title: Riverine food webs and the effects of human disturbance on the contribution and quality of food sources to aquatic consumers in sub-tropical rivers
Creators: name: Tao, Juan
Publisher: name: Griffith University
Year: 2017
Subjects: subject: Sub-tropical rivers
subject: Aquatic consumers
subject: Riverine food webs

description: Understanding flows of energy and nutrients through food webs can provide important insights into mechanisms influencing the structure and function of riverine ecosystems and ecological responses to human activities. However, it is not well understood how changing land use, river impoundment and flow alteration affect the quality and quantity of food resources supporting riverine food webs. The broad aims of this thesis were to determine: (1) the effects of human disturbances on basal food sources and food quality for aquatic consumers in sub-tropical rivers in south-east Queensland (SEQ); and (2) the potential for poor food quality to be causing low recruitment of the threatened Australian lungfish (Neoceratodus forsteri Krefft, 1870). Understanding food webs is important and useful for planning environmental conservation, management and restoration. However, research on food webs has not been uniformly conducted across different parts of the world; it tends to be concentrated in specific areas or ecosystem types, and this may limit our understanding of food webs and ecosystem processes. Therefore, before answering the research questions posed, I first examined trends in food web research over time by analysing publication data from Web of Science (WOS). The review focused on the ecosystem types studied, countries in which the studies were conducted, and which countries collaborated on the studies. A total of 20,239 publications on food webs were examined, and food web research has increased dramatically since the 1990s. Most publications were focused on aquatic ecosystems, and North America and Europe contributed far more studies than Africa and South America. Collaboration among individual authors and countries has become increasingly prevalent. The USA and Canada were consistently the two most productive countries, and had the most frequent collaborations. The results indicate that food web studies from terrestrial ecosystems also require more attention in the future, especially countries from Africa and South America. Importantly, the majority of freshwater food web research has been conducted in temperate systems in the northern hemisphere. The comparatively small number of studies conducted in tropical, sub-tropical or arid freshwater systems may limit our ability to develop a general understanding and synthesis of the key factors influencing freshwater food web processes globally. The relative importance of allochthonous sources versus autochthonous sources to aquatic consumers varies in predictable ways along longitudinal gradients in rivers. However, it remains unclear how this will change under human disturbances, such as river impoundment, flow alteration and agricultural land use. I examined the potential effects of these anthropogenic disturbances on aquatic food webs by using stable isotopes (δ13C and δ15N) from six sites in each of two comparable sub-tropical rivers, the Brisbane River and Mary River in SEQ. The Brisbane River downstream was heavily affected by impoundment and flow alteration compared to the upstream and to the Mary River. Both catchments had similar levels of agricultural land use. The results showed that the main sources of variation in δ13C and δ15N isotopic values of most basal sources, invertebrates, and fish trophic guilds were due to the effects of relative catchment position within rivers, with minimal overall differences between rivers and seasons. Most of the basal sources and consumers showed a general enrichment in δ15N values from upstream to downstream in both rivers, which could be caused by increased anthropogenic nitrogen inputs and changed nitrogen cycling in riparian soils. Mixing model analyses revealed that most of the consumers examined in both rivers were supported by autochthonous carbon sources, with no detectable influence of flow alteration or land use. In contrast, the reliance of consumers on food sources was changed to a higher contribution from autochthonous pelagic sources in Brisbane River sites downstream of the dam. Fatty acids (FA) play an important role in aquatic food webs and are potentially powerful biomarkers for studying the structure and function of ecosystems. However, there has been limited research characterizing FA profiles among potential food sources and how they vary spatially along natural and anthropogenic disturbance gradients in rivers. Quantifying these major sources of variation in FA profiles of aquatic food sources is a critical pre-requisite to understanding variation in food quality for fish and other higher consumers and can contribute to more effective aquatic ecosystem conservation and management. I investigated the FA composition of nine potential food sources for higher consumers (including primary producers and invertebrate primary consumers) collected from eight sites in two sub-tropical rivers (Brisbane River and Mary River) subjected to varying degrees of human disturbance. Different food sources had distinctive FA profiles, and several functionally important unsaturated FA were found to bioaccumulate with increasing trophic position. Partial redundancy analysis (pRDA) was conducted to separate the effects of food source category (taxa) and environment (site). Taxa and site together explained 70.4 % of the variation in FA profiles, though taxa alone accounted for most of this (90.6 %). I found significant spatial variation in FA composition for several food sources that were potentially related to flow alteration, but the effects were weak and not uniform across taxa. The Australian lungfish, Neoceratodus forsteri Krefft, 1870, is federally listed threatened species whose long-term persistence is at risk due to land use intensification, water resource development, and other human pressures. Changes in the availability of high-quality food resources for this species may impact recruitment and contribute to population declines. I analysed the fatty acid (FA) composition of lungfish eggs and fin tissues from two locations with contrasting flow alteration resulting from a large impoundment in the Brisbane River. I hypothesised that flow alteration alters the FA composition of important dietary items for N. forsteri which will translate to the body tissues and eggs. The contribution of each food source was estimated with mixing models using carbon and nitrogen stable isotopes. Although no negative effect on FA composition on tissues was detected from flow alteration, the stable isotope analysis demonstrated FA difference in lungfish eggs could be attributed to changes in their diet. To conclude, my studies found that: (1) current understanding on aquatic food web processes could be biased towards temperate systems and should be modified through conducting more studies in other systems; (2) consumers in south-east Queensland sub-tropical rivers were largely supported by algal basal sources, regardless of the influence of human disturbance; (3) basal food sources and primary aquatic consumers varied in nutritional quality and possessed distinctive FA profiles, but spatial variation in FA of individual food sources was relatively low and unaffected by human disturbances associated with dams and land use; (4) the low availability of high-quality food sources could potentially be the reason for low concentrations of FAs in lungfish eggs and potentially explain the low recruitment success of this threatened species; (5) the Wivenhoe dam increased the contribution of autochthonous pelagic sources to consumers in the Brisbane River sites downstream of the dam, but did not show a negative effect on lungfish egg quality; (6) catchment land use increased the δ15N values of sources and consumers probably through increasing anthropogenic nutrient inputs and changing nitrogen cycling in riparian soils. My studies indicate that a food web approach, focusing in particular on high-quality food sources that sustain the growth and reproduction of consumers, can inform river ecosystem conservation and management.