Dominic Valdez

Seasonal changes in water quality and macrophytes and the impact of cattle on tropical floodplain waterholes

The present study indicates the critical role of hydrologic connectivity in floodplain waterholes in the wet–dry tropics of northern Australia. These waterbodies provide dry-season refugia for plants and animals, are a hotspot of productivity, and are a critical part in the subsistence economy of many remote Aboriginal communities. We examined seasonal changes in water quality and aquatic plant cover of floodplain waterholes, and related changes to variation of waterhole depth and visitation by livestock. The waterholes showed declining water quality through the dry season, which was exacerbated by more frequent cattle usage as conditions became progressively drier, which also increased turbidity and nutrient concentrations. Aquatic macrophyte biomass was highest in the early dry season, and declined as the dry season progressed. Remaining macrophytes were flushed out by the first wet-season flows, although they quickly re-establish later during the wet season. Waterholes of greater depth were more resistant to the effects of cattle disturbance, and seasonal flushing of the waterholes with wet-season flooding homogenised the water quality and increased plant cover of previously disparate waterholes. Therefore, maintaining high levels of connectivity between the river and its floodplain is vital for the persistence of these waterholes.

High-quality algae attached to leaf litter boost invertebrate shredder growth

Algae are higher-quality food resources than allochthonous plant litter for stream invertebrates, in part, because of their higher content of polyunsaturated fatty acids (PUFAs). We tested the hypothesis that algal biofilms on the surfaces of leaf litter improve the nutritional quality of terrestrial inputs for invertebrate shredders. We used a laboratory feeding experiment with 2 light levels (open and shaded) and 2 nutrient regimes (ambient and enriched) to manipulate the algal biofilms that form on leaf surfaces (Lophostemon confertus). We assessed how these treatments affected the fatty acid (FA) composition of these biofilms and the somatic growth of a stream invertebrate shredder (Anisocentropus bicoloratus, Trichoptera) that feeds on the conditioned leaf litter. Shredders reached a significantly larger size when nutrients were added, and leaf mass loss was significantly greater in these treatments than in treatments without nutrients. Shredder growth was affected significantly by leaf PUFA content, and variations in shredder PUFA content were consistent with those in leaf PUFAs. Our results suggest that high-quality algae attached to leaf litter regulated the PUFA composition and improved the somatic growth of these shredders. Our data provide evidence that the availability of high-quality algae enhances dietary use of low-quality riparian leaf litter in stream food webs.

Body size drives allochthony in food webs of tropical rivers

Food web subsidies from external sources (“allochthony”) can support rich biological diversity and high secondary and tertiary production in aquatic systems, even those with low rates of primary production. However, animals vary in their degree of dependence on these subsidies. We examined dietary sources for aquatic animals restricted to refugial habitats (waterholes) during the dry season in Australia’s wet–dry tropics, and show that allochthony is strongly size dependent. While small-bodied fishes and invertebrates derived a large proportion of their diet from autochthonous sources within the waterhole (phytoplankton, periphyton, or macrophytes), larger animals, including predatory fishes and crocodiles, demonstrated allochthony from seasonally inundated floodplains, coastal zones or the surrounding savanna. Autochthony declined roughly 10% for each order of magnitude increase in body size. The largest animals in the food web, estuarine crocodiles (Crocodylus porosus), derived ~80% of their diet from allochthonous sources. Allochthony enables crocodiles and large predatory fish to achieve high biomass, countering empirically derived expectations for negative density vs. body size relationships. These results highlight the strong degree of connectivity that exists between rivers and their floodplains in systems largely unaffected by river regulation or dams and levees, and how large iconic predators could be disproportionately affected by these human activities.

