胶州湾底层水营养盐的分布特征及有机污染状况分析

根据胶州湾底层水营养盐的调查资料,分析了胶州湾底层水中营养盐的分布规律、营养盐的限制因素、水体富营养化水平和有机污染状况。结果表明胶州湾底层水营养盐含量、底层水富营养化判断值与底层水有机污染指数的平面分布非常相似,均从东北向西南递减,研究区域富营养化程度达45．45％,且胶州湾中浮游植物的生长主要受控于溶解无机磷的限制。     

南印度洋海区营养盐的分布特征

探讨了南印度洋海区总有机磷(TOP)、总有机氮(TON)以及溶解无机营养盐的分布规律。分析结果表明：研究海区内溶解无机营养盐受水体中生物活动和物理过程的综合影响,表层水体由于生物活动的消耗,其磷酸盐等无机营养元素的含量一般是采样水深范围内最低的;中深层水体由于生物活动的降低以及有机质矿化作用的影响,无机营养元素的变化范围较小。表层水体中TOP和TON含量占TP和TN的主要部分,说明表层水体中的氮和磷主要以有机态形式存在,且沿着37.8°S从西向东,TOP和TON的含量以及TOP/TP和TON/TN的比值呈降低的趋势。研究海区叶绿素a的分析结果表明,初级生产力的变化可能是控制研究海区TON和TOP空间分布的主要因素。不同形态氮、磷营养元素的相关分析表明,有机营养盐和无机营养盐之间互为补充,且表层水体中有机氮和磷是水体初级生产所需营养盐的重要来源,总氮、总磷的关系表明研究区初级生产力并不受氮、磷的限制。     

西太平洋Y3海山对营养盐的影响及其生态环境效应

作为大洋典型地貌特征的海山,其邻近的海洋生境并不被人熟知。依据2014年冬季对热带西太平洋Y3海山及其邻近海域的综合调查,首次探讨了该海山区海水中营养盐的分布特征及其与生态环境的耦合关系。结果表明Y3海山区是典型的热带寡营养海域。在50~125 m的深度,温跃层和高盐区双重作用对底部高浓度营养盐向上输送的阻碍,是导致真光层上部营养盐浓度较低的重要原因;同时,浮游植物和异养细菌的消耗也加剧了该区域营养盐的缺乏。Y3海山突出的地形地貌对营养盐的分布产生重要的影响,海山对流经的海流产生阻碍,形成上升流,使底部高浓度的营养盐突破温跃层和高盐区的阻碍向上输送,从而使海山周围营养盐的平均浓度高于远离海山的海域。相关性分析显示,Y3海山区真光层中营养盐浓度与温度、盐度、叶绿素a浓度和异养细菌丰度分别呈负相关、正相关、负相关(除NO₂-N外)和负相关关系。海山区特殊的营养盐分布及其与生态环境的耦合是其成为一种独特的大洋生态系统的重要因素。     

乌江渡水库中溶解性硅的时空分布特征

在2003年10月-2004年9月期间每月一次采集了乌江干流上乌江渡水库大坝前开阔水域中的表层水;并于2003年10月、2004年4月和7月在同一采样点采集了分层水样及其底部的柱状沉积物。分别测定了其中溶解性硅(DSi)和叶绿素a(Chl-a)的浓度,同时还现场测定了水体中的温度(T),溶解氧(DO)和pH值。结果表明:乌江渡水库表层水中DSi的浓度范围为0.53~3.96 mg/L,平均值为1.74 mg/L;但沉积物孔隙水中DSi浓度大约是上覆水体中DSi浓度的7倍。分层期间,水体中DSi浓度在垂直方向上随水深增加而升高,而孔隙水中DSi浓度随沉积深度先增加后降低。同时还发现乌江渡水库中DSi与叶绿素a之间存在较好的反相关关系,这表明该水库中DSi的含量和分布可能主要受到浮游植物尤其是硅藻的生物活动调节。     

