW TDN concentrations decreased (p<0.05) from 9.1 (n=61) to 8.6 μM (n=60) from spring (May 25 through June 19) to summer (July 1 through July 27). The seasonal decrease in surface water TDN concentrations corresponded to increases in TDN concentrations in deeper water masses within the Polynya. Most of the TDN increase in deep water was in the form of DON. A possible explanation is that PON was dissolved (partially remineralized) in the water column at mid depths, causing increases in the DON concentration. Transfer of N from PySW (with a short residence time in the polynya area) to Polynya Intermediate Water and deep waters of the Norske and Westwind Trough with multi-year residence times keeps N from leaving the polynya area. In spring, nutrients from degradation of OM in PyIW could support primary production. The role of PyIW as an OM trap could be important in supporting primary production in the polynya area.     

Growth Dynamics in Posidonia oceanica (L.) Delile

Abstract. Amounts of photosynthate in the rhizomes, in photosynthetically inactive basal parts of the leaves and different old leaf blades were examined. Winter leaf growth was supported by mobilisation of starch in the rhizome. This winter growth enables Posidonia oceanica to utilize the increased energy influx in early spring via the substantial leaf area already developed and to approach highest productivity in spring. During summer and autumn considerable concentrations of soluble carbohydrates were found in the leaves and rhizomes. Starch was stored in the rhizomes in concentrations of up to 6.8 % of dry weight. Levels of nitrogen and free amino acids were correlated with growth rates. The percentage of total nitrogen present as free amino acid-nitrogen decreased from November (35 %) to summer (less than 1 %), by which time leaf growth had stopped. In contrast to this, amounts of organic anions were low in winter and spring and reached their maximum in summer.
Within a shoot, sites of leaf growth were characterized by high amounts of total nitrogen and free amino acids (innermost leaves), while carbon Fixation was highest in the leaves #2–#4. Particularly high concentrations of soluble carbohydrates were found midway along leaf blades.
It is evident that this unusual growth rhythm of Posidonia oceanica was only possible because of the ability to store considerable amounts of carbon and nitrogen in the rhizomes.
The results of this investigation demonstrate on the one hand that the chemical composition of the plant is strongly correlated with growth and production, and on the other hand that it is dependent on environmental factors, such as energy influx and temperature, which change with season.     

控释海水植物肥中氮、磷的测定

为评价控释肥的肥效，探讨、建立了相应的氮、磷测定方法。用硫酸作消解剂，消解液可以同时被用来测定氮、磷。钼锑抗坏血酸络合物作为显色剂，用紫外可见分光光度计测定磷的浓度；用改良式微量凯氏定氮蒸馏滴定法测定氮的含量。该方法灵敏、准确，易操作。用海水浸泡自制的控释海水植物肥样品(编号：MRF-0201．中速型)，运用建立的氮、磷测定方法测定浸取液中的氮、磷含量，实验证明该方法可以用于研究控释海水植物肥在海水中的氮、磷的化学动力学行为。     

氮、磷对金藻的增殖效应

本文通过室内培养试验，研究了水体中氮、磷含量变化及氮、磷交互作用对金藻增殖的影响，并比较了不同形态的氮盐对该藻增殖的反应。试验得出该藻生长的氮氨（氯化铵）最适浓度水平为150μmol－N／L，相应的无机磷的最适浓度水平为10μmol-P/L；有机氮（尿素）的最适浓度水平为250μmol－N／L，相应的无机磷最适度水平为5μmol－P／L。氨氮和尿素的试验中，金藻的生长趋势相似，但是相对应浓度下，氨氮的增殖效应大于尿素。氮的形态不同时，藻类对同一种磷的利用也可能产生差异。     

Nitrogen and phosphorus species in the coastal and shelf waters of Southeastern North Carolina, Mid-Atlantic U.S. coast