The effect of light and nutrients on algal food quality and their consequent effect on grazer growth in subtropical streams

Recent investigators have emphasized that food quality has greater importance than food quantity for herbivore somatic growth. Herbivore growth is thought to be regulated by light and nutrients through changes in food elemental ratios, but our knowledge of the mechanisms driving grazer growth in stream ecosystems is scarce. We manipulated light (through shading) and nutrients (fertilizer addition) in 3 headwater streams in southeastern Queensland, Australia, and measured the responses of natural grazer communities. The growth responses of Austrophlebioides and Helicopsyche to light and nutrients differed with their body size and periphyton food quality. Large larvae were more sensitive to increased light and nutrient availability than were small larvae. Light induced a significant negative effect on Austrophlebioides growth, and this impact increased with increasing nutrient addition. Nutrients had a pronounced positive effect on Helicopsyche growth regardless of light intensity. These responses reflected changes in food quality. Periphyton food quality in terms of C∶N played a more decisive role in grazer growth than algal food quantity (as chlorophyll a). The growth of large larvae was significantly greater under low light intensity and nutrient-enriched conditions where high-quality food was more abundant than under high-light and low-nutrient conditions where periphyton food quality was low. Our results suggest that changes in riparian vegetation or nutrient inputs can significantly influence grazer growth through changes in periphyton C∶N. We recommend that future investigators of foodweb responses to riparian canopy change or nutrient enrichment on stream ecosystems pay more attention to the effects of algal food quality rather than quantity.

Reconciling the role of organic matter pathways in aquatic food webs by measuring multiple tracers in individuals

Few studies measure multiple ecological tracers in individual organisms, thus limiting our ability to differentiate among organic matter source pathways and understand consequences of dietary variation and the use of external subsidies in complex food webs. We combined two tracers, stable isotope (SI) ratios and fatty acids (FA), to investigate linkages among ecological compartments (water column, benthos, riparian zone) in food webs in waterholes of a dryland river network, the Border Rivers in southwestern Queensland, Australia. Comprehensive analyses of sources (plankton, periphyton, leaf litter, riparian grasses) and animals (benthic insects, mollusks, large crustaceans, fishes) for SI and FA showed that all three zones contribute to animal biomass, depending on species and life stage. Large fishes derived a subsidy from the riparian/floodplain zone, likely through the consumption of terrestrial and semi-aquatic insects and prawns that fed on detritivorous insects. Importantly, post-larval bony bream (Nematalosa erebi) and golden perch (Macquaria ambigua) were tightly connected to the water column, as evidenced by 13C-depleted, 15N-enriched isotope ratios and a high content of plankton-derived polyunsaturated fatty acids (eicosapentaenoic acid [EPA; 20:53] and docosahexaenoic acid [DHA; 22:6003]). These observations were consistent with expectations from nutritional requirements of fish early life stages and habitat changes associated with maturity. These results highlight the importance of high-quality foods during early development of fishes, and show that attempting to attribute food-web production to a single source pathway overlooks important but often subtle subsidies that maintain viable populations. A complete understanding of food-web dynamics must consider both quantity and quality of different available organic matter sources. This understanding can be achieved with a combined SI and FA approach, but more controlled dietary studies are needed to estimate how FA profiles are modified by animals when consuming a diverse range of diets of variable quality.

Temporal Uncoupling between Energy Acquisition and Allocation to Reproduction in a Herbivorous-Detritivorous Fish.

Although considerable knowledge has been gathered regarding the role of fish in cycling and translocation of nutrients across ecosystem boundaries, little information is available on how the energy obtained from different ecosystems is temporally allocated in fish bodies. Although in theory, limitations on energy budgets promote the existence of a trade-off between energy allocated to reproduction and somatic growth, this trade-off has rarely been found under natural conditions. Combining information on RNA:DNA ratios and carbon and nitrogen stable-isotope analyses we were able to achieve novel insights into the reproductive allocation of diamond mullet (Liza alata), a catadromous, widely distributed herbivorous-detritivorous fish. Although diamond mullet were in better condition during the wet season, most reproductive allocation occurred during the dry season when resources are limited and fish have poorer body condition. We found a strong trade-off between reproductive and somatic investment. Values of δ13C from reproductive and somatic tissues were correlated, probably because δ13C in food resources between dry and wet seasons do not differ markedly. On the other hand, data for δ15N showed that gonads are more correlated to muscle, a slow turnover tissue, suggesting long term synthesis of reproductive tissues. In combination, these lines of evidence suggest that L. alata is a capital breeder which shows temporal uncoupling of resource ingestion, energy storage and later allocation to reproduction.

