Agricultural runoff is a major non-point source of phosphorus (P) and nitrogen (N) in aquatic environments and often contributes to coastal eutrophication. Chemical extraction techniques that measure P and N in runoff and sediments have provided little information on biologically available P and N, particularly when soils/sediments move from an agricultural setting to freshwater, estuarine and marine systems. The aims of this thesis were to a) develop algal bioassay techniques for assessing bioavailability of P in marine and freshwater sediments, b) apply algal bioassays to determine the role of suspended sediments as P and N sources c) examine how sediment type and sediment load affect P bioavailability and identify the chemical analysis of soil P that best reflects bioavailable P, and d) determine nutrient bioavailability of agricultural soil runoff in estuarine and coastal marine ecosystems. An accurate and repeatable sediment bioassay was developed using non-axenic uni-algal cultures of the euryhaline diatom Skeletonema costatum and freshwater chlorophyte Selenastrum capricornutum. Bioassays were conducted in 250 mL Erlenmeyer flasks, with 100 mg L-1 of simulated sediment and 100 mL of modified growth media, which were continuously shaken. Suspended sediments were simulated from 7 major soil types of the Maroochy River catchment in Australia, a coastal catchment influenced by agriculture. Simulated sediments were used as the sole P source in replicate batch 7-14 d bioassays in which changes in in vivo Chl a fluorescence were measured. Maximum algal biomass in bioassays was significantly correlated to total sediment P content of the added sediment in marine and freshwater. Bioassay methodology was adapted to assess N as well as P bioavailability in simulated suspended sediments using Skeletonema costatum. S. costatum biomass was significantly influenced by sediment type and correlated with the Total P content of the sediment. Conversely, S. costatum response did not correlate to total nitrogen (TN), ammonium (NH4+), nitrate (NO3-) and easily mineralisable N content of the sediment. Sediment type did significantly influence response, suggesting that sorption/desorption properties of the sediment influenced N bioavailability. Suspended sediments appear to be a significant source of bioavailable P as S. costatum biomass was enhanced by their addition. Conversely, the lack of stimulation by sediments when used as a N source implies that suspended sediments are not an immediate source of bioavailable N, although they do contribute to the sediment nutrient pool once deposited. The influence of sediment type and sediment load on phosphorus (P) bioavailability was compared using Skeletonema costatum bioassays. The amount of sediment and associated P load influenced the maximum algal biomass reached, with low sediment loads (&lt 100 mg L-1) capable of stimulating high algal biomass. Maximum algal biomass correlated with sediment iron-oxide extractable inorganic P (FeO-P) and inorganic P (Pi) extracted by sodium bicarbonate for all sediment types. Total P (TP) did not discriminate at low TP concentrations and organic P (Po) extracted by sodium bicarbonate only correlated with algal biomass when bioavailable P was low. Thus FeO-P and Pi are the most representative analyses of bioavailable P. FeO-P can also be used to accurately measure rate of sediment P desorption, which significantly influences algal growth rate. Nutrient bioavailability of agricultural soil runoff was investigated in the Maroochy River catchment in southeast Queensland Australia, a coastal catchment influenced by agriculture. Suspended sediments, river and estuarine sediments and deposited sediment in the near-shore coastal zone were collected and analysed for nutrient bioavailability using chemical analyses and S. costatum bioassays. Suspended sediments in the Maroochy River, which consisted of silt and clay sized particles, had elevated FeO-P and TP concentrations comparable to fertilised soil. Similarly the deposited sediment sampled offshore to the river mouth had elevated total P, FeO-P and total N concentrations which were much greater than the underlying marine sediment. The deposited offshore sediment contained mainly silt sized particles and appeared to be terrigenous in origin as it was more similar in composition (TP, FeO-P, TN, total carbon, total aluminium, total iron, total silica) to estuarine suspended sediments and terrestrial soils, than the underlying marine sediment. This study demonstrated that fine silt sized particles, rich in bioavailable P and N, are being transported and deposited offshore during erosion events. These findings highlight the importance of effective land management and erosion control measures that reduce the suspended sediment load. Although agricultural practices are continually improving to reduce soil erosion, practices need to encompass a broader approach that emphasizes the link between land and sea. Effective management must consider agricultural productivity as well as the environmental impact or ‘algal bloom potential’ that a soil may have when eroded into a waterway. Soils high in bioavailable nutrients must be targeted and managed effectively, particularly in areas prone to erosion.