Net Nutrient Flux Research Articles

Effect of site of starch digestion on portal nutrient net fluxes in steers

Processing of maize grain is known to modulate the site of starch digestion, thus the nature and amount of nutrients delivered for absorption. We assessed the effect of site of starch digestion on nutrient net fluxes across portal-drained viscera (PDV). Three steers, fitted with permanent digestive cannulas and blood catheters, successively received two diets containing 35 % starch as dent maize grain. Diets differed according to maize presentation: dry and cracked (by-pass, BP) v. wet and ground (control, C). Ruminal physicochemical parameters were not significantly affected. Between C and BP, the decrease in ruminal starch digestion was compensated by an increase in starch digestion in the small intestine. The amount of glucose and soluble alpha-glucoside reaching the ileum was not affected. The amount of glucose disappearing in the small intestine increased from 238 to 531 g/d between C and BP, but portal net flux of glucose remained unchanged (-97 g/d). The portal O2 consumption and net energy release were not significantly affected, averaging 16 % and 57 % of metabolizable energy intake, respectively. The whole-body glucose appearance rate, measured by jugular infusion of [6,6-2H2]glucose, averaged 916 g/d. The present study shows that the increase in the amount of glucose disappearing in the small intestine of conventionally fed cattle at a moderate intake level induces no change in portal net flux of glucose, reflecting an increase in glucose utilization by PDV. That could contribute to the low response of whole-body glucose appearance rate observed at this moderate level of intestinal glucose supply.

Seasonal variations of organic-carbon and nutrient transport through a tropical estuary (Tsengwen) in southwestern Taiwan

This paper reports the fluvial fluxes and estuarine transport of organic carbon and nutrients from a tropical river (Tsengwen River), southwestern Taiwan. Riverine fluxes of organic carbon and nutrients were highly variable temporally, due primarily to temporal variations in river discharge and suspended load. The sediment yield of the drainage basin during the study period (1995-1996, 616 tonne km(-2) year(-1)) was ca. 15 times lower than that of the long-term (1960-1998) average (9379 tonne km2 year(-1)), resulting mainly from the damming effect and historically low record of river water discharge (5.02 m3 s(-1)) in 1995. The flushing time of river water in the estuary varied from 5 months in the dry season to >4.5 days in the wet season and about 1 day in the flood period. Consequently, distributions of nutrients, dissolved organic carbon (DOC) and particulate organic carbon (POC) were of highly seasonal variability in the estuary. Nutrients and POC behaved nonconservatively but DOC behaved conservatively in the estuary. DOC fluxes were generally greater than POC fluxes with the exception that POC fluxes considerably exceeded DOC fluxes during the flood period. Degradation of DOC and POC within the span of flushing time was insignificant and may contribute little amount of CO2 to the estuary during the wet season and flood period. Net estuarine fluxes of nutrients were determined by riverine fluxes and estuarine removals (or additions) of nutrients. The magnitude of estuarine removal or addition for a nutrient was also seasonally variable, and these processes must be considered for net flux estimates from the river to the sea. As a result, nonconservative fluxes of dissolved inorganic phosphorus (deltaDIP) from the estuary are -0.002, -0.09 and -0.59 mmol m(-2) day(-1), respectively, for dry season, wet season and flood period, indicating internal sinks of DIP during all seasons. Due to high turbidity and short flushing time of estuarine water, deltaDIP in the flood period may be derived largely from geochemical processes rather than biological removal, and this deltaDIP should not be included in an annual estimate of carbon budget. The internal sink of phosphorus corresponds to a net organic carbon production (photosynthesis-respiration, p-r) during dry (0.21 mmol m(-2) day(-1)) and wet (9.5 mmol m(-2) day(-1)) seasons. The magnitude of net production (p-r) is 1.5 mol m(-2) year(-1), indicating that the estuary is autotrophic in 1995. However, there is a net nitrogen loss (nitrogen fixation-denitrification < 0) in 1995, but the magnitude is small (-0.17 mol m(-2) year(-1)).

