Why is Nitrogen 15 found almost exclusively in marine environments?
Let's start that the sentence found in the source you provided:


  the nitrogen isotope 15, which is found almost exclusively in marine environments


is misleading and simply incorrect.

Now to some basics. When we discuss isotopes in environmental sciences, we never use the absolute values of an isotope content. Instead we use isotope ratios:

$$\text{Ratio (R)} = \frac{\text{heavy isotope abundance}}{\text{light isotope abundance}}$$

However, the ratios themself are not very useful, so the delta ($\delta$) notation is used:

$$\delta[^0\!\!/\!_{00}] = \left(\frac{R_{\mathrm{sample}}}{R_{\mathrm{standard}}}-1\right)\times1000$$

All standards have the delta value equal $0$ by convention. There is an international standard material for each isotope, for example for $\ce{^13C/^12C}$ ratio it is VPDB (Vienna PeeDee Belemnite), and for $\ce{^15N/^14N}$ it is atmospheric nitrogen. The latter standard was chosen because the content of the $\ce{^15N}$ isotope in atmospheric nitrogen is uniform throughout the world and it does not vary significantly over time. Many chemical, biochemical and physical processes cause the so-called isotopic fractionation to occur - it results in the increased (enrichment) or decreased (depletion) isotope ratios.

OK, we know the terminology, so now to the point. In general, the metabolic processes of living organisms cause enrichment of nitrogen. The extent of the enrichment depends on the trophic level (position in the food chain) and dietary sources of nitrogen. The highest enrichment is found in terrestial herbivores, while marine organisms usually show lower $\delta\ce{^14N}$ values. Given those facts the cited sentence makes no sense, and is probably a result of a misinterpretation of some scientific paper.

There is a lot of literature on the subject if you are interested. I can give you one good reference as a starting point:

M.A. Vanderklift, S. Ponsard. Sources of variation in consumer-diet
$\delta\ce{^15N}$ enrichment: a meta-analysis. Oecologia (2003) 136:169–182 DOI 10.1007/s00442-003-1270-z.