滇池湖滨带叶绿素a与营养盐空间分布特征

为探讨滇池湖滨带叶绿素a与营养盐空间分布特征及其对滇池水体富营养化的影响,在滇池湖滨带设置21个采样点进行水质调查,采集滇池水样,分析测定总氮(TN)、总磷(TP)、化学需氧量(CODCr)、叶绿素a(Chla)和悬浮固体(SS)。结果表明:滇池湖滨带水体TN、TP、CODCr、Chla、SS浓度均值的空间分布存在差异,叶绿素a空间分布呈现北高南低,西高东低的趋势。滇池湖滨带的北部和东部水体Chla与TN极显著相关,TN具有强变异性。南部和西部TN与CODCr、TP显著相关,南部Chla与营养盐空间变异性较小,污染风险相对较小。按照"一区一策"的原则,滇池北部和东部湖滨带富营养化以控制TN的水平为主,南部、西部的富营养化防控则需要控制整体营养盐元素浓度。     

海洋中氮营养盐循环及其模型研究

对某一海区营养盐的去向、不同形态间的相互转化及其与生物相关的过程的研究是研究整个海洋生态系统的基础和关键。氮是海洋环境中主要的营养元素之一,并被认为是大部分海区的限制营养元素。人们对于氮在海洋环境中的循环过程的研究随着分析方法及对化学和生物知识的掌握和理解而不断加深。模型研究是研究海洋生态系统的一个有效方法,营养盐循环模型是其中重要的环节,随着理论和观测数据的补充而不断发展。综述了海洋环境中氮营养盐循环主要过程的实验研究,主要包括:浮游植物的吸收、氮的再生、微生物在其循环中的作用、不同形态氮的化学转化、水体-沉积物界面等,及其相关过程近十年来模型化研究的进展。     

用三维荧光谱图分析原水输送过程中藻类和有机物的变化

采用新型分析技术三维荧光谱图方法对深圳东江水源工程水体中藻类进行了荧光分析。实验结果表明,水体中有较丰富的藻类,其中蓝藻、绿藻呈优势藻种,实验期间沿程水体叶绿素a浓度在1.71~16.43μg/L之间,藻密度在72.92万~1 375万个/L之间。三维荧光方法能较好表征水体中生长藻类种类和数量,三维荧光强度F(A.U)与叶绿素a浓度Chla(μg/L)、藻密度N(104个/L)呈较好的线性关系。三维荧光方法能较好的鉴别水体中可溶性有机物腐殖酸和蛋白质,并表征其含量多少。     

梅梁湾、大太湖夏季和冬季CDOM特征及可能来源分析

基于2004年夏季水华暴发期和冬季在梅梁湾及大太湖各2次采样,分析了夏季、冬季CDOM的特征及其可能的来源,发现夏季CDOM吸收系数、叶绿素a浓度均明显高于冬季,DOC浓度、CDOM吸收系数a(355)的变化范围分别为5.17～12.42 mg/L、2.57～6.77 m-1,最大值均出现在冬季(12月15日)的直湖港入湖口.CDOM吸收系数与DOC浓度、定标后的荧光值一般都存在显著正相关,但夏季由于受浮游植物降解的影响,与DOC浓度和荧光的相关性明显低于冬季.表征CDOM组成和来源的参数比吸收系数、M值、S值存在显著的季节差异,夏季吸收系数a*(355)值明显要大于冬季,而S值、M值则要小于冬季.夏季水华暴发时CDOM吸收系数与叶绿素a浓度空间分布较为一致,吸收系数与叶绿素a浓度存在正相关,浮游植物降解产物可能是水体中CDOM的重要来源;相反,冬季CDOM吸收系数呈现从梁溪河入湖口、湾内往湾口递减的趋势,其来源可能主要以陆源为主,受入湖河流的影响较大.     

近海营养盐和微量元素的大气沉降

受气候变化和人类活动的影响,传输和沉降到全球近海的大气污染物急剧增加.1997年后对近海营养盐和微量元素大气沉降的众多研究表明,通过大气沉降至近海的氮和磷分别为13～73 mmol N/(m2·a)和0.11～1.6 mmol P/(m2·a),微量元素的沉降通量具有显著的时空变化特征,在不同海区最高可相差3个数量级.对于很多近海包括东海(East China Sea)和黄海(Yellow Sea),大气沉降的营养盐和部分微量元素可能超出了其河流输入量.大气沉降除了对近海富营养化有重要贡献之外,其事件性特征可使初级生产力在短期内大幅度增加,从而影响赤潮发生.微量元素沉降还可能抑制某些藻类生长,对初级生产力和生态系统结构产生更为复杂的影响.未来研究重点是准确估算近海各物质的大气沉降通量,了解其对浮游植物生长的影响机制.     