Effect of Level of Metabolizable Protein on Splanchnic Flux of Amino Acids in Lactating Dairy Cows

The response of splanchnic tissue metabolism to different levels of metabolizable protein (MP) was measured in 6 catheterized multiparous lactating Holstein cows. Three diets, balanced to provide similar energy intakes and increasing amounts of MP (g/d)—1922 (low), 2264 (medium), and 2517 (high)—were fed during 21-d experimental periods according to a replicated Latin square. On d 18, 19, or 20, six hourly blood samples were collected simultaneously from the portal and hepatic veins plus an artery to determine net fluxes of nutrients across the portal-drained viscera and the liver. Yields of milk and protein increased, as did urinary N excretion with increasing MP. Portal absorption of essential amino acids (EAA) increased linearly with increasing MP supply, as did liver removal of His, Met, and Phe. In contrast, liver removal of the branched-chain AA (BCAA) and lysine was unaffected by diets. With increasing MP, the ratio of milk output to postliver supply of BCAA, Thr, and Lys decreased linearly, indicating oxidation of these AA in the peripheral tissues. Concomitant to a decreased catabolism of EAA in the liver (His, Met, Phe, and Thr) and/or in peripheral tissues (BCAA, Lys, and Thr), the efficiency of transfer of absorbed EAA into milk protein decreases markedly as protein supply increases. The efficiency of transfer of absorbed AA into milk also varies greatly between AA. These 2 important factors should be taken into account when building predictive schemes for milk protein output.

Effects of ruminally protected betaine and choline on net flux of nutrients across the portal-drained viscera and liver of meat goat wethers consuming diets differing in protein concentration

Six Boer × Spanish goat wethers (43 ± 5.1 kg BW) were used in an experiment with a 2 × 3 factorial arrangement of treatments to investigate effects of dietary CP level (9% and 15% DM) and supplementation with ruminally protected betaine or choline (0.9% DM) on plasma concentrations and net fluxes of oxygen, ammonia N, non-esterified fatty acids (NEFA), triacylglycerols (TG) and cholesterol across the portal-drained viscera (PDV) and liver. Neither betaine nor choline affected blood flow, packed cell volume, hemoglobin concentration or oxygen consumption. Blood flow and oxygen consumption were greater ( P < 0.05) for 15% versus 9% dietary CP. Arterial plasma ammonia N concentration was greater ( P < 0.05) for 9% versus 15% CP. Compared with Control, choline supplementation decreased ( P < 0.05) PDV release and hepatic uptake of ammonia N with the 15% CP diet, whereas betaine decreased ( P < 0.05) PDV release and hepatic uptake of ammonia N with 9% dietary CP. With 9% dietary CP, the concentration of NEFA in arterial, hepatic venous and portal venous plasma ranked ( P < 0.05) choline < Control < betaine; with 15% CP, NEFA concentration also was greater ( P < 0.05) for betaine versus Control, although the magnitude of difference was smaller than with 9% CP. The only treatment effect on NEFA net fluxes was greater ( P < 0.05) hepatic uptake with 9% CP than with 15%. Plasma TG concentrations also were increased ( P < 0.05) by betaine with 9% dietary CP, whereas choline did not have any influence with either dietary CP level. Concentrations and net fluxes of cholesterol were similar among treatments. In conclusion, these data indicate that potential effects of ruminally protected betaine on performance of ruminants might involve changes in lipid metabolism, with the magnitude of alteration varying with dietary CP level.

Splanchnic energy expenditure and net nutrient flux in sheep fed diets with varying concentrate:forage ratios

Splanchnic energy expenditure and net nutrient flux in sheep fed diets with varying concentrate:forage ratios

Digestion and nutrient net fluxes across the rumen, and the mesenteric- and portal-drained viscera in sheep fed with fresh forage twice daily: net balance and dynamic aspects.

