Phosphorus Excretion Rates Research Articles

Teaching Nutrient Cycling and Climate Change Concepts Using Excretion Experiments with Common Fish

Many high school students learn about nutrient cycling during biology, environmental science, and agriculture classes. These lessons often focus on soil and plants, and nutrient cycling is usually taught independently from climate change. Scientists know that animals, including fish, can have strong effects on nutrient cycling (i.e., nitrogen and phosphorus) in ecosystems. Additionally, research has shown that nitrogen and phosphorus excretion rates of animals increase with water temperatures. We worked with high school students to design and conduct nutrient excretion experiments using common fish (zebrafish) to explore the impact of climate change on nutrient cycling. This allowed students to have hands-on laboratory experience. In 2021, we worked with students participating in a residential summer program in Georgia. Meanwhile, in 2022, students enrolled in the local high school visited the university campus on two occasions to participate in the experiments, and we once again worked with students in Georgia. Students from all three groups showed an increased understanding of the role of animals in nutrient cycling and ways climate change may impact these processes, despite variable results from the excretion experiments. Students also showed increased understanding of science processes and were more likely to feel like part of the science community. We believe that these experiments can be done in high school classrooms to expand students’ understanding of the scientific process, nutrient cycling, and climate change.

Copper and Gold Nanoparticles Increase Nutrient Excretion Rates of Primary Consumers.

Freshwater ecosystems are exposed to engineered nanoparticles through municipal and industrial wastewater-effluent discharges and agricultural nonpoint source runoff. Because previous work has shown that engineered nanoparticles from these sources can accumulate in freshwater algal assemblages, we hypothesized that nanoparticles may affect the biology of primary consumers by altering the processing of two critical nutrients associated with growth and survivorship, nitrogen and phosphorus. We tested this hypothesis by measuring the excretion rates of nitrogen and phosphorus of Physella acuta, a ubiquitous pulmonate snail that grazes heavily on periphyton, exposed to either copper or gold engineered nanoparticles for 6 months in an outdoor wetland mesocosm experiment. Chronic nanoparticle exposure doubled nutrient excretion when compared to the control. Gold nanoparticles increased nitrogen and phosphorus excretion rates more than copper nanoparticles, but overall, both nanoparticles led to higher consumer excretion, despite contrasting particle stability and physiochemical properties. Snails in mesocosms enriched with nitrogen and phosphorus had overall higher excretion rates than ones in ambient (no nutrients added) mesocosms. Stimulation patterns were different between nitrogen and phosphorus excretion, which could have implications for the resulting nutrient ratio in the water column. These results suggest that low concentrations of engineered nanoparticles could alter the metabolism of consumers and increase consumer-mediated nutrient recycling rates, potentially intensifying eutrophication in aquatic systems, for example, the increased persistence of algal blooms as observed in our mesocosm experiment.

Monthly Abundance Patterns and the Potential Role of Waterbirds as Phosphorus Sources to a Hypertrophic Baltic Lagoon

Compared to external loads from tributaries and sediment recycling, the role of waterbirds as phosphorus (P) sources in estuaries is overlooked. We performed monthly ship-based surveys of waterbird abundance in the Lithuanian part of the Curonian Lagoon, calculated their potential P excretion, and compared its relevance to the riverine inputs. Phosphorus excretion rates for the censused species were assessed accounting for variations of body weights, daily feces production and their P content, and assigning species to different feeding and nutrient cycling guilds. During the study period (March–November 2018), 32 waterbird species were censused, varying in abundance from ~18,000–32,000 (October–November) to ~30,000–48,000 individuals (June–September). The estimated avian P loads during the whole study period varied between 3.6 and 25 tons, corresponding to an area load between 8.7 and 60.7 mg P m−2. Waterbird release of reactive P to the system represented a variable but not negligible fraction (1%–12%) of total external P loads, peaking in June–September and coinciding with cyanobacterial blooms. This study is the first in the Baltic Sea region suggesting that waterbirds are potentially important P sources to phytoplankton in large estuaries, in particular, during low discharge periods.