强弱风浪扰动下太湖的营养盐垂向分布特征

在一次风速12m/s的强风浪过程中及在连续多天弱风浪之后,对太湖梅梁湾一浅水区营养盐、悬浮物等的垂向分布进行了观测和分析。结果表明,在水底沉积物约20cm的情况下,强风浪期间与弱风浪期间相比,湖水中悬浮物浓度提高了10倍,总磷浓度提高了3 6倍。而强风浪期间与弱风浪期间的水体溶解性总磷(DTP)、溶解性活性磷(SRP)的浓度无显著差异。说明尽管强风浪过程引起沉积物大量悬浮,水体悬浮颗粒态营养盐显著增高,但是由于悬浮过程营养盐释放与沉降机制作用十分复杂,活性营养盐的浓度未必能提高。无论强风浪还是弱风浪期间,水体的表层至水土界面上50cm层的悬浮物浓度、营养盐浓度没有明显的分层现象,但明显低于水土界面上50cm内的悬浮物浓度和总磷浓度。无论是强风浪期间还是弱风浪期间,表层到底层水体SRP浓度无显著差异。     

Nutrient limitation in the lower Housatonic River estuary

One of the most serious threats to freshwater and marine ecosystems is high rates of anthropogenic nutrient loading, particularly nitrogen (N) and phophorus (P). One of the major freshwater sources of nutrients to Long Island Sound (LIS) is the Housatonic River (HR). Current management plans that call for reducing N inputs without reducing P inputs may change the N: P ratio in the water column and the pattern of algal nutrient limitation and species composition in the tidal portion of the river. To assess the current pattern of algal nutrient limitation in the HR estuary, nutrient bioassays were conducted in spring, summer, and fall at 5 sites throughout the tidal portion and adjacent LIS. Diatoms were a dominant taxon at all sites throughout the sampling period. Other seasonally important taxa include cyanobacteria, cryptophytes, and euglenoids. Phytoplankton in LIS were always strongly N limited and were co-limited by P in spring. During low flow (summer), phytoplankton in the lower HR estuary were N limited. Phytoplankton in the middle reaches showed no evidence of N or P limitation and were likely limited by other factors. In spring, phytoplankton in the upper HR estuary were P limited. Periods of N or P limitation were better correlated with periods of lower concentrations of nitrate or phosphate than with differences in N: P ratio. These results suggest that decreases in N concentration could increase the prevalence of N limitation throughout the estuary that in turn may reduce phytoplankton biomass and alter species composition of the phytoplankton.     

Alternation of factors limiting phytoplankton production in the Cape Fear River Estuary

Phytoplankton nutrient limitation experiments were performed from 1994 to 1996 at three stations in the Cape Fear River Estuary, a riverine system originating in the North Carolina piedmont. Nutrient addition bioassays were conducted by spiking triplicate cubitainers with various nutrient combinations and determining algal response by analyzing chlorophyll a production and ¹⁴C uptake daily for 3 d. Ambient chlorophyll a, nutrient concentration, and associated physical data were collected throughout the estuary as well. At a turbid, nutrient-rich oligohaline station, significant responses to nutrient additions were rare, with light the likely principal factor limiting phytoplankton production. During summer at a mesohaline station, phytoplankton community displayed significant nitrogen (N) limitation, while both phosphorus (P) and N were occasionally limiting in spring with some N+P co-limitation. Light was apparently limiting during fall and winter when the water was turid and nutrient-rich, as well as during other months of heavy rainfall and runoff. A polyhaline station in the lower estuary had clearer water and displayed significant responses to nutrient additions during all enrichment experiments. At this site N limitation occurred in summer and fall, and P limitation (with strong N+P co-limitation) occurred in winter and spring. The data suggest there are two patterns controlling phytoplankton productivity in the Cape Fear system: 1) a longitudinal pattern of decreasing light limitation and increasing nutrient sensitivity along the salinity gradient, and 2) a seasonal alternation of N limitation, light limitation, and P limitation in the middle-to-lower estuary. Statistical analyses indicated upper watershed precipitation events led to increased flow, turbidity, light attenuation, and nutrient loading, and decreased chlorophyll a and nutrient limitation potential in the estuary. Periods of low rainfall and river flow led to reduced estuarine turbidity, higher chlorophyll a, lower ambient nutrients, and more pronounced nutrient limitation.     