Digestion and portal net flux of nutrients were studied in sheep fed twice daily with fresh orchard-grass. Digestive flows were measured in six fistulated sheep using the double-marker technique. Three sheep were fitted with catheters and blood-flow probes, allowing nutrient net flux measurements across the portal-drained viscera (PDV), the mesenteric-drained viscera (MDV) and the rumen. Total tract apparent digestion of N was similar to portal net appearance of N, calculated as the sum of free amino acids (FAA), peptide amino acids (PAA), NH3, and urea net fluxes. PAA accounted for 25 % of non-protein amino acid net release across the PDV. With the exception of glycine and glutamate, the small intestine was the main contributor to this PAA net release. The essential amino acid (EAA) apparent disappearance between the duodenum and the ileum was lower than the net appearance of EAA (FAA + PAA) across the MDV. The value of PDV:MDV flux of free EAA was, on average, 78 %. The rumen accounted for 30 % of the net uptake of EAA by the PDV tissues not drained by the mesenteric vein. Rumen net release of acetate, propionate, butyrate, 3-hydroxybutyrate, and lactate accounted for 70, 55, 46, 77 and 52 %, respectively, of their portal net releases. Conversely, the small intestine was a net consumer of arterial acetate and 3-hydroxybutyrate. Dynamic study of nutrient net fluxes across the PDV showed that throughout a feeding cycle, the liver faced a constant flux of amino acids (AA), whereas volatile fatty acid and NH3 net fluxes varied in response to the meal. The present study specified, in forage-fed sheep, the partitioning of nutrient net fluxes across the PDV and the role of peptides in portal net release of AA.

Eddy‐driven sources and sinks of nutrients in the upper ocean: Results from a 0.1° resolution model of the North Atlantic

A nitrate‐based model of new production is incorporated into eddy‐resolving (0.1°) simulations of the North Atlantic. The biological model consists of light and nutrient limited production within the euphotic zone and relaxation of the nitrate field to climatology below. Sensitivity of the solutions to the parameters of the biological model is assessed in a series of simulations. Model skill is quantitatively evaluated with observations using an objective error metric; simulated new production falls within the range of observed values at several sites throughout the basin. Results from the “best fit” model are diagnosed in detail. Mean and eddying components of the nutrient fluxes are separated via Reynolds decomposition. In the subtropical gyre, eddy‐driven vertical advection of nutrients is sufficient to overcome the mean wind‐driven downwelling in the region and fuels a significant fraction of the annual new production in that area. In contrast, eddies constitute a net sink of nutrients in the subpolar gyre. Geostrophic adjustment to deep winter convection through mesoscale processes causes a net flux of nutrients out of the euphotic zone; the magnitude of this sink is sufficient to counterbalance the mean wind‐driven upwelling of nutrients over much of the region. On the basis of these simulations it appears that the oceanic mesoscale has major impacts on nutrient supply to, and removal from, the euphotic zone.

Splanchnic Metabolism of Dairy Cows During the Transition From Late Gestation Through Early Lactation

Blood flow and net nutrient fluxes for portal-drained viscera (PDV) and liver (total splanchnic tissues) were measured at 19 and 9 d prepartum and at 11, 21, 33, and 83 d in milk (DIM) in 5 multiparous Holstein-Friesian cows. Cows were fed a grass silage-based gestation ration initially and a corn silage-based lactation ration peripartum and postpartum. Meals were fed at 8-h intervals and hourly (n = 8) measures of splanchnic metabolism were started before (0730h and 0830h) feeding at 0830h. Dry matter intakes (DMI) at 19 and 9 d prepartum were not different. Metabolism changes measured from 19 to 9 d prepartum were lower arterial insulin and acetate, higher arterial nonesterified fatty acids and increased net liver removal of glycerol. After calving, PDV and liver blood flow and oxygen consumption more than doubled as DMI and milk yield increased, but 85 and 93% of the respective increases in PDV and liver blood flow at 83 DIM had occurred by 11 DIM. Therefore, factors additional to DMI must also contribute to increased blood flow in early lactation. Most postpartum changes in net PDV and liver metabolism could be attributed to increases in DMI and digestion or increased milk yield and tissue energy loss. Glucose release was increasingly greater than calculated requirements as DIM increased, presumably as tissue energy balance increased. Potential contributions of lactate, alanine, and glycerol to liver glucose synthesis were greatest at 11 DIM but decreased by 83 DIM. Excluding alanine, there was no evidence of an increased contribution of amino acids to liver glucose synthesis is required in early lactation. Increased net liver removal of propionate (69%), lactate (20%), alanine (8%), and glycerol (4%) can account for increased liver glucose release in transition cows from 9 d before to 11 d after calving.