EOS789, a novel pan-phosphate transporter inhibitor, is effective for the treatment of chronic kidney disease–mineral bone disorder

Chronic kidney disease is characterized as impaired renal function along with the imbalance and dysregulation of mineral metabolism; recognized as chronic kidney disease-mineral and bone disorder. Hyperphosphatemia, characterized by altered phosphate homeostasis along with elevated fibroblast growth factor-23 and intact parathyroid hormone, is such an alteration of mineral metabolism. We discovered a novel inhibitor, EOS789, that interacts with several sodium-dependent phosphate transporters (NaPi-IIb, PiT-1, and PiT-2) known to contribute to intestinal phosphate absorption. This inhibitor dose-dependently increased the fecal phosphorus excretion rate and inversely decreased the urinary phosphorus excretion rate in normal rats, suggesting inhibition of intestinal phosphorus absorption. In rats with adenine-induced hyperphosphatemia, EOS789 markedly decreased the serum phosphate, fibroblast growth factor-23, and intact parathyroid hormone below values found in normal control rats. Notably, this pan-phosphate transporter inhibitor exhibited a more potent effect on serum phosphate than a NaPi-IIb-selective inhibitor in rats with hyperphosphatemia indicating that PiT-1 and PiT-2 play important roles in intestinal phosphate absorption. Moreover, in a long-term study, EOS789 sustained the suppression of serum phosphorus in parallel with fibroblast growth factor-23 and intact parathyroid hormone and ameliorated ectopic calcification of the thoracic aorta. Additionally, EOS789 treatment also ameliorated kidney deterioration in rats with progressive kidney injury, probably due to the strict phosphate control. Thus, EOS789 has potent efficacy against hyperphosphatemia and its complications and could provide a significant benefit to patients who are ineffectively treated with phosphate binders.

Effects of oilseed meals on pellet characteristics, faecal matter production, postprandial ammonia and phosphorus excretion rates in Nile tilapia, Oreochromis niloticus

ABSTRACT This study assessed the physical characteristics, faecal, ammonia and phosphorus waste production rates of mixtures of groundnut, cottonseed, copra and soybean meals as a replacement for fishmeal in Nile tilapia diets. Six experimental diets were formulated such that the oilseed mixtures contributed 80% of total dietary protein. A fishmeal-based diet served as the control. The diets were randomly assigned in triplicates to the experimental tanks, containing 15 juvenile tilapia with a bulk weight of ∼1 kg. Ammonia and phosphorus excretion rates of the diets were monitored at 3-hour intervals over 24 h. The pellet characteristics, faecal and nutrient waste production were assessed. The control diet recorded a mean bulk density of 376.00 ± 5.19 g L−1 which was lower than the oilseed-based diets which ranged from 550.67 ± 3.06 g L−1 to 614.27 ± 2.08 g L−1. There was a positive correlation (p < 0.0001; r = 0.93) between the sinking rates of pellets and their bulk densities. The oilseed-based diets recorded 4–45% higher faecal matter production compared to the control diet. Fish fed Diet 3 had the lowest Total Ammonia-nitrogen (TAN) excretion rate of 106.14 ± 1.95 mg kg−1 while the control diet recorded the highest TAN excretion of 162.31 ± 4.39 mg kg−1. Accumulated dissolved phosphorus was 64.52–74.09% lower in the oilseed-based diets compared to the control.