Nutrient Limitation on Phytoplankton Growth in the Upper Barataria Basin,Louisiana: Microcosm Bioassays

The Davis Pond Diversion (DPD) was constructed to divert Mississippi River (MR) water into the Barataria Basin to reduce the salinity in support of wetland restoration on the Louisiana coast. To assess the phytoplankton nutrient limitation in adjacent water systems and potential impacts of DPD, 12 seasonal nutrient-phytoplankton bioassay experiments were conducted from October 2003 to July 2004 using the natural phytoplankton assemblages from freshwater and brackish-water lakes, Cataouatche and Salvador, LA (USA), which receive Mississippi River water from the DPD, and from a nearby freshwater lake, Lac des Allemands, that does not. Dissolved inorganic nitrogen (N), phosphorus (P), and silicate (Si) were added with different combinations at Redfield ratios in 10-l microcosms. Nitrogen was found to be the sole or primary limiting nutrient in all 12 experiments. N and P colimitations were found in seven of 12 experiments, but N was always the stronger limiting factor. P limitation was never observed to be the sole limiting nutrient. The results showed that a low concentration of P and a relatively high concentration of N do not necessarily indicate only P limitation in these lakes. Lake Cataouatche and Lake Salvador were dominated by centric diatoms, and Anabaena spp. were detected at high levels, particularly in summer. Lac des Allemands was generally dominated by N-fixing Anabaena spp. and other cyanobacteria, and their biomass responded significantly to N addition but not to P addition, indicating that nitrogen fixation in Lac des Allemands may be inhibited by other factors such as iron. Our bioassay results demonstrate that whether a water body is N- or P-limited is the consequence of the nutrient status and not the salinity regime. The results suggest that the addition of nutrient-rich waters via diversions of Mississippi River water into these lakes might increase the frequency of algal blooms, including noxious and toxic freshwater cyanobacteria.     

Going West: Nutrient Limitation of Primary Production in the Northern Gulf of Mexico and the Importance of the Atchafalaya River

To investigate controls on phytoplankton production along the Louisiana coastal shelf, we mapped salinity, nutrient concentrations (dissolved inorganic nitrogen (DIN) and phosphorus (P_i), silicate (Si)), nutrient ratios (DIN/P_i), alkaline phosphatase activity, chlorophyll and ¹⁴C primary productivity on fine spatial scales during cruises in March, May, and July 2004. Additionally, resource limitation assays were undertaken in a range of salinity and nutrient regimes reflecting gradients typical of this region. Of these, seven showed P_i limitation, five revealed nitrogen (N) limitation, three exhibited light (L) limitation, and one bioassay had no growth. We found the phytoplankton community to shift from being predominately N limited in the early spring (March) to P limited in late spring and summer (May and July). Light limitation of phytoplankton production was recorded in several bioassays in July in water samples collected after peak annual flows from both the Mississippi and Atchafalaya Rivers. We also found that organic phosphorus, as glucose-6-phosphate, alleviated P limitation while phosphono-acetic acid had no effect. Whereas DIN/P_i and DIN/Si ratios in the initial water samples were good predictors of the outcome of phytoplankton production in response to inorganic nutrients, alkaline phosphatase activity was the best predictor when examining organic forms of phosphorus. We measured the rates of integrated primary production (0.33?C7.01 g C m^?2 d^?1), finding the highest rates within the Mississippi River delta and across Atchafalaya Bay at intermediate salinities. The lowest rates were measured along the outer shelf at the highest salinities and lowest nutrient concentrations (<0.1 ??M DIN and Pi). The results of this study indicate that P_i limitation of phytoplankton delays the assimilation of riverine DIN in the summer as the plume spreads across the shelf, pushing primary production over a larger region. Findings from water samples, taken adjacent the Atchafalaya River discharge, highlighted the importance of this riverine system to the overall production along the Louisiana coast.     