Effect of Supply of Metabolizable Protein on Splanchnic Fluxes of Nutrients and Hormones in Lactating Dairy Cows

The effect of the supply of metabolizable protein on splanchnic fluxes of nutrients and hormones was measured in six catheterized late-lactation Holstein cows in a crossover design. Two isonitrogenous diets (16.3% CP), but differing in rumen protein degradability and estimated metabolizable protein (MP) supply (1654g/d, Lo-MP; 1930g/d, Hi-MP) were fed, each over a 35-d experimental period. On d 34 or 35, net fluxes of nutrients and hormones across the portal-drained viscera, the liver, and total splanchnic tissues were determined. Portal absorption of total, essential, nonessential, and branched-chain amino acids (AA) increased with the Hi-MP diet. Approximately 76% of the additional metabolizable protein supply was recovered as extra AA-N absorption in the portal vein. Liver removal of AA was not different between diets, and this resulted in a greater net release across the splanchnic tissues for the Hi-MP diet. This extra AA supply provided substrates for the observed increased milk protein yield for the Hi-MP diet. Fractional efficiencies of conversion of absorbed individual essential AA into milk protein ranged from 0.42 to 0.68. The corresponding efficiencies for utilization of postsplanchnic AA supply were 0.42 to 1.80. Provision of methionine, phenylalanine, and histidine beyond the liver were similar to outputs in milk protein but the other essential AA were supplied to peripheral tissues in excess of milk output, indicative of oxidative mechanisms in nonhepatic tissues. Net fluxes of glucose, NH3-N, and urea were not affected by the diets. Neither arterial concentrations of insulin, somatotropin, or IGF-1, nor net transfers across the portal-drained viscera or liver of insulin, were affected by the diets. Although portal release of glucagon was not different between the diets, a smaller proportion was removed by the liver on the Hi-MP diet. Metabolism of AA across the splanchnic tissue bed is a major determinant of the quantity and the profile of AA delivered to peripheral tissues.

Effects of Abomasal Vegetable Oil Infusion on Splanchnic Nutrient Metabolism in Lactating Dairy Cows

Changes in the metabolism of nutrients by the portal-drained viscera (PDV) and liver may contribute to the reduction in dry matter intake (DMI) and other production responses generally observed in lactating dairy cows fed supplemental long-chain fatty acids (LCFA). In the present study, effects of a 7-d abomasal infusion of vegetable oil on arterial concentration and splanchnic (PDV and liver) metabolism of nutrients were measured in six cows at 55 (early lactation [ELAC]) and 111 (midlactation [MLAC]) d postpartum. Cows were fed for ad libitum DMI at 8-h intervals, and blood samples for measurement of splanchnic metabolism were obtained over 8h beginning 2h before feeding at 0830h. Blood flow for the PDV and liver was increased by oil infusion and was greater in ELAC, despite similar-feed DMI during blood sampling. Increased blood flow in ELAC was associated with greater liver oxygen removal and glucose release that accompanied greater milk yield. In contrast, oil infusion had no effect on splanchnic oxygen use. Greater blood flow during oil infusion may have been due to specific effects of intestinal LCFA supply on PDV blood flow. Net PDV release and liver removal of branched-chain volatile fatty acids (VFA) and ammonia were increased by oil infusion. Net PDV release of longer-chain (4 and 5C) VFA and NEFA was greater in ELAC, but net PDV flux of other nutrients was not affected by lactation stage, possibly due to the similarity of feed DMI. Oil infusion increased arterial concentration and net PDV release and liver removal of NEFA, and it decreased net liver release and arterial concentration of glucose. Effects of oil infusion on liver glucose release were associated with decreased daily DMI. In ELAC, arterial concentration and net liver removal of NEFA were also increased, but liver release of glucose was greater than in MLAC. Oil infusion and ELAC both increased net liver removal of l-lactate. The resulting decrease in net total splanchnic release of l-lactate in ELAC reflects decreased tissue energy balance of the cows. Generally, stage of lactation and relative milk yield had greater effects on metabolism of the liver than the PDV, in which metabolism was largely dictated by DMI. In the present study, there was little evidence to suggest an effect of stage of lactation on the metabolic response of the PDV and liver to postruminal LCFA supply.