The diversity of small-scale wetlands, high salinity tidal creeks, salt marshes, estuaries, and a wide and shallow shelf with the Gulf Stream close to the break makes the coastal zone of south-eastern North Carolina (U.S.) a natural laboratory for the study of the cycling of nitrogen (N) and phosphorus (P) in coastal and shelf waters. We assessed the summer concentrations, forms, and ratios for each N (total dissolved N, nitrate + nitrite, ammonium and dissolved organic N) and P (total dissolved P, o-phosphate and dissolved organic P) pool as these nutrients travel from tidal creeks, salt marshes and two large estuaries to Long and Onslow Bays. Additionally, we measured ancillary physical (temperature, salinity and turbidity) and chemical (dissolved oxygen, chlorophyll a and pH) water properties. Highest concentrations of all individual N and P compounds were found in the upper parts of each tributary and were attributed to loads from agricultural and urban sources to the coastal watersheds, continuing downstream to receiving estuaries. In all areas, dissolved organic N and P species were predominant constituents of the total dissolved N and P pools (64–97% and 56–93%, respectively). The lower parts of estuaries and surface shelf waters were characterized by oceanic surface values, indicating removal of N and P downstream in all tributaries. The different watershed and hydrological characteristics also determined the different speciation of N and P pools in each estuary. Despite a high level of anthropogenic pressure on the uppermost coastal waters, there is self-regulation in this coastal ecosystem with respect to human perturbations; i.e. significant amounts of the N and P load are retained within estuarine and nearshore waters without reaching the shelf.     

High nutrient pulses, tidal mixing and biological response in a small California estuary: Variability in nutrient concentrations from decadal to hourly time scales

Elkhorn Slough is a small estuary in Central California, where nutrient inputs are dominated by runoff from agricultural row crops, a golf course, and residential development. We examined the variability in nutrient concentrations from decadal to hourly time scales in Elkhorn Slough to compare forcing by physical and biological factors. Hourly data were collected using in situ nitrate analyzers and water quality data sondes, and two decades of monthly monitoring data were analyzed. Nutrient concentrations increased from the mid 1970s to 1990s as pastures and woodlands were converted to row crops and population increased in the watershed. Climatic variability was also a significant factor controlling interannual nutrient variability, with higher nutrient concentrations during wet than drought years. Elkhorn Slough has a Mediterranean climate with dry and rainy seasons. Dissolved inorganic nitrogen (DIN) concentrations were relatively low (10–70 μmol L⁻¹) during the dry season and high (20–160 μmol L⁻¹) during the rainy season. Dissolved inorganic phosphorus (DIP) concentrations showed the inverse pattern, with higher concentrations during the dry season. Pulsed runoff events were a consistent feature controlling nitrate concentrations during the rainy season. Peak nitrate concentrations lagged runoff events by 1 to 6 days. Tidal exchange with Monterey Bay was also an important process controlling nutrient concentrations, particularly near the mouth of the Slough. Biological processes had the greatest effect on nitrate concentrations during the dry season and were less important during the rainy season. While primary production was enhanced by nutrient pulses, chlorophyll a concentrations were not. We believe that the generally weak biological response compared to the strong physical forcing in Elkhorn Slough occurred because the short residence time and tidal mixing rapidly diluted nutrient pulses.     

对虾、青蛤和江蓠混养系统氮磷收支的实验研究

为减少养殖污水中营养盐的排放量,降低对近海水质的污染,采用海水陆基围隔实验法,对凡纳滨对虾(Litope-naeus vannamei)、青蛤(Cyclina sinesis)和菊花心江蓠(Gracilaria lichevoides)不同混养系统的氮(N)、磷(P)收支状况进行了实验研究。结果表明:实验中的单养组投放的饵料占N总输入的75.5%,占P总输入的93.6%,混养组投放的饵料占N总输入的54.7%~64.8%,占P总输入的81.6%~88.7%;在支出项目中,单养组和混养组收获的养殖生物分别占N总输入的25.0%和44.4%~51.5%,占P总收入的11.2%和25.1%~31.8%;底泥沉积N,P为其主要支出项目,单养组和混养组分别占总输入的45.1%和30.0~32.5%,68.3%和50.3%~57.1%,其中混养组N,P的底泥沉积量低于单养组。     