Changes in the body chemical composition and the excretion of nitrogen and phosphorus during long-term starvation of Antarctic fish Notothenia coriiceps and Notothenia rossii

This study examined the effect- of starvation on the chemical composition (total protein, crude fat, dry matter, ash) and the excretion of nitrogen (N) and phosphorus (P) of the Antarctic fish Notothenia coriiceps and Notothenia rossii. Starvation resulted in a decrease in lipid content in both species. In Notothenia coriiceps the body fat content decreased from 2.3% to 1.88% and in Notothenia rossii from 0.82% to 0.6%. Excretion rates of N and P were highest on the first day of starvation, then gradually decreased. Average excretion rates (in µmol N or P h−1 100 g−1) of ammonium nitrogen, organic nitrogen, reactive phosphorus, and organic phosphorus respectively were higher for N. coriiceps (8.71, 2.93, 1.09, 0.46) than N. rossii (6.28, 4.94, 0.54, 0.13). During starvation, the proportion of TON in the total excreted nitrogen was on average 43.5% for N. rossi and 24.1% for N. coriiceps, and remained at a relatively stable level. The study confirmed the natural adaptation of the studied fish to the long-term lack of food, and that the excreted loads of N and P may be a source of biogens for autotrophs and serve as a chemical signal in trophochemoreception. The inter-species differences in the excretion of N and P could be due to the differences in fish size, gonad stage of maturity, and nutritional preferences.

Biomass loss and change in species dominance shift stream community excretion stoichiometry during severe drought

Abstract Animals contribute significantly to nutrient cycling through excretion, but most studies consider their effects under relatively benign abiotic conditions. Disturbances such as drought may alter animals’ nutrient contributions through shifts in species composition and biomass. Headwater streams are particularly vulnerable to extreme climate events and thus might show rapid changes in stream biota and their ecosystem effects. We tested how biomass and subsequent ecosystem effects (nutrient cycling) of an intermittent prairie stream community changed during a drought. We quantified the biomass and contributions to nutrient cycling for assemblages comprising fishes, crayfish, and tadpoles in 12 isolated pools over 3 months encompassing the harshest drought on record for Kings Creek, KS, U.S.A. We predicted that macroconsumer biomass would decline with pool surface area and that differences in macroconsumer biomass and taxonomic composition would lead to different contributions of pool assemblages to nutrient cycling. The biomass of pool assemblages declined with decreasing pool size, a pattern apparently driven by mortality, emigration, or metamorphosis. We also observed a change in assemblage structure of drying pools during drought relative to pool size, shifting dominance toward species with more drought‐resistant traits. Accordingly, assemblage nitrogen (N) excretion rates declined as pool biomass was reduced, leading to a 58% reduction in N available to epilithic biofilms. Phosphorus (P) excretion rates declined from June to July, but increased in August, as species with high P excretion rates maintained similar proportional biomass and biomass of a non‐native fish increased. Molar N:P of pool assemblage excretion declined significantly throughout the drought and coincided with loss of southern redbelly dace (Chrosomus erythrogaster: Cyprinidae). Animal‐mediated nutrient cycling was altered by the loss of biomass and stoichiometric traits of taxa that differed in their occurrences and ability to tolerate abiotic conditions during drought. Elevated availability of dissolved N in isolated pools may increase N uptake rates by biofilms during drought conditions, indicating the importance of N excreted by aggregated macroconsumers, especially those with unique stoichiometric traits. While the significance of shifts in the composition of freshwater communities to ecosystems is not entirely known, additional losses in ecosystem function and changes in community structure may follow episodes of severe drought.

High interspecific variation in nutrient excretion within a guild of closely related caddisfly species

AbstractUnderstanding the amount of variation in functional traits between closely related species within guilds is critical for understanding links between community composition and ecosystem processes. Nutrient excretion is an important link between animals and their environments, and aquatic invertebrate communities can supply a considerable proportion of ecosystem nutrient demand via excretion. We quantified nitrogen (N) and phosphorus (P) excretion rates of 10 species of larval caddisflies that inhabit high‐elevation ponds and wetlands to determine the magnitude of variation in nutrient excretion within this guild. We found considerable interspecific variation in biomass‐specific excretion of nitrogen (eightfold differences), phosphorus (sevenfold differences), and the stoichiometric N:P ratios (fivefold differences). Through a meta‐analysis, we compared the variation within this guild to the variation found in other family‐level species assemblages to determine the overall range in the variation of nutrient excretion that could be expected across guilds and to determine whether the variation in this caddisfly guild is comparatively extreme, average, or low. The meta‐analysis revealed a large range in variation among guilds, and comparatively, the variation within this caddisfly guild is high for N excretion and intermediate for P excretion. The considerable variation within guilds revealed by our meta‐analysis suggests that functional redundancy among guild members is difficult to predict. Thus, some natural or human‐caused species gains or losses within biological groupings such as guilds and trophic levels could have little or no effect on ecosystem processes, whereas others could have very large effects.

