ALGAL GROWTH POTENTIAL AND NUTRIENT LIMITATION IN SPRING IN THREE-GORGES RESERVOIR, CHINA

In China, especially in Three-Gorges Reservoir, our knowledge of the algal growth potential and nutrient limitation was still limited. In the spring of 2006, the water column ratios of total nitrogen/total phosphorus were investigated and algal bioassays performed to determine algal growth potential of waters and nutrient limitation of mainstream and Xiangxi Bay of Three-Gorges Reservoir. The results showed sampling sites in mainstream were co-limited by N and P or P-limited alone, and sites in Xiangxi Bay were N-limited alone. Fe likely played an important role in determining the appearance and disappearance of algal blooms of Three-Gorges Reservoir. Native algae, Pseudokirchneriella subcapitata and Cyclotella meneghiniana, had high growth potential in Three-Gorges Reservoir.

Spatial analysis for spring bloom and nutrient limitation in Xiangxi bay of three Gorges Reservoir

The spatial and temporal dynamics of physical variables, inorganic nutrients and phytoplankton chlorophyll a were investigated in Xiangxi Bay from 23 Feb. to 28 Apr. every six days, including one daily sampling site and one bidaily sampling site. The concentrations of nutrient variables showed ranges of 0.02-3.20 mg/L for dissolved silicate (Si); 0.06-2.40 mg/L for DIN (NH4N + NO2N + NO3N); 0.03-0.56 mg/L for PO4P and 0.22-193.37 mu g/L for chlorophyll a, respectively. The concentration of chlorophyll a and inorganic nutrients were interpolated using GIS techniques. The results indicated that the spring bloom was occurred twice in space during the whole monitoring period (The first one: 26 Feb.-23 Mar.; the second one: 23 Mar.-28 Apr.). The concentration of DIN was always high in the mouth of Xiangxi Bay, and PO4P was high in the upstream of Xiangxi Bay during the whole bloom period. Si seems no obvious difference in space in the beginning of the spring bloom, but showed high heterogeneity in space and time with the development of spring bloom. By comparing the interpolated maps of chlorophyll a and inorganic variables, obvious consumptions of Si and DIN were found when the bloom status was serious. However, no obvious depletion of PO4P was found. Spatial regression analysis could explained most variation of Chl-a except at the begin of the first and second bloom. The result indicated that Si was the factor limiting Chl-a in space before achieved the max area of hypertrophic in the first and second bloom period. When Si was obviously exhausted, DIN became the factor limiting the Chl-a in space. Daily and bidaily monitoring of Site A and B, representing for high DIN: PO4P ratio and low DIN:PO4P ratio, indicated that the concentration of Si was decreased with times at both site A and B, and the dramatically drop of DIN was found in the end monitoring at site B. Multiple stepwise regression analysis indicated that Si was the most important factor affect the development of spring bloom both at site A and B in time series.

Frequency limitation in the calibration of microwave test fixtures

The problem of frequency limitation arising from the calibration of asymmetric and symmetric test fixtures has been investigated. For asymmetric test fixtures, a new algorithm based on the thru-short-match (TSM) method is outlined. It is found that the conventional TSM method does not have any inherent frequency limitation, but using the same procedure with an unknown match may lead to the said problem. This limitation can be avoided by using a different algorithm. The various calibration methods for symmetric test fixtures using known standards are also discussed and the origin of the frequency limitation is identified. Several ways in avoiding the problem are proposed. There is good agreement between the theories and experimental data.

Threshold behavior in backgating in GaAs metal-semiconductor field-effect transistors: Induced by limitation of channel-substrate junction to leakage current

An analytical model is proposed to understand backgating in GaAs metal-semiconductor field-effect transistors (MESFETs), in which the effect of channel-substrate (CS) junction is included. We have found that the limitation of CS junction to leakage current will cause backgate voltage to apply directly to CS junction and result in a threshold behavior in backgating effect. A new and valuable expression for the threshold voltage has been obtained. The corresponding threshold electric field is estimated to be in the range of 1000-4000 V/cm and for the first time is in good agreement with reported experimental data. More, the eliminated backgating effect in MESFETs that are fabricated on the GaAs epitaxial layer grown at low temperature is well explained by our theory. (C) 1997 American Institute of Physics.

Frequency limitation in calibrating microwave test fixtures

The problem of frequency limitation arising in calibration of the test fixtures is investigated in this paper. It is found that at some frequencies periodically, the accuracy of the methods becomes very low, and. the denominators of the expressions of the required S-parameters approach zero. This conclusion can be drawn whether-the test fixtures, are symmetric or not. A good agreement between theory and experiment is obtained.