In Situ Response of Phytoplankton to Nutrient Additions in a Tropical Coastal Lagoon, (La Mancha,Veracruz, Mexico)

An experimental in situ microcosm study was conducted in the tropical lagoon La Mancha (Gulf of Mexico) to determine whether or not nutrient limitation occurs and to examine the direct effect of an inorganic nutrient pulse on the phytoplankton community structure. The phytoplankton community response to the addition of four treatments with different combinations of nitrogen (N), phosphorus (P), and silica (Si) (+N-NH₄ ⁺, +P-PO₄ ^?, +Si-SO₃, and N:P16) showed that phytoplankton was N-limited as indicated by an increase in phytoplankton biomass (i.e., chlorophyll a) (range, 8–34 mg m^?3) during the dry season in two consecutive years (2006 and 2007). Picophytoplankton abundance significantly increased in the +N treatment (145.46 10³ cells L^?1), while microphytoplankton reached a maximum abundance (68.38 10³ cells L^?1) in the N:P16 treatment. Phytoplankton composition changed from a community initially dominated by dinoflagellates (e.g., Prorocentrum spp.) to another dominated by diatoms (Thalassiosira and Nitzschia longissima) in the N:P16 treatment. The +N treatment significantly increased Synechococcus sp. growth rates (1.3 divisions per day) (picocyanobacteria). Biomarker pigments measured in the experimental microcosms confirmed observed changes in phytoplankton groups. Our results reveal that La Mancha lagoon is a N-limited coastal system during the dry season and provides evidence of the temporal species successional patterns and mechanisms regulating the phytoplankton community response to nutrient enrichment pulses in this already eutrophic coastal lagoon.     

Changes in Nutrient and Carbon Availability and Temperature as Factors Controlling Bacterial Growth in the Northern Baltic Sea

The effects of inorganic nutrient (N and P) and glucose C treatments on bacterial growth were followed for 3 days in natural surface and deep water bacterial samples during the main post-spring bloom stages of phytoplankton growth in the northern Baltic Sea. In addition, the importance of photochemical degradation of dissolved organic matter on bacterial growth was investigated vertically (0.1–2.0 m) and spatially, in a salinity gradient from river mouth to open sea. Bacterial production was consistently C limited in the surface layer, with N or both N and P as the secondary limiting nutrients from spring to early summer and in late summer, respectively. In deep water, bacterial growth showed combined temperature and C limitation, and in spring, this also appeared to be true with surface samples. The effect of 1-day sunlight pre-treatment varied from no effect up to a 44% production increase, with clear bacterial production responses only being seen at the surface (10 cm depth). The implications of bacterial C limitation for the structure and function of the surface plankton ecosystem, including its CO₂ exchange with the atmosphere are discussed.     

Nutrient limitation of the macroalgaEnteromorpha intestinalis collected along a resource gradient in a highly eutrophic estuary

We conducted a laboratory experiment to quantify nutrient (nitrogen and phosphorus) limitation of macroalgae collected along a gradient in water column nutrient availability in Upper Newport Bay estuary, a relatively nutrient-rich system in southern California, United States. We collectedEnteromorpha intestinalis and water for use in the experiment from five sites ranging from the lower end of the estuary to the head. Initial algal tissue N and P concentrations and molar N∶P ratios—as well as water column NO₃ and total Kjeldahl nitrogen (TKN)—increased along a spatial gradient from the lower end toward the head. Water column soluble reactive phosphorus (SRP) varied among sites as well but did not follow a pattem of increasing from the seaward end toward the head. Algae from each site were assigned to one of four experimental treatments: control (C), nitrogen enrichment (+N), phosphorus enrichment (+P), and nitrogen and phosphorus enrichment (+N+P). Each week for 3 wk we replaced the water in each unit with the appropriate treatment water to mimic a poorly flushed estuary. After 3 wk, the degree of nutrient limitation ofE. intestinalis varied spatially with distance from the head of the estuary. Growth ofE. intestinalis collected from several sites increased with N enrichment alone and increased further when P was added in combination with N This indicated that N was limiting and that when N was sufficient, P became limiting. Sites from whichE. intestinalis exhibited nutrient limitation spanned the range of background water column NO₃ (12.9±0.4 to 55.2±2.1 μM) and SRP (0.8±0.0 to 2.9±0.2 μM) concentrations. Algae that were N limited had initial tissue N levels ranging from 1.18±0.03 to 2.81±0.08% dry weight and molar N∶P ratios ranging from 16.75±0.39 to 26.40±1.98.     