A global database of nitrogen and phosphorus excretion rates of aquatic animals.

Animals can be important in modulating ecosystem-level nutrient cycling, although their importance varies greatly among species and ecosystems. Nutrient cycling rates of individual animals represent valuable data for testing the predictions of important frameworks such as the Metabolic Theory of Ecology (MTE) and ecological stoichiometry (ES). They also represent an important set of functional traits that may reflect both environmental and phylogenetic influences. Over the past two decades, studies of animal-mediated nutrient cycling have increased dramatically, especially in aquatic ecosystems. Here we present a global compilation of aquatic animal nutrient excretion rates. The dataset includes 10,534 observations from freshwater and marine animals of N and/or P excretion rates. These observations represent 491 species, including most aquatic phyla. Coverage varies greatly among phyla and other taxonomic levels. The dataset includes information on animal body size, ambient temperature, taxonomic affiliations, and animal body N:P. This data set was used to test predictions of MTE and ES, as described in Vanni and McIntyre (2016; Ecology DOI: 10.1002/ecy.1582).

Stoichiometric traits of stickleback: Effects of genetic background, rearing environment, and ontogeny.

Phenotypes can both evolve in response to, and affect, ecosystem change, but few examples of diverging ecosystem‐effect traits have been investigated. Bony armor traits of fish are good candidates for this because they evolve rapidly in some freshwater fish populations, and bone is phosphorus rich and likely to affect nutrient recycling in aquatic ecosystems. Here, we explore how ontogeny, rearing environment, and bone allocation among body parts affect the stoichiometric phenotype (i.e., stoichiometric composition of bodies and excretion) of threespine stickleback. We use two populations from distinct freshwater lineages with contrasting lateral plating phenotypes (full vs. low plating) and their hybrids, which are mostly fully plated. We found that ontogeny, rearing environment, and body condition were the most important predictors of organismal stoichiometry. Although elemental composition was similar between both populations and their hybrids, we found significant divergence in phosphorus allocation among body parts and in phosphorus excretion rates. Overall, body armor differences did not explain variation in whole body phosphorus, phosphorus allocation, or phosphorus excretion. Evolutionary divergence between these lineages in both allocation and excretion is likely to have important direct consequences for ecosystems, but may be mediated by evolution of multiple morphological or physiological traits beyond plating phenotype.

Feeding and nutrient excretion of the New Zealand freshwater mussel Echyridella menziesii (Hyriidae, Unionida): implications for nearshore nutrient budgets in lakes and reservoirs

Native freshwater mussels (Bivalvia: Unionoida) can represent a large portion of benthic biomass, but their functional role is still poorly understood. We sampled Echyridella menziesii (Hyriidae) in six lakes from two regions of the North Island of New Zealand to develop general allometric relationships for predicting filtration, excretion and biodeposition rates. Experimental containers were set up in the field (no-flow conditions) to measure filtration and biodeposition using natural phytoplankton communities, and excretion in filtered lake water. Filtration rates were 0.02–1.3 L mussel−1 h−1 and increased with increasing mussel size (R2 = 0.13, P = 0.023). Stable isotope analysis suggests that mussels also assimilate food from non-planktonic origins. Nitrogen excretion rates were 4–50 μg N mussel−1 h−1 and increased with mussel size (R2 = 0.70, P 0.3). In contrast, phosphorus excretion rates did not vary with mussel size, and were much lower in Rotorua than Waikato lakes (2 vs 5 μg P mussel−1 h−1). The reason for this regional difference is unclear, but suggests that mussels could contribute different N:P ratios to nearshore nutrient budgets in different types of lakes. Biodeposits represented 50–70% of the N, and 25–70% of the P recycled. Echyridella filters and excretes nutrients at rates similar to those measured in North American and European mussels (Unionidae, Margaritiferidae). Mussels could be important contributors of nutrients in areas where they are abundant, and their inclusion into nutrient budget models could improve the predictions of nearshore nutrient fluxes in lakes and reservoirs.