Effects of nutrient supply and dietary bulk on O2 uptake and nutrient net fluxes across rumen, mesenteric- and portal-drained viscera in ewes.

We assessed the effects of nutrient supply and dietary bulk, both increasing with hay intake, on O2 uptake and nutrient net fluxes across the portal-(PDV) and mesenteric- (MDV) drained viscera, and the rumen in adult ewes. Four ewes, fitted with a ruminal cannula, with catheters in the mesenteric artery, the portal, mesenteric and right ruminal veins, and with a blood flow probe around the right ruminal artery, were used in a 4 x 4 Latin square design. Treatments consisted of 500 g DM/d hay (LL, low bulk and low nutrient supply), 500 g DM/d hay + infused nutrients (LH, low bulk and high nutrient supply), 750 g DM/d hay + infused nutrients (MH, medium bulk and high nutrient supply), and 1,000 g DM/d hay (HH, high bulk and high nutrient supply). Infused nutrients consisted of volatile fatty acids (VFA) and casein dissolved in salts and infused continuously in the rumen to provide the same amount of metabolizable energy (7.6 MJ/d) and digestible protein (63 g/d) for LH, MH, and HH. Both increases in bulk and nutrient supply increased O2 uptake in the MDV and PDV. Dietary bulk stimulated mainly blood flow, whereas nutrient supply stimulated mainly O2 extraction rate. The O2 uptake by the rumen was not significantly affected by hay intake, although blood flow increased due to nutrient supply. Increase in hay intake had no effects on portal net release of lactate and net uptake of glucose but increased VFA, 3-D-hydroxybutyrate, ammonia, and amino acids (AA) net release and urea net uptake across PDV. The increase in portal nutrient net fluxes with hay intake was entirely related to the increase of nutrient supply for VFA, 3-D-hydroxybutyrate, ammonia, and urea, irrespective of the amount of casein infused for AA. Dietary bulk had no effect on total energy net release in the portal vein. We conclude that despite the increase in portal O2 uptake, increasing dietary bulk had no significant impact on portal recovery of energy. In ruminal tissues, which were the main site of energy absorption, O2 uptake appeared low and was not sensitive to dietary manipulation. In contrast, in mesenteric tissues, which contribute poorly to energy absorption with forage diets, O2 uptake appeared high and very sensitive to dietary manipulation.

Economics of visceral energy metabolism in ruminants: Toll keeping or internal revenue service?

Measurements across a range of productive states show that the portal-drained viscera and liver, or the total splanchnic tissues, account for 40 to 50% of body oxygen consumption or heat. This high rate of metabolism is in part attributable to high rates of protein turnover and thus, AA utilization, as well as other “service” functions supporting nutrient assimilation and “waste management.” This metabolic intensity and the anatomic position of absorptive and liver tissues have led to the assumption that the tissues that assimilate and process incoming nutrients from the diet exact a toll in payment for their entry. This “toll” is believed to reduce the extent to which absorbed nutrients gain admission to the arterial blood pool and reach “productive” organs such as the mammary gland or skeletal muscle. Measurements of net nutrient flux generally support this concept of splanchnic metabolism “restricting entry” and, thus, dictating supply. On a net basis the appearance of the major carbon-based nutrients absorbed into the portal vein is typically low compared to their rate of disappearance from the gut lumen. An alternative interpretation is that this low net recovery of absorbed nutrients across splanchnic tissues is attributable to extensive metabolism of nutrients from the arterial pool, which masks true rates of absorption. In this scenario, any tax to support community services is paid using internal funds. Measurements of nutrient kinetics, based on isotopic labeling, support the latter scenario. In the case of the liver, catabolism of AA is driven in part by supply and demand, with overpopulation dealt with by deportation, restructuring, or the metabolic equivalent of cremation. Similarly, relative rates of AA metabolism by the gut and mammary gland vary with requirement. Metabolism of many energy-yielding nutrients varies with supply, demand, and the need for community services such as waste management.

