Light and temperature are the principal drivers of ecosystem function influencing nutrient cycling, energy flows, and food web dynamics. Solar irradiance controls stream thermodynamics, and in-stream temperature governs the metabolic rate of stream biota. The loss of riparian vegetation can lead to unpredictable changes in fish biomass due to variations in metabolic requirements and energy availability. In New Zealand, unshaded pasture streams have been shown to support greater fish biomass, leading to questions about supply and demand synchronies and energy sources that support fish biomass under differing light and temperature regimes. This thesis focuses on the ecological processes in relation to the interactions among stream biota and their physical and chemical environment linking freshwater fish biomass, food web dynamics and energy availability. Using a variety of field assessments, I investigate food web dynamics and characterise energy flow with respect to land use and longitudinal riparian fragmentation in mountainous Taranaki streams, New Zealand. I further analyse the influence of alternative energy sources derived from terrestrial and marine environments and their subsidiary role in supporting fish biomass. Fish densities and biomasses were five-fold greater in pasture than in forest streams and significant correlations were found with light intensity and water temperature (p < 0.05). Body mass to abundance (M-N) allometry was similar between land uses, but the effects of increased light and temperature in pasture streams likely resulted in increased abundance for the equivalent body sizes across the entire community. Stable isotope analyses (SIA) revealed evidence for energetic and functional food web alterations, in response to land use. Changes in food sources were reflected by the transition in physical variables at the forest to pasture boundary. Here, non-predatory invertebrates showed a distinct food dominance transition between land uses, predominantly assimilating leaf litter (77%) in forest and shifting to periphyton (73%) in pasture. Periphyton biomass was significantly greater in pasture streams and was the most important food source for crayfish (Paranephrops planifrons), contributing 76% to crayfish diet in forest and 97% to crayfish diet in pasture. This highlights the importance of periphyton for energy transfer to higher trophic levels in streams. Fish showed a distinct dietary reliance on both crayfish and terrestrial invertebrates in both land uses. Unexpectedly, terrestrial invertebrate inputs were five-fold greater at pasture sites when compared to forest sites (p < 0.05). Since terrestrial invertebrates are an important food source consumed by New Zealand fish species, these terrestrially derived food sources may play a significant role in the observed fish biomass. This research highlighted the role of terrestrially derived invertebrates in providing a significant subsidiary energy source, irrespective of land use. Temperature plays a fundamental role in metabolic rates and energetic requirements of fish, emphasising the importance of temperature-driven changes in supply-demand synchronies. Importantly, summer water temperatures at pasture sites were closer to the thermal preferences of New Zealand fish species. This suggests that higher temperatures increase metabolic scope and food requirements. However, there must be sufficient food supply to compensate for rising metabolic demands of fish. On a longitudinal scale, Taranaki streams showed localised variations in energy utilisation in response to riparian fragmentation. Non-predatory invertebrates showed a food dominance transition back to allochthonous sources at lower sites, corresponding with greater riparian vegetation cover downstream. Transitions in source dominance were also observed in longfin eel (Anguilla dieffenbachii) and shortfin eel (Anguilla australis) diets, where the proportional dominance shifted from aquatic invertebrates to terrestrial invertebrates at vegetated downstream sites. These data were more reflective of recent conceptualised models derived from the Riverine Ecosystem Synthesis (RES), rather than the River Continuum Concept (RCC), with localised processes influencing pathways of energy transfer. There is limited research on the contribution of marine-derived nitrogen (MDN) by diadromous New Zealand fish, which may provide an important subsidiary source of nutrients for stream production. MDN was detected in migratory inanga larvae (Galaxias maculatus) and shrimp (Paratya curvirostris), with these species showing comparable δ¹⁵N that reflected a period of marine residence. The incorporation of MDN was not expressed in the food web, however, most likely due to low densities of inanga and shrimp in the study reaches. High δ¹⁵N in inanga shows the potential for MDN to be incorporated into stream food webs where significant seasonal whitebait migrations occur. This research provides critical insight into the drivers behind fish biomass, highlighting temperature-driven supply and demand synchronies and the importance of resource availability in sustaining New Zealand fish populations.