Predicting nutrient excretion of aquatic animals with metabolic ecology and ecological stoichiometry: a global synthesis.

The metabolic theory of ecology (MTE) and ecological stoichiometry (ES) are both prominent frameworks for understanding energy and nutrient budgets of organisms. We tested their separate and joint power to predict nitrogen (N) and phosphorus (P) excretion rates of ectothermic aquatic invertebrate and vertebrate animals (10,534 observations worldwide). MTE variables (body size, temperature) performed better than ES variables (trophic guild, vertebrate classification, body N:P) in predicting excretion rates, but the best models included variables from both frameworks. Size scaling coefficients were significantly lower than predicted by MTE (<0.75), were lower for P than N, and varied greatly among species. Contrary to expectations under ES, vertebrates excreted both N and P at higher rates than invertebrates despite having more nutrient-rich bodies, and primary consumers excreted as much nutrients as carnivores despite having nutrient-poor diets. Accounting for body N:P hardly improved upon predictions from treating vertebrate classification categorically. We conclude that basic data on body size, water temperature, trophic guild, and vertebrate classification are sufficient to make general estimates of nutrient excretion rates for any animal taxon or aquatic ecosystem. Nonetheless, dramatic interspecific variation in size-scaling coefficients and counter-intuitive patterns with respect to diet and body composition underscore the need for field data on consumption and egestion rates. Together, MTE and ES provide a powerful conceptual basis for interpreting and predicting nutrient recycling rates of aquatic animals worldwide.

Stoichiometric implications of a biphasic life cycle.

Animals mediate flows of elements and energy in ecosystems through processes such as nutrient sequestration in body tissues, and mineralization through excretion. For taxa with biphasic life cycles, the dramatic shifts in anatomy and physiology that occur during ontogeny are expected to be accompanied by changes in body and excreta stoichiometry, but remain little-explored, especially in vertebrates. Here we tested stoichiometric hypotheses related to the bodies and excreta of the wood frog (Lithobates sylvaticus) across life stages and during larval development. Per-capita rates of nitrogen (N) and phosphorus (P) excretion varied widely during larval ontogeny, followed unimodal patterns, and peaked midway through development (Taylor-Kollros stages XV and XII, respectively). Larval mass did not increase steadily during development but peaked at stage XVII and declined until the termination of the experiment at stage XXII. Mass-specific N and P excretion rates of the larvae decreased exponentially during development. When coupled with population-biomass estimates, population-level excretion rates were greatest at stages VIII-X. Percent carbon (C), N, and C:N of body tissue showed weak trends across major life stages; body P and C:P, however, increased sixfold during development from egg to adult. Our results demonstrate that intraspecific ontogenic changes in nutrient contents of excretion and body tissues can be significant, and that N and P are not always excreted proportionally throughout life cycles. These results highlight the dynamic roles that species play in ecosystems, and how the morphological and physiological changes that accompany ontogeny can influence ecosystem-level processes.