Ileal absorptive adaptation to jejunal resection and extrinsic denervation: implications for living-related small bowel transplantation

Net absorption of water, electrolytes, and simple nutrients decreases early after jejunoileal autotransplantation (extrinsic denervation) in a canine model but recovers toward normal by 8 weeks. However, the ability of the extrinsically denervated ileum to adapt after total jejunectomy, which would be relevant as a model of segmental small bowel transplantation, remains unknown. Two groups of five dogs each were studied before and 2 weeks and 12 weeks after 50% proximal enterectomy. A control group remained neurally intact, whereas the other group underwent extrinsic denervation (Ext Den) of the remaining ileum. Using a perfusion technique, net absorption of water, electrolytes, and five simple nutrients (glucose, arginine, glutamine, and oleic and taurocholic acids) was measured at the three time points. Ileal morphometry was also evaluated. All dogs developed diarrhea, which resolved by 12 weeks in all but two of the Ext Den dogs. Weight in both groups was decreased at 2 weeks ( P <0.05), returned to normal at 12 weeks in control dogs, but remained low in Ext Den dogs ( P <0.05). Maximal weight loss was greater in the Ext Den group ( P <0.05). No consistent or important differences in net absorptive fluxes of water, electrolytes, or simple nutrients were noted either within or between groups at any time point. Villous height, crypt depth, and longitudinal muscle width increased significantly at 12 weeks after jejunectomy in the Ext Den dogs, but not in the control dogs ( P <0.05). Extrinsic denervation of the ileum results in persistent weight loss after proximal 50% enterectomy. Despite diarrhea, only minor changes in electrolyte absorption occur, and ileal net absorption of simple nutrients remains unaffected. The ileum of extrinsically denervated dogs undergoes a more prominent morphometric adaptation after jejunectomy. Extrinsic denervation necessitated by small bowel transplantation, independent of immune effects, does not appear to suppress the ileal adaptive response to maintain net absorption of water, electrolytes, and simple nutrients.

Nutrient Dynamics in Vegetated and Unvegetated Areas of a Southern Everglades Mangrove Creek

Flow-through flumes were used to quantify net areal fluxes of nutrients in the fringe mangrove zone of lower Taylor River in the southern Everglades National Park. We also quantified net areal fluxes along the open water portion of the channel to determine the relative importance of either zone (vegetated vs. unvegetated) in the regulation of nutrient exchange in this system. Taylor River's hydrology is driven mainly by precipitation and wind, as there is little influence of tide. Therefore, quarterly samplings of the vegetated and unvegetated flumes were slated to include typical wet season and dry season periods, as well as between seasons, over a duration of two years. Concentrations of dissolved and total organic carbon (DOC and TOC) were highest during the wet season and similar to one another throughout the study, reflecting the low particulate loads in this creek. Dissolved inorganic nitrogen (nitrate+nitrite+ammonium) was 10–15% of the total nitrogen (TN) content, with NO−x and NH+4 showing similar concentration ranges over the 2-year study. Soluble reactive phosphorus (SRP) was usually <0·05μM, while total phosphorus (TP) was typically an order of magnitude higher. Net areal fluxes were calculated from nutrient concentration change over the length of the flumes. Most flux occurred in the vegetated zone. Dissolved inorganic nitrogen and DOC were usually taken up from the water column; however, we saw no seasonal pattern for any constituent over the course of this study. Total nutrients (TOC, TN, and TP) showed little net exchange and, like SRP, had fluxes that shifted irregularly throughout the study. Despite the lack of a clear seasonal pattern, there was a great deal of consistency between vegetated flumes, especially for NO−x and NH+4, and fluxes in the vegetated flumes were generally in the same direction (import, export, or no net flux) during a given sampling. These findings suggest that the fringe mangrove zone is of considerable importance in regulating nutrient dynamics in lower Taylor River. Furthermore, the influence of this zone may at times extend into northeast Florida Bay, as the bay is the primary recipient of water and nutrients during the wet season.