Estuarine crocodiles in a tropical coastal floodplain obtain nutrition from terrestrial prey

The estuarine crocodile (Crocodylus porosus) is one of the largest and most widespread crocodilians in the world. Although considered an apex species, the role of the estuarine crocodile in aquatic foodwebs is poorly understood; we know what crocodiles ingest, but not what nourishes them. In this study, we used a combination of stable isotope measurements (δ13C, δ15N, and δ34S) and direct feeding observations to identify the source of nutrition of estuarine crocodiles in Kakadu National Park, Northern Australia. Our results show that most crocodiles sampled (size 850 – 4200mm, with 76% of them being > 2.5 m) consume a large variety of prey, however a large proportion of their nutrition is derived from terrestrial prey. Introduced species such as water buffaloes (Bubalus bubalis) and pigs (Sus scrofa) could contribute between 53 and 84% to the nutrition of the sampled crocodiles. The isotopic composition of large crocodiles (total length > 3 m) suggested possible increase in marine prey consumption with size (R2 = 0.30; p = 0.005). Additionally, we found crocodiles sampled in the dry season had on average higher terrestrial contributions compared to crocodiles sampled during the wet season (84.1 ± 2.4% versus 55.4 ± 7.0%). Overall, we found that terrestrial prey are important source of nutrition for many crocodiles in this region where introduced herbivorous mammals are abundant.

High rates of organic carbon processing in the hyporheic zone of intermittent streams

Organic carbon cycling is a fundamental process that underpins energy transfer through the biosphere. However, little is known about the rates of particulate organic carbon processing in the hyporheic zone of intermittent streams, which is often the only wetted environment remaining when surface flows cease. We used leaf litter and cotton decomposition assays, as well as rates of microbial respiration, to quantify rates of organic carbon processing in surface and hyporheic environments of intermittent and perennial streams under a range of substrate saturation conditions. Leaf litter processing was 48% greater, and cotton processing 124% greater, in the hyporheic zone compared to surface environments when calculated over multiple substrate saturation conditions. Processing was also greater in more saturated surface environments (i.e. pools). Further, rates of microbial respiration on incubated substrates in the hyporheic zone were similar to, or greater than, rates in surface environments. Our results highlight that intermittent streams are important locations for particulate organic carbon processing and that the hyporheic zone sustains this fundamental process even without surface flow. Not accounting for carbon processing in the hyporheic zone of intermittent streams may lead to an underestimation of its local ecological significance and collective contribution to landscape carbon processes.

Correction: Estuarine crocodiles in a tropical coastal floodplain obtain nutrition from terrestrial prey

[This corrects the article DOI: 10.1371/journal.pone.0197159.].

Groundwater supports intermittent-stream food webs

Stream food webs are changing in response to anthropogenic stressors, such as nutrient enrichment and water extraction. One way to monitor these changes is to examine shifts in the relative importance of autotrophic and detrital energetic pathways in food webs. This understanding is particularly pertinent in intermittent streams whose prevalence is projected to increase because of a changing climate and groundwater extraction. We used the stable-isotope values (δ13C and δ15N) of autotrophic and detrital basal resources and the tissue of consumers to examine the relative importance of these 2 energetic pathways during wet and dry phases in 5 groundwater-influenced intermittent streams. In addition, we used surface-water biogeochemical data to estimate the use of groundwater-derived dissolved inorganic C (DIC) in food webs. Autotrophic basal resources were the dominant C source for primary consumers during wet (>95%, on average) and dry (>77%, on average) hydrological phases. 13C-depleted autotrophs and consumers and negative correlations between the δ13C values of autotrophs and consumers with aqueous CO2 concentrations, indicated that primary producers probably are assimilating microbial respired CO2 from groundwater. Overall, we suggest that groundwater discharge is probably central to foodweb productivity in these streams by extending the duration of wetted habitat for autotrophic growth during dry phases and by delivering dissolved resources that potentially contribute to the energetic base of producers and consumers. Consequently, extended periods of reduced flow (and cease-to-flow events) caused by groundwater extraction or changed climatic conditions may reduce the overall productivity of groundwater-influenced intermittent streams.