Assessing the effects of guppy life history evolution on nutrient recycling: from experiments to the field

Summary Trait evolution can occur in response to anthropogenic alterations to ecosystems and can occur on timescales similar to those of ecological processes suggesting that it could alter ecosystem function. In this study, we characterise the effects of life history evolution on nutrient recycling using the Trinidadian guppy (Poecilia reticulata) as a model system. Guppy life history traits evolve in response to predation pressure. When predation pressure is removed, guppy population density and average body size of the population also increase. Therefore, the evolution of guppy life histories involves changes in individual traits and demographic characteristics, both of which can alter nutrient recycling. The relative contributions of these variables to guppy‐driven nutrient recycling are unknown. We synthesise data from published experiments to disentangle how differences in individual traits, population characteristics and environmental conditions contribute to differences in guppy excretion rates. Individual guppies adapted to the absence of predators [low‐predation (LP) guppies] have lower nitrogen and phosphorus excretion rates than individual guppies adapted to predators [high‐predation (HP) guppies]. However, LP guppy populations excrete twice as much nitrogen as HP populations because of their larger average body size and higher population densities. We compare these findings to guppy excretion data collected from HP and LP sites in four rivers in Trinidad. Phenotypic and population differences in excretion rates are consistent with those observed in the experiments. Our study demonstrates that life history evolution can alter nutrient recycling in freshwater ecosystems. Characterizing the combined effects of traits and demographics is essential for understanding the effects of life history evolution on ecosystem processes.

Biogeochemical hotspots: temporal and spatial scaling of the impact of freshwater mussels on ecosystem function

Summary In streams, the creation of nutrient‐transformation hotspots by aggregated organisms may have heterogeneous and strong cumulative influences on stream nutrient dynamics. Here, we examine the potential for aggregations of freshwater mussels to create such hotspots. We measured nitrogen (N) and phosphorus (P) excretion rates of six mussel species and body tissue composition of eight. We combined these data with population densities of surveyed mussel beds in the Kiamichi River, OK, to estimate reach‐scale and stream‐scale nutrient recycling and storage. Additionally, we estimated the temporal variability in the magnitude of mussel nutrient recycling combining volumetric excretion at a reach scale with discharge and temperature data. Mussel beds constituted 1.45% of the area of the Kiamichi River. Mussel nutrient remineralisation varied greatly across beds (11.1–699.5 μmol N m−2 h−1 and 0.8–53.0 μmol P m−2 h−1), because of varying mussel densities. The community‐wide average excretion N:P (molar) of the mussel communities was 29.57, with higher excretion N:P significantly associated with higher abundances of Actinonaias ligamentina. Total nutrient storage per bed varied two orders of magnitude (6.3–631.7 kg N and 2.3–227.5 kg P) between mussel beds. Moreover, areal nutrient storage varied among the beds (11.2–133.7 mg N m−2, 4.1–48.9 mg P m−2) with the majority of nutrient storage in a long‐term store, shell (c. 87% of total N storage, c. 95% of total P storage). Freshwater mussels can be important to nutrient dynamics through nutrient regeneration and the creation of storage hotspots. However, the importance of nutrient remineralisation varies dramatically in response to organism patchiness, flow conditions and background nutrient concentrations.

Migratory fish excretion as a nutrient subsidy to recipient stream ecosystems

Summary Consumer excretion can be an important component of nutrient cycling in aquatic ecosystems. Uncertainty concerning the functional role of many freshwater organisms remains, including those with migratory life‐history strategies that may introduce nutrients to recipient systems. We quantified the magnitude, variability and importance of excretion by migratory fish in the context of stream nutrient cycling. In 2011–12, we collected data from tributaries of a central Utah reservoir used by two potamodromous fishes (Bonneville cut‐throat trout – BCT, Oncorhynchus clarkii utah; kokanee salmon – KOK, Oncorhynchus nerka) with temporally separated spawning migrations. To quantify the contribution of the two migratory freshwater fishes to nutrient cycling, we extrapolated measurements of per capita nitrogen (N) and phosphorus (P) excretion rates to the population level within the local environmental context of two tributaries. We observed differences in excretion subsidies between species and tributaries. BCT excretion rates and ratios were significantly greater than those for KOK. Estimates of the ratio of population‐level migrant excretion to tributary nutrient export were highly variable through time and between tributaries. These estimates were influenced by spatiotemporal hydrologic variation and positively related to ratios of migrant biomass to discharge. During migrations, daily migrant excretion loading comprised a maximum of 6–859% and 1–388% of tributary NH4‐N and soluble reactive phosphorus (SRP) export, respectively. Measurements of nutrient uptake suggested that migrant excretion could meet a substantial portion of ecosystem nutrient demand. Migrant excretion fluxes comprised 46–188% of ecosystem NH4‐N demand and varied between streams and species. In contrast, the proportion of SRP demand supplied by migrant excretion (34–37%) was relatively invariant. These results demonstrate an important role for potamodromous fishes as nutrient sources in recipient ecosystems. Furthermore, our data support predictions about when and where effects of fish‐derived nutrients will be strongest, thereby advancing the understanding of context‐dependent migratory fish effects in riverine ecosystems. Although widespread and common, potamodromous fishes are overlooked but important organisms capable of substantially affecting stream nutrient cycling.