Increasing Severity of Phytoplankton Nutrient Limitation Following Reductions in Point Source Inputs to the Tidal Freshwater Segment of the James River Estuary

We conducted monthly bioassay experiments to characterize light and nutrient use efficiency of phytoplankton communities from the chlorophyll-a maximum located in the tidal freshwater region of the James River Estuary. Bioassay results were interpreted in the context of seasonal and inter-annual variation in nutrient delivery and biomass yield using recent and long-term data. Bioassay experiments suggest that nutrient limitation of phytoplankton production has increased over the past 20 years coinciding with reductions in point source inputs and estuarine dissolved nutrient concentrations. Despite increasing nutrient stress, chlorophyll concentrations have not declined due to more efficient nutrient usage. Greater CHLa yield (per unit of N and P) may be due to feedback mechanisms by which the presence of toxin-producing cyanobacteria inhibits grazing by benthic and pelagic filter-feeders. Seasonal patterns in nutrient limitation indicate that phytoplankton in the James respond to variations in inflow concentrations of dissolved nutrients. This association gives rise to an atypical pattern whereby the severity of nutrient limitation diminishes with low discharge in late summer due to minimal dilution of local point sources inputs by riverine discharge. We suggest that this may be a common feature of estuaries located in proximity to urbanized areas.     

Seasonal growth stimulation of sub-temperate estuarine phytoplankton to nitrogen and phosphorus: An outdoor microcosm experiment

A study of nutrient limitation of phytoplankton biomass production with emphasis on nitrate-nitrogen (NO₃ ^?) and ortho-phosphate-phosphorus (PO₄ ^3?) was conducted in Perdido Bay, Alabama-Florida. The experimental design employed 18-1 outdoor microcosms operated in a static renewal mode. Phytoplankton growth responses (i.e., growth stimulation) measured as chlorophyll a (chl a) fell into three principal categories: primary P stimulation occurred mostly during the cooler months at the upper bay (tidal brackish) and mid bay (lower mesohaline) stations; a total of 12 out of 36 experiments; primary N stimulation occurred mostly during the warmer months primarily at the mid-bay station and infrequently at the upper and lower bay stations (upper mesohaline); a total of 7 out of 36 experiments; and N+P costimulation occurred primarily during the warmer months in the upper bay and mid bay and during both warmer and cooler months of the lower bay; a total of 17 out of 36 experiments. Primary P stimulation was generally associated with high ratios of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphate (DIP) (ratio range: 18 to 288). Conversely, primary N stimulation was associated with decreasing DIN:DIP ratios (range 8–46). Redfield ratios of particulate organic N (PON) to particulate organic P (POP) often indicated N limitation (i.e., values often less than 10). PON:chl a ratios often indicated N sufficiency, but three occasions were noted where PON:POP and PON:chl a ratios were not congruent. It is difficult to reconcile the inorganic and organic N and P ratios with the relatively low DIP and DIN concentrations. The phytoplankton assemblage appeared not to be strongly nutrient-limited but, given a nutrient increase, responded differentially to N and P, both seasonally and along the longitudinal salinity gradient. Grazing pressure in concert with nutrient limitation was advanced as an hypothesis to explain N+P co-limitation.     

Phytoplankton productivity response to nutrient concentrations, light availability and temperature along an Australian estuarine gradient

Phytoplankton productivity and the factors that influence it were studied in the Logan River and southern Moreton Bay, a large embayment on the east coast of Australia. Phytoplankton productivity, dissolved and total nutrient concentrations, and turbidity were determined throughout high and low rainfall periods to characterize light and nutrient influences on productivity. Turbidity and nutrient concentrations were highest at upriver sites, but productivity was highest at the river mouth and within the river plume. Phytoplankton productivity peaked after rainfall events (>150 mg C m^?3 h^?1), commensurate with a decrease in dissolved nitrogen concentrations. Productivity responses to increased nutrient concentrations and light availability were determined in laboratory incubations. During summer, productivities at the bay sites were stimulated by nitrogen (N) enrichment, while productivities at upriver sites were stimulated by phosphorus (P) addition. Light stimulation of productivities was more pronounced at upriver sites than bay sites. The relative magnitude of nutrient and light stimulation of productivities indicate a predominance of light limitation upriver, significant N limitation within the Logan River plume, and little effect of light, N, or P at sites beyond the Logan River plume. Productivity decreased with seasonal decreases in temperature. Lower water temperatures in winter probably helped determine maximum rates of phytoplankton productivity. The combination of light and N limitation of productivity during summer, and temperature limitation during winter, account for low areal productivities (<0.6 g C m^?2 d^?1), compared with other rivers and estuaries worldwide.