Transport of mass, heat, salt, and nutrients in the southern California Current System: Annual cycle and interannual variability

Net fluxes of mass, heat, salt, nutrients, oxygen, and chlorophyll into a control volume within the southern California Current System (CCS) were computed from data collected on 55 cruises over a 14 year period (1984–1997). This analysis builds on an earlier work [Roemmich, 1989] by using an additional 39 cruises over 10 years, allowing for reliable estimates of the temporal variability in the fluxes on seasonal and interannual timescales and a reduction in the corresponding error budgets. A close balance was found between geostrophic convergence and Ekman divergence for the 14 year, seasonal, and interannual cruise subsets using three different wind products. Wind data taken concomitantly with the hydrographic sampling provided the best balance and hence the best flux estimates. The southern CCS was found to be a region with higher evaporation over precipitation and net heat gain by the ocean from the atmosphere (86 W m−2 in the 14 year mean) in all seasons. Significant variability in both the Ekman and geostrophic transports and the net property fluxes was found to be related to low‐frequency (interpentadal and El Niño‐Southern Oscillation timescale) changes in the dominant wind and circulation patterns in the CCS. Variability in primary productivity, estimated from the derived nutrient fluxes, accompanied the environmental changes. Application of this model to the ongoing data collection will further reduce the error bars on the fluxes and will allow for continued monitoring of changes in the physical and biological structure of the southern CCS.

Effect of underfeeding on metabolism of portal-drained viscera in ewes

We investigated whether short-term underfeeding could induce adaptative mechanisms in portal-drained viscera (PDV) that would allow nutrients to be spared for vital functions in adult ewes. Six ewes (three of them fitted with catheters in the mesenteric artery and portal and mesenteric veins) were fed, in a double Latin square design (2 weeks per experimental period), a regrowth of natural grassland hay at 143 (high; H), 88 (medium; M) and 51 (low; L) % of their energy maintenance requirements. The digestibility of the diet was measured in all six ewes and the net portal fluxes of nutrients in the three catheterized ewes. The organic matter content and N digestibility of the diet were not affected by underfeeding. Urinary and faecal N losses and N balance were linearly related to feed intake. Arterial concentration of acetate was linearly related to feed intake. Arterial concentrations of the other volatile fatty acids, 3-hydroxybutyrate, lactate, glucose, NH3, urea and total amino acids were not affected by underfeeding. Arterial concentration of non-esterified fatty acids (NEFA) increased with underfeeding. The portal net release of all volatile fatty acids, 3-hydroxybutyrate and NH3 were linearly related to intake. The portal net flux of both essential and non-essential amino acids, and thus total amino acids, remained unchanged between levels H and M, and decreased between levels M and L. A significant net uptake for glycine and total non-essential amino acids occurred at level L. The portal net uptake of glucose, urea, glutamate and glutamine, and the portal net release of lactate and NEFA were not affected by underfeeding. Summation of portal energy fluxes indicated that 51% of the metabolizable energy intake was recovered in the portal blood with the three levels of intake. In conclusion, no quantitative adaptation to spare energy, in terms of percentage of intake, occurred in PDV of short-term underfed ruminants, but the pattern of absorption of energetic nutrients was modified.

Metabolismo de los lípidos durante el embarazo

El flujo neto de nutrientes, electrolitos, vitaminas, y otros compuestos de la madre a el feto es necesario para asegurar un suministro constante para el metabolismo energético fetal así como para el crecimiento y diferenciación de tejidos. En la formación de sistemas fetales como el nervioso y en el balance de requerimientos extra de energía en el embarazo avanzado es indispensable la utilización de los lípidos. En la presente revisión podemos recordar los mecanismos básicos y la secuencia de eventos metabólicos relacionados con estos durante la gestación.

Effect of composition of ruminally-infused short-chain fatty acids on net fluxes of nutrients across portal-drained viscera in underfed ewes