Ryther revisited: nutrient excretions by fishes enhance productivity of pelagic Sargassum in the western North Atlantic Ocean

The sustained biomass of pelagic Sargassum in nutrient-poor waters of the Sargasso Sea has long been a paradox in biological oceanography. To better understand the mechanisms supporting growth of Sargassum over its broad geographic range in the western North Atlantic, we measured growth rate, gross productivity, and C:N:P ratios of both Sargassum natans and Sargassum fluitans from a variety of neritic (Caribbean Sea, Straits of Florida, Gulf Stream) and oceanic (Sargasso Sea) locations. In neritic areas, the abundance of associated fishes was quantified with a purse seine net designed to minimize fish avoidance of sampling gear. Abundant fish species were also collected for measurements of ammonium and soluble reactive phosphorus excretion rates. Low growth rates and productivity of both S. natans and S. fluitans were associated with high C:N and C:P ratios in oceanic populations in the Sargasso Sea, confirming strong nutrient-limitation in this oligotrophic gyre. In comparison, both species from the neritic areas had higher productivity and growth rates and lower C:N and C:P ratios, indicating relatively nutrient-enriched growth. Sargassum windrows in neritic locations had high abundances of associated fishes (mean of 128 fishes/kg wet weight Sargassum), especially juvenile filefish Stephanolepsis hispidus (Monacanthidae) and jacks (Carangidae). High excretion rates of ammonium and soluble reactive phosphorus were associated with these mutualistic fishes, which can provide nutrients needed to sustain growth and biomass of Sargassum. These findings suggest that new production of Sargassum occurs in neritic waters of the western North Atlantic Ocean and Gulf of Mexico, where mutualistic relationships with fishes, especially juvenile filefishes and carangids, contribute to nutrient supply and growth.

Effects of temperature on the nitrogen and phosphorus excretion of &lt;i&gt;Corbicula fluminea&lt;/i&gt;