Breaking seasonal limitation: year-round sporogenesis in the brown alga Laminaria saccharina by blocking the transport of putative sporulation inhibitors

Year-round induction of sporogenesis of Laminaria saccharina was performed by mechanically blocking the transport of the putative sporulation inhibitors produced by the blade meristem and culturing the plants in constant short days. Sporogenesis was successfully induced by removal of the blade meristem, either by cultivating distal blade fragments or by performing a transverse cut in the frond. The earliest sorus formation after artificial induction was 10 days. The age of the sporophytes used for induction was 6-11 months or 2 years in tank-grown or field-collected sporophytes, respectively. Zoospores were successfully released in all cases. Thus, by year-round artificial induction of sporogenesis, (1) sporeling production of L. saccharina and thereafter sporophyte cultivation could be achieved without seasonal limitation, and (2) the life cycle of L. saccharina (from spore to spore) could be completed within 8 months under controlled conditions. (C) 2004 Elsevier B.V. All rights reserved.

Potential P limitation leads to excess N in the pearl river estuarine coastal plume

There is excess nitrate (NO3) in the Pearl River coastal plume in the southern waters of Hong Kong in summer. We hypothesize that phosphorus (P) limitation controls the utilization of excess NO3 due to the high N:P ratio in the Pearl River. To test this hypothesis, we conducted two 1-day cruises on July 13 and 19, 2000 to examine the response of the phytoplankton to P additions with respect to changes in biomass, uptake of nutrients and nutrient uptake ratios using a batch incubation of natural water samples collected from the Pearl River estuary and adjacent coastal waters. At a station (E1, salinity =5) in the Pearl River estuary, the N/P ratio at the surface was 46:1, (64 muM DIN: 1.3 muM PO4) and decreased to 24:1 (12 muM DIN: 0.5 muM PO4) downstream at a station (Stn 26, salinity =26) in the coastal plume south of Hong Kong. Without a P addition, NO3 in the water samples collected at E1 could not be depleted during a 9 day incubation (similar to20 muM NO3 remaining). With a P addition, NO3 disappeared completely on day 6 with the depletion of the added PO4 (2-3 muM). This was also true for a station, E4 (salinity= 15) further downstream, but within the estuary. At Stn 26, in the coastal plume south of Hong Kong, NO3 (similar to11.5 muM) was eventually depleted without the addition of PO4, but it took 8 days instead of 5 days for Stn E4. The uptake ratio of dissolved inorganic nitrogen (DIN) to PO4, without a P addition was 51:1, 43:1 and 46:1 for Stns E1, E4 and 26, respectively. With a P addition, the DIN/PO4 uptake ratio decreased to 20:1, 14:1 and 12:1, respectively, for the 3 stations. These results clearly indicate potential P limitation to utilization of NO3 in the Pearl River estuary, resulting in excess NO3 in waters of the coastal plume downstream of the estuary, some of which would eventually be transported offshore. High uptake ratios of N:P without a P addition (43N:1P) suggest that phytoplankton have a nitrogen uptake capacity in excess of the Redfield ratio of 16N: 1P by 2.5-3 times. The value of 2.5-3 times was likely a maximum that should have contained a contribution of P released from desorption of P from sediments or from regeneration by zooplankton grazing and bacterial activity during the incubation of natural water samples. Without a P addition, however, phytoplankton biomass did not increase. This means that P turnover rates or regeneration may allow phytoplankton to take up additional N in excess of the Redfield ratio and store it, but without increasing the algal biomass. Therefore, high ambient N:P ratios in excess of the Redfield ratio do indicate potential P limitation to phytoplankton biomass in this estuarine coastal ecosystem. (C) 2004 Elsevier Ltd. All rights reserved.

Interannual fluctuations in floral longevity, pollinator visitation and pollination limitation of an alpine plant (Gentiana straminea Maxim., Gentianaceae) at two altitudes in the Qinghai-Tibetan Plateau

Prolonged floral longevity and bumblebees as dominate pollinators in alpine ecosystem have been suggested to overcome pollination limitation of alpine plants arising from the decrease of pollinator activity with increasing altitude. However, this conclusion has never been examined in the Qinghai-Tibetan Plateau (QTP), the highest and largest plateau in the world. In this study, we intended to test year-to-year correlations between floral longevity, visiting frequency and pollen limitation of this species between two populations (at 3200 m and 4000 m) of Gentiana straminea in this plateau. Pollinator exclusion elongated both male and female phases greatly at both sites, and durations of both male and female phases in natural condition varied greatly over three years. The visiting frequency of bumblebees varied greatly at the higher altitude, but seemed to be stable at the lower altitude. Seed production was pollination limited in both populations in most studied years. The floral durations, pollinator frequency and pollination limitation showed no significant and consistent variations with the increase of altitude. The previous hypothesis that the prolonged floral longevity of alpine plants can compensate for low levels of pollinator visitation therefore could not be confirmed, and our results further suggested that in the QTP platform, the altitude shows no consistent effect on the reproductive performance of this species, despite that the fluctuation of visit frequency intensified at the higher altitude.