Four ewes, each fitted with a rumen cannula and with catheters in the mesenteric artery and portal and mesenteric veins, received continuous intrarumen infusions of water or of short-chain fatty acids (SCFA). SCFA infusions were isoenergetic (83 kJ/h) and provided rumen molar proportions (acetate:propionate:butyrate) of 70:20:10, 50:40:10 or 50:20:30. The rumen SCFA production rate with the basal diet was 90.0, 23.1 and 8.8 mmol/h for acetate, propionate and butyrate respectively. Portal net fluxes indicated that 74, 67 and 22-30% of infused acetate, propionate and butyrate respectively, reached the portal vein. Portal net release of beta-hydroxybutyrate increased with SCFA infusions, irrespective of the amount of butyrate infused. Portal net release of lactate decreased with high-butyrate infusion. Portal net uptake of glucose increased with the SCFA infusions. In ewes infused with water, a portal net uptake of total amino acids (AA) was observed. SCFA infusions decreased the uptake of nonessential AA (glutamate, glycine, but not glutamine) and increased the net release of tyrosine and essential AA (isoleucine, leucine). Portal net fluxes of AA were similar with both high-acetate and high-propionate infusions. Lower net uptake of glutamine and net release of most essential AA and some nonessential AA were observed with the high-butyrate infusion. Energetic summation of portal net release was not significantly different between the three SCFA infusions, although it tended to be lower with high-butyrate infusion. This may be related to the higher trophic effect of butyrate on the digestive mucosa.

Influences of physical processes on the ecosystem in Jiaozhou Bay: A coupled physical and biological model experiment

In this paper we have used a three‐dimensional coupled physical and biological model to examine the marine ecosystem of Jiaozhou Bay. Physical processes included (1) the M2 tide, (2) river discharges, and (3) winds. The biological model described a simple, phosphorous‐based, lower trophic food web system. The model results showed that tidal mixing had a direct impact on temporal and spatial distributions of nutrients and phytoplankton as well as on shellfish aquaculture. Nutrients and phytoplankton were well mixed vertically by tidal motion. Their concentrations were highest around northwestern and northern regions of the bay near river sources and decreased with water depth from the inner bay to the outer bay. A phytoplankton bloom can occur around the northwestern coast due to the “accumulation” of nutrients under southeasterly wind conditions. The flux analysis suggests that in summer the nutrients in Jiaozhou Bay was directly supplied and maintained by physical processes, but the temporal variation of phytoplankton was controlled dominantly by biological processes associated with nutrient uptake, grazing by zooplankton, and consumption by shellfish. Shellfish aquaculture can modify the bay‐scale balance of the ecosystem in Jiaozhou Bay. The loss of phytoplankton in shellfish aquaculture sites tended to be compensated by advection and diffusion from the surrounding waters. A large consumption of phytoplankton by shellfish can cause a net flux of phytoplankton from the Yellow Sea to Jiaozhou Bay, although there was a net nutrient flux flowing out of the bay. The physical mechanism for the water exchange between Jiaozhou Bay and the Yellow Sea was controlled mainly by a chaotic process associated with nonlinear interactions between oscillating tidal currents and double residual eddies.

Benthic flux of biogenic elements on the Southeastern US continental shelf: influence of pore water advective transport and benthic microalgae

In situ, paired light and dark benthic flux chamber incubations were used to estimate the exchange of nutrients, oxygen and inorganic carbon across the sediment – water interface of the South Atlantic Bight (SAB) continental shelf. The results indicate that physically forced non-diffusive pore water transport and benthic primary production (BPP) by sea floor microalgae exert a major influence on benthic exchange rates on the mid- and outer-continental shelf (depths of 14–40 m). Light fluxes to the sea floor and sediment photosynthetic pigment distributions determined on two, widely spaced cross-shelf transects suggest that BPP may occur over 84% of the SAB continental shelf area. Microalgal gross BPP rates at all study sites averaged 400±260 mg C m −2 d −1 between May and September 1996 while water column primary productivity averaged 682±176 mg C m −2 d −1, implying a total primary productivity for this region of approximately 1100 mg C m −2 d −1 (1.6 times the water column productivity alone). The results are also consistent with the advective transport of pore waters. Benthic flux chambers appear to retard this exchange, affecting the accuracy of derived net fluxes. Given our inability to relate pore water gradients to fluxes in non-diffusive regimes and to mimic natural advective transport in intact core incubations, traditional techniques such as pore water gradient diffusion calculations or shipboard core incubations also may not provide accurate flux estimates. Because of these limitations, fundamental questions remain concerning the processes that control nutrient inventories in pore waters and the magnitude of the net benthic flux of nutrients on the sandy SAB shelf.