海洋沉积物中氮的形态及其生态学意义

海洋沉积物作为生源要素氮的重要源与汇，在其生物地球化学循环中起着至关重要的作用。该项研究旨在查清海洋沉积物中氮的存在形态及其生态学意义。氮是生物生命活动必需的营养要素，海洋中的氮往往是海洋初级生产力的限制因子，氮的吸收与再生释放在估算海洋新生产力和生源要素的生物地球化学循环率上也有重要贡献。海洋沉积物中的氮作为海水中氮的重要供给源是海洋生物赖以生存的重要物质基础，对维持海洋生态平衡、修复失衡的海洋生态环境具有重要意义。但由于不同海区海洋沉积物的来源和环境不同，氮的形态和含量亦不相同，可被生物吸收和利用的数量就不相同，因此造成了不同海区生物种群和环境不同，进而影响生态环境。因此，研究海洋沉积物中氮的形态，了解各个形态与沉积物中生物种群及与环境的关系，确定其生物和化学活性，査清氮不同形态的生态学功能，对于深入探讨海洋生物资源可持续利用的方法和策略具有重要意义。本文主要阐述海洋沉积物中氮的存在形式与分布、氮的早期成岩和去营养化作用、硝化和反硝化作用以及氮与生物特定种群的关系等，探讨了影响海洋沉积物中的氮循环的主要因素，并分析了海洋沉积物中的氮与生态系的关系，以期对研究氮的海洋生物地球化学过程有所帮助。     

Nutrient Cycling and a Stoichiometric Model in Epidavros, a Deep Basin in the Aegean Sea

Abstract. The nutrient cycling of Epidavros, a deep basin in the Saronikos Gulf, was studied in relation to various environmental factors during 1973–1976 at a station characterized by stagnant conditions. The regeneration of nutrients was related to the consumption of oxygen, and a seasonal nutrient cycle occurred with low nutrient concentrations in the spring and summer, followed by high nutrient levels in autumn and winter. In addition high values of nitrate and silicate were observed in the deeper waters, which tended to be anoxic, although the water masses were renewed during spring 1974. The distribution pattern of nutrients together with nutrient ratios were compared with previous studies of the same and neighbouring areas as well as of other isolated basins. A stoichiometric model indicates that plankton organisms in the Epidavros basin have approximate atomic ratios for C: N: P of 150: 14:1, while the ratio of change for nitrogen and phosphorus in the water is only 8.8:1 by atoms. This is probably because of the slow rate of regeneration of nitrogenous material and/or assimilation and regeneration in organic forms. The water/plankton relation in the Epidavros basin appears to be very similar to that in the Baltic Sea.     

Carbon and nitrogen primary productivity in three North Carolina estuaries

Uptake of inorganic carbon and ammonium by the plankton community of three North Carolina estuaries was measured using ¹⁴C and ¹⁵N isotope methods. At 0% light, C appeared to be lost via respiration, and at increasing light levels uptake of inorganic carbon increased linearly, saturated (mean I_k = 358±30 μEin m⁻² s⁻¹), and frequently showed inhibition at the highest light intensities. At 0% light NH₄⁺ uptake was significantly greater than zero and was frequently equivalent to uptake in the light (light independent); at increasing light levels NH₄⁺ uptake saturated (mean I_k = 172±44 μEin m⁻² s⁻¹) and frequently indicated strong inhibition. Light-saturated uptake rates of inorganic carbon and NH₄⁺ were a function of chlorophyll a (r² = 0·7−0·9); average assimilation numbers were 625 nmol CO₂ (μg chl. a)⁻¹ h⁻¹ and 12·9 nmol NH₄⁺ (μg chl. a)⁻¹ h⁻¹ and were positively correlated with temperature (r² = 0·3−0·7). The ratio of dark to light-saturated NH₄⁺ uptake tended to be near 1·0 for large algal populations at low NH₄⁺ concentrations, indicating near light independence of uptake; whereas the ratio was lower for the opposite conditions. These data are interpreted as indicative of nitrogen stress, and it is suggested that uptake of NH₄⁺ deep in the euphotic zone and at night are mechanisms for balancing the C:N of cellular pools. A 24-h study using summed short-term incubations confirmed this; the cumulative C:N of CO₂ and NH₄⁺ uptake during the daylight period was 10–20, whereas over the 24-h period the ratio was 6 due to dark NH₄⁺ uptake. Annual carbon and nitrogen primary productivity were respectively estimated as 24 and 4·0 mol m⁻² year⁻¹ for the South River estuary, 42 and 7·3 mol m⁻² year⁻¹ for the Neuse River estuary, and 9·6 and 1·6 mol m⁻² year⁻¹ for the Newport River estuary.