在不同温度下,对太湖河蚬进行了室内短期氮、磷释放模拟实验.结果表明,温度对河蚬(Corbicula fluminea)不同形态氮、磷的排泄都有重要的影响.总磷(TP)、总氮(TN)、总溶解性氮(TDN)、总溶解性磷(TDP)和正磷酸盐(PO₄^3--P)排泄率随温度的升高都显著升高,而亚硝态氮(NO₂^--N)排泄率先升高后下降.铵态氮(NH₄⁺-N)排泄率在5和15℃间变化不显著,温度对硝态氮(NO₃^--N)排泄率有影响但不显著.同时实验发现河蚬为排氨动物,NH₄⁺-N排泄占TN排泄的50.78%~100%,TDN排泄占TN排泄的89.14%~100%,而NO₃^--N、NO₂^--N虽都有检出,但所占比例相对较小.在河蚬磷排泄中,TDP占TP比例范围为83.01%~100%,PO₄^3--P在36.60%~96.59%之间,且所占比例都有随温度升高而增加的趋势.同时对排泄率与干重和温度之间的关系进行了回归分析,发现不同氮、磷排泄率(NO₃^--N和NO₂^--N除外)与温度和干重的关系均符合方程R(X)=aW^b·e^c^T＋d,决定系数R²都在0.967以上.;A microcosm was established to simulate the short-term nutrient excretion of the Corbicula fluminea which was sampled in the Gonghu Bay, Lake Taihu. The C. fluminea was cultivated at 5, 15 and 25℃ after 24 hours' acclimation. The results showed that temperature greatly affected the nitrogen and phosphorus excretion rate of the C. fluminea. The excretion rate of total phosphorus(TP), total nitrogen(TN), total dissolved nitrogen(TDN), total dissolved phosphorus(TDP) and phosphate phosphorus(PO₄^3--P) represented a notable increase with the temperature increasing, however, the excretion rate of nitrite nitrogen(NO₂^--N) increased first and then reduced with the increasing temperature. The change in the excretion of ammonium nitrogen(NH₄⁺-N) between 5 and 15℃ was not significant. Temperature had a non-significant effect on nitrate nitrogen(NO₃^--N). We also found that the C. fluminea was ammonotelic benthos, and NH₄⁺-N was the main nitrogenous excretory product which occupied for 50.78%-100% of the TN. TDN accounted for 89.14%-100% of the TN. Although NO₃^--N and NO₂^--N were detected, the amount of them is very little. In the phosphorus excretion, the proportion of TDP and PO₄^3--P to TP were 83.01%-100% and 36.60%-96.59%, respectively, and the proportion of them increased with the rising temperature. Regression analysis between the dry weight, temperature and the excretion rates indicated that the regression equation R(X)=aW^b·e^c^T+d was adaptive to all types of the nitrogen and phosphorus excretion except NO₃^--N and NO₂^--N, and the R square reached over 0.967.

Feeding, respiration, and excretion of the Black Sea Noctiluca scintillans MacCartney in summer

Studies were conducted at the end of June 2011 in the coastal region of the northeastern part of the Black Sea. The bulk of the Noctiluca scintillans population was observed in the thermocline and reached a density of 40000 ind./m3. Analysis of digestive vacuoles content showed that Noctiluca could consume cells of Neoceratium tripos and N. furca, which had been considered inedible for Black Sea zooplankton, as well as temporary cysts of dinoflagellates, presumably of the toxic genus Alexandrium. The Noctiluca population consumed in total 10–30% of the abundance of temporary cysts, 2–29% of primary production, and 2–9% of potential Calanus euxinus egg production. For the first time, the excretion rates of ammonium nitrogen and mineral phosphorus were measured for N. scintillans. Our calculations showed that in summer, excretion by Noctiluca contributed from 4 to 18% and from 15 to 53% of phytoplankton total nitrogen and phosphorus requirements, respectively. The specific growth rate of Noctiluca (0.17–0.35) in summer, estimated from data on the daily food intake and respiration rate, was close to the values obtained in spring.

Invasive fishes generate biogeochemical hotspots in a nutrient-limited system.

Fishes can play important functional roles in the nutrient dynamics of freshwater systems. Aggregating fishes have the potential to generate areas of increased biogeochemical activity, or hotspots, in streams and rivers. Many of the studies documenting the functional role of fishes in nutrient dynamics have focused on native fish species; however, introduced fishes may restructure nutrient storage and cycling freshwater systems as they can attain high population densities in novel environments. The purpose of this study was to examine the impact of a non-native catfish (Loricariidae: Pterygoplichthys) on nitrogen and phosphorus remineralization and estimate whether large aggregations of these fish generate measurable biogeochemical hotspots within nutrient-limited ecosystems. Loricariids formed large aggregations during daylight hours and dispersed throughout the stream during evening hours to graze benthic habitats. Excretion rates of phosphorus were twice as great during nighttime hours when fishes were actively feeding; however, there was no diel pattern in nitrogen excretion rates. Our results indicate that spatially heterogeneous aggregations of loricariids can significantly elevate dissolved nutrient concentrations via excretion relative to ambient nitrogen and phosphorus concentrations during daylight hours, creating biogeochemical hotspots and potentially altering nutrient dynamics in invaded systems.